Title: The Evolution of Man
Author: Haeckel, Ernst
Language: English
As this book started as an ASCII text book there are no pictures available.


*** Start of this LibraryBlog Digital Book "The Evolution of Man" ***


The Evolution of Man

A POPULAR SCIENTIFIC STUDY

by Ernst Haeckel

Translated from the Fifth (enlarged) Edition by Joseph McCabe

[Issued for the Rationalist Press Association, Limited]

WATTS & CO.
17 Johnson’s Court, Fleet Street, London, E.C.
1912

From the painting by Franz von Lenbach, 1899


Contents

 LIST OF ILLUSTRATIONS
 GLOSSARY
 TRANSLATOR’S PREFACE
 TABLE: CLASSIFICATION OF THE ANIMAL WORLD

 Chapter I. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION
 Chapter II. THE OLDER EMBRYOLOGY
 Chapter III. MODERN EMBRYOLOGY
 Chapter IV. THE OLDER PHYLOGENY
 Chapter V. THE MODERN SCIENCE OF EVOLUTION
 Chapter VI. THE OVUM–THE AMŒBA
 Chapter VII. CONCEPTION
 Chapter VIII. THE GASTRÆA THEORY
 Chapter IX. THE GASTRULATION OF THE VERTEBRATE
 Chapter X. THE CŒLOM THEORY
 Chapter XI. THE VERTEBRATE CHARACTER OF MAN
 Chapter XII. THE EMBRYONIC SHIELD–GERMINATIVE AREA
 Chapter XIII. DORSAL BODY–VENTRAL BODY
 Chapter XIV. THE ARTICULATION OF THE BODY
 Chapter XV. FŒTAL MEMBRANES AND CIRCULATION
 Chapter XVI. STRUCTURE OF THE  LANCELET AND THE SEA-SQUIRT
 Chapter XVII. EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT
 Chapter XVIII. DURATION OF THE HISTORY OF OUR STEM
 Chapter XIX. OUR PROTIST ANCESTORS
 Chapter XX. OUR WORM-LIKE ANCESTORS
 Chapter XXI. OUR FISH-LIKE ANCESTORS
 Chapter XXII. OUR FIVE-TOED ANCESTORS
 Chapter XXIII. OUR APE ANCESTORS
 Chapter XXIV. EVOLUTION OF THE NERVOUS SYSTEM
 Chapter XXV. EVOLUTION OF THE SENSE-ORGANS
 Chapter XXVI. EVOLUTION OF THE ORGANS OF MOVEMENT
 Chapter XXVII. EVOLUTION OF THE ALIMENTARY SYSTEM
 Chapter XXVIII. EVOLUTION OF THE VASCULAR SYSTEM
 Chapter XXIX. EVOLUTION OF THE SEXUAL ORGANS
 Chapter XXX. RESULTS OF ANTHROPOGENY

 INDEX



LIST OF ILLUSTRATIONS

Fig. 1. The human ovum
Fig. 2. Stem-cell of an echinoderm
Fig. 3. Three epithelial cells
Fig. 4. Five spiny or grooved cells
Fig. 5. Ten liver-cells
Fig. 6. Nine star-shaped bone-cells
Fig. 7. Eleven star-shaped cells
Fig. 8. Unfertilised ovum of an echinoderm
Fig. 9. A large branching nerve-cell
Fig. 10. Blood-cells
Fig. 11. Indirect or mitotic cell-division
Fig. 12. Mobile cells
Fig. 13. Ova of various animals
Fig. 14. The human ovum
Fig. 15. Fertilised ovum of hen
Fig. 16. A creeping amœba
Fig. 17. Division of an amœba
Fig. 18. Ovum of a sponge
Fig. 19. Blood-cells, or phagocytes
Fig. 20. Spermia or spermatozoa
Fig. 21. Spermatozoa of various animals
Fig. 22. A single human spermatozoon
Fig. 23. Fertilisation of the ovum
Fig. 24. Impregnated echinoderm ovum
Fig. 25. Impregnation of the star-fish ovum
Figs. 26–27. Impregnation of sea-urchin ovum
Fig. 28. Stem-cell of a rabbit
Fig. 29. Gastrulation of a coral
Fig. 30. Gastrula of a gastræad
Fig. 31. Gastrula of a worm
Fig. 32. Gastrula of an echinoderm
Fig. 33. Gastrula of an arthropod
Fig. 34. Gastrula of a mollusc
Fig. 35. Gastrula of a vertebrate
Fig. 36. Gastrula of a lower sponge
Fig. 37. Cells from the primary germinal layers
Fig. 38. Gastrulation of the amphioxus
Fig. 39. Gastrula of the amphioxus
Fig. 40. Cleavage of the frog’s ovum
Figs. 41–44. Sections of fertilised toad ovum
Figs. 45–48. Gastrulation of the salamander
Fig. 49. Segmentation of the lamprey
Fig. 50. Gastrulation of the lamprey
Fig. 51. Gastrulation of ceratodus
Fig. 52. Ovum of a deep-sea bony fish
Fig. 53. Segmentation of a bony fish
Fig. 54. Discoid gastrula of a bony fish
Figs. 55–56. Sections of blastula of shark
Fig. 57. Discoid segmentation of bird’s ovum
Figs. 58–61. Gastrulation of the bird
Fig. 62. Germinal disk of the lizard
Figs. 63–64. Gastrulation of the opossum
Figs. 65–67. Gastrulation of the opossum
Figs. 68–71. Gastrulation of the rabbit
Fig. 72. Gastrula of the placental mammal
Fig. 73. Gastrula of the rabbit
Figs. 74–75. Diagram of the four secondary germinal layers
Figs. 76–77. Cœlomula of sagitta
Fig. 78. Section of young sagitta
Figs. 79–80. Section of amphioxus-larvæ
Figs. 81–82. Section of amphioxus-larvæ
Figs. 83–84. Chordula of the amphioxus
Figs. 85–86. Chordula of the amphibia
Figs. 87–88. Section of cœlomula-embryos of vertebrates
Figs. 89–90. Section of cœlomula-embryo of triton
Fig. 91. Dorsal part of three triton-embryos
Fig. 92. Chordula-embryo of a bird
Fig. 93. Vertebrate-embryo of a bird
Figs. 94–95. Section of the primitive streak of a chick
Fig. 96. Section of the primitive groove of a rabbit
Fig. 97. Section of primitive mouth of a human embryo
Figs. 98–102. The ideal primitive vertebrate
Fig. 103. Redundant mammary glands
Fig. 104. A Greek gynecomast
Fig. 105. Severance of the discoid mammal embryo
Figs. 106–107. The visceral embryonic vesicle
Fig. 108. Four entodermic cells
Fig. 109. Two entodermic cells
Figs. 110–114. Ovum of a rabbit
Figs. 115–118. Embryonic vesicle of a rabbit
Fig. 119. Section of the gastrula of four vertebrates
Figs 120. Embryonic shield of a rabbit
Figs. 121–123. Dorsal shield and embryonic shield of a rabbit.
Fig. 124. Cœlomula of the amphioxus
Fig. 125. Chordula of a frog
Fig. 126. Section of frog-embryo
Figs. 127–128. Dorsal shield of a chick
Fig. 129. Section of hind end of a chick
Fig. 130. Germinal area of the rabbit
Fig. 131. Embryo of the opossum
Fig. 132. Embryonic shield of the rabbit
Fig. 133. Human embryo at the sandal-stage
Fig. 134. Embryonic shield of rabbit
Fig. 135. Embryonic shield of opossum
Fig. 136. Embryonic disk of a chick
Fig. 137. Embryonic disk of a higher vertebrate
Figs. 138–142. Sections of maturing mammal embryo
Figs. 143–146. Sections of embryonic chicks
Fig. 147. Section of embryonic chick
Fig. 148. Section of fore-half of chick-embryo
Figs. 149–150. Sections of human embryos
Fig. 151. Section of a shark-embryo
Fig. 152. Section of a duck-embryo
Figs. 153–155. Sole-shaped embryonic disk of chick
Figs. 156–157. Embryo of the amphioxus
Figs. 158–160. Embryo of the amphioxus
Figs. 161–162. Sections of shark-embryos
Fig. 163. Section of a Triton-embryo
Figs. 164–166. Vertebræ
Fig. 167. Head of a shark-embryo
Figs. 168–169. Head of a chick-embryo
Fig. 170. Head of a dog-embryo
Fig. 171. Human embryo of the fourth week
Fig. 172. Section of shoulder of chick-embryo
Fig. 173. Section of pelvic region of chick-embryo
Fig. 174. Development of the lizard’s legs
Fig. 175. Human-embryo five weeks old
Figs. 176–178. Embryos of the bat
Fig. 179. Human embryos
Fig. 180. Human embryo of the fourth week
Fig. 181. Human embryo of the fifth week
Fig. 182. Section of tail of human embryo
Figs. 183–184. Human embryo dissected
Fig. 185. Miss Julia Pastrana
Figs. 186–190. Human embryos
Fig. 191. Human embryos of sixteen to eighteen ays
Figs. 192–193. Human embryo of fourth week
Fig. 194. Human embryo with its membranes
Fig. 195. Diagram of the embryonic organs
Fig. 196. Section of the pregnant womb
Fig. 197. Embryo of siamang-gibbon
Fig. 198. Section of pregnant womb
Figs. 199–200. Human fœtus–placenta
Fig. 201. Vitelline vessels in germinative area
Fig. 202. Boat-shaped embryo of the dog
Fig. 203. Lar or white-handed gibbon
Fig. 204. Young orang
Fig. 205. Wild orang
Fig. 206. Bald-headed chimpanzee
Fig. 207. Female chimpanzee
Fig. 208. Female gorilla
Fig. 209. Male giant-gorilla
Fig. 210. The lancelet
Fig. 211. Section of the head of the lancelet
Fig. 212. Section of an amphioxus-larva
Fig. 213. Diagram of preceding
Fig. 214. Section of a young amphioxus
Fig. 215. Diagram of a young amphioxus
Fig. 216. Transverse section of lancelet
Fig. 217. Section through the middle of the lancelet
Fig. 218. Section of a primitive-fish embryo
Fig. 219. Section of the head of the lancelet
Figs. 220. Organisation of an ascidia
Figs. 221. Organisation of an ascidia
Figs. 222–224. Sections of young amphioxus-larvæ
Fig. 225. An appendicaria
Fig. 226. Chroococcus minor
Fig. 227. Aphanocapsa primordialis
Fig. 228. Protamœba
Fig. 229. Original ovum-cleavage
Fig. 230. Morula
Figs. 231–232. Magosphæra planula
Fig. 233. Modern gastræads
Figs. 234–235. Prophysema primordiale
Figs. 236–237. Ascula of gastrophysema
Fig. 238. Olynthus
Fig. 239. Aphanostomum langii
Figs. 240–241. A turbellarian
Figs. 242–243. Chætonotus
Fig. 244. A nemertine worm
Fig. 245. An enteropneust
Fig. 246. Section of the branchial gut
Fig. 247. The marine lamprey
Fig. 248. Fossil primitive fish
Fig. 249. Embryo of a shark
Fig. 250. Man-eating shark
Fig. 251. Fossil angel-shark
Fig. 252. Tooth of a gigantic shark
Figs. 253–255. Crossopterygii
Fig. 256. Fossil dipneust
Fig. 257. The Australian dipneust
Figs. 258–259. Young ceratodus
Fig. 260. Fossil amphibian
Fig. 261. Larva of the spotted salamander
Fig. 262. Larva of common frog
Fig. 263. Fossil mailed amphibian
Fig. 264. The new zealand lizard
Fig. 265. Homœosaurus pulchellus
Fig. 266. Skull of a permian lizard
Fig. 267. Skull of a theromorphum
Fig. 268. Lower jaw of a primitive mammal
Figs. 269–270. The ornithorhyncus
Fig. 271. Lower jaw of a promammal
Fig. 272. The crab-eating opossum
Fig. 273. Fœtal membranes of the human embryo
Fig. 274. Skull of a fossil lemur
Fig. 275. The slender lori
Fig. 276. The white-nosed ape
Fig. 277. The drill-baboon
Figs. 278–282. Skeletons of man and the anthropoid apes
Fig. 283. Skull of the java ape-man
Fig. 284. Section of the human skin
Fig. 285. Epidermic cells
Fig. 286. Rudimentary lachrymal glands
Fig. 287. The female breast
Fig. 288. Mammary gland of a new-born infant
Fig. 289. Embryo of a bear
Fig. 290. Human embryo
Fig. 291. Central marrow of a human embryo
Figs. 292–293. The human brain
Figs. 294–296. Central marrow of human embryo
Fig. 297. Head of a chick embryo
Fig. 298. Brain of three craniote embryos
Fig. 299. Brain of a shark
Fig. 300. Brain and spinal cord of a frog
Fig. 301. Brain of an ox-embryo
Fig. 302. Brain of a human embryo
Fig. 303. Brain of a human embryo
Fig. 304. Brain of the rabbit
Fig. 305. Bead of a shark
Figs. 306–310. Heads of chick-embryos
Fig. 311. Section of mouth of human embryo
Fig. 312. Diagram of mouth-nose cavity
Figs. 313–314. Heads of human embryo
Figs. 315–316. Face of human embryo
Fig. 317. The human eye
Fig. 318. Eye of the chick embryo
Fig. 319. Section of eye of a human embryo
Fig. 320. The human ear
Fig. 321. The bony labyrinth
Fig. 322. Development of the labyrinth
Fig. 323. Primitive skull of human embryo
Fig. 324. Rudimentary muscles of the ear
Figs. 325–326. The human skeleton
Fig. 327. The human vertebral column
Fig. 328. Piece of the dorsal cord
Figs. 329–330. Dorsal vertebræ
Fig. 331. Intervertebral disk
Fig. 332. Human skull
Fig. 333. Skull of new-born child
Fig. 334. Head-skeleton of a primitive fish
Fig. 335. Skulls of nine primates
Figs. 336–338. Evolution of the fin
Fig. 339. Skeleton of the fore-leg of an amphibian
Fig. 340. Skeleton of gorilla’s hand
Fig. 341. Skeleton of human hand
Fig. 342. Skeleton of hand of six mammals
Figs. 343–345. Arm and hand of three anthropoids
Fig. 346. Section of fish’s tail
Fig. 347. Human skeleton
Fig. 348. Skeleton of the giant gorilla
Fig. 349. The human stomach
Fig. 350. Section of the head of a rabbit-embryo
Fig. 351. Shark’s teeth
Fig. 352. Gut of a human embryo
Figs. 353–354. Gut of a dog embryo
Figs. 355–356. Sections of head of lamprey
Fig. 357. Viscera of a human embryo
Fig. 358. Red blood-cells
Fig. 359. Vascular tissue
Fig. 360. Section of trunk of a chick-embryo
Fig. 361. Merocytes
Fig. 362. Vascular system of an annelid
Fig. 363. Head of a fish-embryo
Figs. 364–366. The five arterial arches
Figs. 367–370. The five arterial arches
Figs. 371–372. Heart of a rabbit-embryo
Figs. 373–374. Heart of a dog-embryo
Figs. 375–377. Heart of a human embryo
Fig. 378. Heart of adult man
Fig. 379. Section of head of a chick-embryo
Fig. 380. Section of a human embryo
Figs. 381–382. Sections of a chick-embryo
Fig. 383. Embryos of sagitta
Fig. 384. Kidneys of bdellostoma
Fig. 385. Section of embryonic shield
Figs. 386–387. Primitive kidneys
Fig. 388. Pig-embryo
Fig. 389. Human embryo
Figs. 390–392. Rudimentary kidneys and sexual organs
Figs. 393–394. Urinary and sexual organs of salamander
Fig. 395. Primitive kidneys of human embryo
Figs. 396–398. Urinary organs of ox-embryos
Fig. 399. Sexual organs of water-mole
Figs. 400–401. Original position of sexual glands
Fig. 402. Urogenital system of human embryo
Fig. 403. Section of ovary
Figs. 404–406. Graafian follicles
Fig. 407. A ripe graafian follicle
Fig. 408. The human ovum



GLOSSARY


Acrania: animals without skull (_cranium_).

Anthropogeny: the evolution (_genesis_) of man (_anthropos_).

Anthropology: the science of man.

Archi-: (in compounds) the first or typical—as, archi-cytula,
archi-gastrula, etc.

Biogeny: the science of the genesis of life (_bios_).

Blast-: (in compounds) pertaining to the early embryo (_blastos_ = a
bud); hence:—
    Blastoderm: skin (_derma_) or enclosing layer of the embryo.
    Blastosphere: the embryo in the hollow sphere stage.
    Blastula: same as preceding.
    Epiblast: the outer layer of the embryo (ectoderm).
    Hypoblast: the inner layer of the embryo (endoderm).

Branchial: pertaining to the gills (_branchia_).

Caryo-: (in compounds) pertaining to the nucleus (_caryon_); hence:—
    Caryokineses: the movement of the nucleus.
    Caryolysis: dissolution of the nucleus.
    Caryoplasm: the matter of the nucleus.

Centrolecithal: see under Lecith-.

Chordaria and Chordonia: animals with a dorsal chord or back-bone.

Cœlom or Cœloma: the body-cavity in the embryo; hence:—
    Cœlenterata: animals without a body-cavity.
    Cœlomaria: animals with a body-cavity.
    Cœlomation: formation of the body-cavity.

Cyto-: (in compounds) pertaining to the cell (_cytos_); hence:—
    Cytoblast: the nucleus of the cell.
    Cytodes: cell-like bodies, imperfect cells.
    Cytoplasm: the matter of the body of the cell.
    Cytosoma: the body (_soma_) of the cell.

Cryptorchism: abnormal retention of the testicles in the body.

Deutoplasm: see Plasm.

Dualism: the belief in the existence of two entirely distinct
principles (such as matter and spirit).

Dysteleology: the science of those features in organisms which refute
the “design-argument”.

Ectoderm: the outer (_ekto_) layer of the embryo.

Entoderm: the inner (_ento_) layer of the embryo.

Epiderm: the outer layer of the skin.

Epigenesis: the theory of gradual development of organs in the embryo.

Epiphysis: the third or central eye in the early vertebrates.

Episoma: see Soma.

Epithelia: tissues covering the surface of parts of the body (such as
the mouth, etc.)

Gonads: the sexual glands.

Gonochorism: separation of the male and female sexes.

Gonotomes: sections of the sexual glands.

Gynecomast: a male with the breasts (_masta_) of a woman (_gyne_).

Hepatic: pertaining to the liver (_hepar_).

Holoblastic: embryos in which the animal and vegetal cells divide
equally (_holon_ = whole).

Hypermastism: the possession of more than the normal breasts (_masta_).

Hypobranchial: underneath (_hypo_) the gills.

Hypophysis: sensitive-offshoot from the brain in the vertebrate.

Hyposoma: see Soma.

Lecith-: pertaining to the yelk (_lecithus_); hence:—
    Centrolecithal: eggs with the yelk in the centre.
    Lecithoma: the yelk-sac.
    Telolecithal: eggs with the yelk at one end.

Meroblastic: cleaving in part (_meron_) only.

Meta-: (in compounds) the “after” or secondary stage; hence:—
    Metagaster: the secondary or permanent gut (_gaster_).
    Metaplasm: secondary or differentiated plasm.
    Metastoma: the secondary or permanent mouth (_stoma_).
    Metazoa: the higher or later animals, made up of many cells.
    Metovum: the mature or advanced ovum.

Metamera: the segments into which the embryo breaks up.

Metamerism: the segmentation of the embryo.

Monera: the most primitive of the unicellular organisms.

Monism: belief in the fundamental unity of all things.

Morphology: the science of organic forms (generally equivalent to
anatomy).

Myotomes: segments into which the muscles break up.

Nephra: the kidneys; hence:—
    Nephridia: the rudimentary kidney-organs.
    Nephrotomes: the segments of the developing kidneys.

Ontogeny: the science of the development of the individual (generally
equivalent to embryology).

Perigenesis: the genesis of the movements in the vital particles.

Phagocytes: cells that absorb food (_phagein_ = to eat).

Phylogeny: the science of the evolution of species (_phyla_).

Planocytes: cells that move about (_planein_).

Plasm: the colloid or jelly-like matter of which organisms are
composed; hence:—
    Caryoplasm: the matter of the nucleus (_caryon_).
    Cytoplasm: the matter of the body of the cell.
    Deutoplasm: secondary or differentiated plasm.
    Metaplasm: secondary or differentiated plasm.
    Protoplasm: primitive or undifferentiated plasm.

Plasson: the simplest form of plasm.

Plastidules: small particles of plasm.

Polyspermism: the penetration of more than one sperm-cell into the
ovum.

Pro- or Prot: (in compounds) the earlier form (opposed to Meta);
hence:—
    Prochorion: the first form of the chorion.
    Progaster: the first or primitive stomach.
    Pronephridia: the earlier form of the kidneys.
    Prorenal: the earlier form of the kidneys.
    Prostoma: the first or primitive mouth.
    Protists: the earliest or unicellular organisms.
    Provertebræ: the earliest phase of the vertebræ.
    Protophyta: the primitive or unicellular plants.
    Protoplasm: undifferentiated plasm.
    Protozoa: the primitive or unicellular animals.

Renal: pertaining to the kidneys (_renes_).

Scatulation: packing or boxing-up (_scatula_ = a box).

Sclerotomes: segments into which the primitive skeleton falls.

Soma: the body; hence:—
    Cytosoma: the body of the cell (_cytos_).
    Episoma: the upper or back-half of the embryonic body.
    Somites: segments of the embryonic body.
    Hyposoma: the under or belly-half of the embryonic body.

Teleology: the belief in design and purpose (_telos_) in nature.

Telolecithal: see Lecith-.

Umbilical: pertaining to the navel (_umbilicus_).

Vitelline: pertaining to the yelk (_vitellus_).



PREFACE

[BY JOSEPH MCCABE]


The work which we now place within the reach of every reader of the
English tongue is one of the finest productions of its distinguished
author. The first edition appeared in 1874. At that time the conviction
of man’s natural evolution was even less advanced in Germany than in
England, and the work raised a storm of controversy.
Theologians—forgetting the commonest facts of our individual
development—spoke with the most profound disdain of the theory that a
Luther or a Goethe could be the outcome of development from a tiny
speck of protoplasm. The work, one of the most distinguished of them
said, was “a fleck of shame on the escutcheon of Germany.” To-day its
conclusion is accepted by influential clerics, such as the Dean of
Westminster, and by almost every biologist and anthropologist of
distinction in Europe. Evolution is not a laboriously reached
conclusion, but a guiding truth, in biological literature to-day.

 There was ample evidence to substantiate the conclusion even in the
 first edition of the book. But fresh facts have come to light in each
 decade, always enforcing the general truth of man’s evolution, and at
 times making clearer the line of development. Professor Haeckel
 embodied these in successive editions of his work. In the fifth
 edition, of which this is a translation, reference will be found to
 the very latest facts bearing on the evolution of man, such as the
 discovery of the remarkable effect of mixing human blood with that of
 the anthropoid ape. Moreover, the ample series of illustrations has
 been considerably improved and enlarged; there is no scientific work
 published, at a price remotely approaching that of the present
 edition, with so abundant and excellent a supply of illustrations.
 When it was issued in Germany, a few years ago, a distinguished
 biologist wrote in the _Frankfurter Zeitung_ that it would secure
 immortality for its author, the most notable critic of the idea of
 immortality. And the _Daily Telegraph_ reviewer described the English
 version as a “handsome edition of Haeckel’s monumental work,” and “an
 issue worthy of the subject and the author.”

The influence of such a work, one of the most constructive that Haeckel
has ever written, should extend to more than the few hundred readers
who are able to purchase the expensive volumes of the original issue.
Few pages in the story of science are more arresting and generally
instructive than this great picture of “mankind in the making.” The
horizon of the mind is healthily expanded as we follow the search-light
of science down the vast avenues of past time, and gaze on the uncouth
forms that enter into, or illustrate, the line of our ancestry. And if
the imagination recoils from the strange and remote figures that are
lit up by our search-light, and hesitates to accept them as ancestral
forms, science draws aside another veil and reveals another picture to
us. It shows us that each of us passes, in our embryonic development,
through a series of forms hardly less uncouth and unfamiliar. Nay, it
traces a parallel between the two series of forms. It shows us man
beginning his existence, in the ovary of the female infant, as a minute
and simple speck of jelly-like plasm. It shows us (from analogy) the
fertilised ovum breaking into a cluster of cohering cells, and folding
and curving, until the limb-less, head-less, long-tailed fœtus looks
like a worm-shaped body. It then points out how gill-slits and
corresponding blood-vessels appear, as in a lowly fish, and the
fin-like extremities bud out and grow into limbs, and so on; until,
after a very clear ape-stage, the definite human form emerges from the
series of transformations.

It is with this embryological evidence for our evolution that the
present volume is concerned. There are illustrations in the work that
will make the point clear at a glance. Possibly _too_ clear; for the
simplicity of the idea and the eagerness to apply it at every point
have carried many, who borrow hastily from Haeckel, out of their
scientific depth. Haeckel has never shared their errors, nor encouraged
their superficiality. He insists from the outset that a complete
parallel could not possibly be expected. Embryonic life itself is
subject to evolution. Though there is a general and substantial law—as
most of our English and American authorities admit—that the embryonic
series of forms recalls the ancestral series of forms, the parallel is
blurred throughout and often distorted. It is not the obvious
resemblance of the embryos of different animals, and their general
similarity to our extinct ancestors in this or that organ, on which we
must rest our case. A careful study must be made of the various stages
through which all embryos pass, and an effort made to prove their real
identity and therefore genealogical relation.

This is a task of great subtlety and delicacy. Many scientists have
worked at it together with Professor Haeckel—I need only name our own
Professor Balfour and Professor Ray Lankester—and the scheme is fairly
complete. But the general reader must not expect that even so clear a
writer as Haeckel can describe these intricate processes without
demanding his very careful attention. Most of the chapters in the
present volume (and the second volume will be less difficult) are
easily intelligible to all; but there are points at which the line of
argument is necessarily subtle and complex. In the hope that most
readers will be induced to master even these more difficult chapters, I
will give an outline of the characteristic argument of the work.
Haeckel’s distinctive services in regard to man’s evolution have been:
(1) The construction of a complete ancestral tree, though, of course,
some of the stages in it are purely conjectural, and not final; (2) The
tracing of the remarkable reproduction of ancestral forms in the
embryonic development of the individual. Naturally, he has not worked
alone in either department. The second volume of this work will embody
the first of these two achievements; the present one is mainly
concerned with the latter. It will be useful for the reader to have a
synopsis of the argument and an explanation of some of the chief terms
invented or employed by the author.

The main theme of the work is that, in the course of their embryonic
development, all animals, including man, pass roughly and rapidly
through a series of forms which represents the succession of their
ancestors in the past. After a severe and extensive study of embryonic
phenomena, Haeckel has drawn up a “law” (in the ordinary scientific
sense) to this effect, and has called it “the biogenetic law,” or the
chief law relating to the evolution (_genesis_) of life (_bios_). This
law is widely and increasingly accepted by embryologists and
zoologists. It is enough to quote a recent declaration of the great
American zoologist, President D. Starr Jordan: “It is, of course, true
that the life-history of the individual is an epitome of the
life-history of the race”; while a distinguished German zoologist
(Sarasin) has described it as being of the same use to the biologist as
spectrum analysis is to the astronomer.

But the reproduction of ancestral forms in the course of the embryonic
development is by no means always clear, or even always present. Many
of the embryonic phases do not recall ancestral stages at all. They may
have done so originally, but we must remember that the embryonic life
itself has been subject to adaptive changes for millions of years. All
this is clearly explained by Professor Haeckel. For the moment, I would
impress on the reader the vital importance of fixing the distinction
from the start. He must thoroughly familiarise himself with the meaning
of five terms. _Biogeny_ is the development of life in general (both in
the individual and the species), or the sciences describing it.
_Ontogeny_ is the development (embryonic and post-embryonic) of the
individual (_on_), or the science describing it. _Phylogeny_ is the
development of the race or stem (_phulon_), or the science describing
it. Roughly, _ontogeny_ may be taken to mean embryology, and
_phylogeny_ what we generally call evolution. Further, the embryonic
phenomena sometimes reproduce ancestral forms, and they are then called
_palingenetic_ (from _palin_ = again): sometimes they do not recall
ancestral forms, but are later modifications due to adaptation, and
they are then called _cenogenetic_ (from _kenos_ = new or foreign).
These terms are now widely used, but the reader of Haeckel must
understand them thoroughly.

The first five chapters are an easy account of the history of
embryology and evolution. The sixth and seventh give an equally clear
account of the sexual elements and the process of conception. But some
of the succeeding chapters must deal with embryonic processes so
unfamiliar, and pursue them through so wide a range of animals in a
brief space, that, in spite of the 200 illustrations, they will offer
difficulty to many a reader. As our aim is to secure, not a superficial
acquiescence in conclusions, but a fair comprehension of the truths of
science, we have retained these chapters. However, I will give a brief
and clear outline of the argument, so that the reader with little
leisure may realise their value.

When the animal ovum (egg-cell) has been fertilised, it divides and
subdivides until we have a cluster of cohering cells, externally not
unlike a raspberry or mulberry. This is the _morula_ (= mulberry)
stage. The cluster becomes hollow, or filled with fluid in the centre,
all the cells rising to the surface. This is the _blastula_ (hollow
ball) stage. One half of the cluster then bends or folds in upon the
other, as one might do with a thin indiarubber ball, and we get a
vase-shaped body with hollow interior (the first stomach, or “primitive
gut”), an open mouth (the first or “primitive mouth”), and a wall
composed of two layers of cells (two “germinal layers”). This is the
_gastrula_ (stomach) stage, and the process of its formation is called
_gastrulation_. A glance at the illustration (Fig. 29) will make this
perfectly clear.

So much for the embryonic process in itself. The application to
evolution has been a long and laborious task. Briefly, it was necessary
to show that _all_ the multicellular animals passed through these three
stages, so that our biogenetic law would enable us to recognise them as
reminiscences of ancestral forms. This is the work of Chapters VIII and
IX. The difficulty can be realised in this way: As we reach the higher
animals the ovum has to take up a large quantity of yelk, on which it
may feed in developing. Think of the bird’s “egg.” The effect of this
was to flatten the germ (the _morula_ and _blastula_) from the first,
and so give, at first sight, a totally different complexion to what it
has in the lowest animals. When we pass the reptile and bird stage, the
large yelk almost disappears (the germ now being supplied with blood by
the mother), but the germ has been permanently altered in shape, and
there are now a number of new embryonic processes (membranes,
blood-vessel connections, etc.). Thus it was no light task to trace the
identity of this process of _gastrulation_ in all the animals. It has
been done, however; and with this introduction the reader will be able
to follow the proof. The conclusion is important. If all animals pass
through the curious gastrula stage, it must be because they all had a
common ancestor of that nature. To this conjectural ancestor (it lived
before the period of fossilisation begins) Haeckel gives the name of
the _Gastræa,_ and in the second volume we shall see a number of living
animals of this type (“gastræads”).

The line of argument is the same in the next chapter. After laborious
and careful research (though this stage is not generally admitted in
the same sense as the previous one), a fourth common stage was
discovered, and given the name of the _Cœlomula._ The blastula had one
layer of cells, the _blastoderm_ (_derma_ = skin): the gastrula two
layers, the _ectoderm_ (“outer skin”) and _entoderm_ (“inner skin”).
Now a third layer (_mesoderm_ = middle skin) is formed, by the growth
inwards of two pouches or folds of the skin. The pouches blend
together, and form a single cavity (the body cavity, or cœlom), and its
two walls are two fresh “germinal layers.” Again, the identity of the
process has to be proved in all the higher classes of animals, and when
this is done we have another ancestral stage, the _Cœlomæa._

The remaining task is to build up the complex frame of the higher
animals—always showing the identity of the process (on which the
evolutionary argument depends) in enormously different conditions of
embryonic life—out of the four “germinal layers.” Chapter IX prepares
us for the work by giving us a very clear account of the essential
structure of the back-boned (vertebrate) animal, and the probable
common ancestor of all the vertebrates (a small fish of the lancelet
type). Chapters XI–XIV then carry out the construction step by step.
The work is now simpler, in the sense that we leave all the
invertebrate animals out of account; but there are so many organs to be
fashioned out of the four simple layers that the reader must proceed
carefully. In the second volume each of these organs will be dealt with
separately, and the parallel will be worked out between its embryonic
and its phylogenetic (evolutionary) development. The general reader may
wait for this for a full understanding. But in the meantime the
wonderful story of the construction of all our organs in the course of
a few weeks (the human frame is perfectly formed, though less than two
inches in length, by the twelfth week) from so simple a material is
full of interest. It would be useless to attempt to summarise the
process. The four chapters are themselves but a summary of it, and the
eighty fine illustrations of the process will make it sufficiently
clear. The last chapter carries the story on to the point where man at
last parts company with the anthropoid ape, and gives a full account of
the membranes or wrappers that enfold him in the womb, and the
connection with the mother.

In conclusion, I would urge the reader to consult, at his free library
perhaps, the complete edition of this work, when he has read the
present abbreviated edition. Much of the text has had to be condensed
in order to bring out the work at our popular price, and the beautiful
plates of the complete edition have had to be omitted. The reader will
find it an immense assistance if he can consult the library edition.

JOSEPH MCCABE

_Cricklewood, March, 1906._



HAECKEL’S CLASSIFICATION OF THE ANIMAL WORLD

Unicellular animals (Protozoa) 1. Unnucleated     Bacteria
Protamæbæ     Monera 2. Nucleated     _a._ Rhizopoda     Amœbina
Radiolaria _b._ Infusoria     Flagellata Ciliata 3.
Cell-colonies     Catallacta Blastæada



Unicellular animals (Protozoa) I Cœlenterata,
or Zoophytes.
Animals without
body-cavity,
blood, or anus.     _a._ Gastræads     Gastremaria Cyemaria _b._
Sponges     Protospongiæ Metaspongiæ _c._ Cnidaria (stinging animals)
Hydrozoa Polyps Medusæ _d._ Platodes (flat-worms)     Platodaria
Turbullaria Trematoda Cestoda
II
Cœlomaria or
Bilaterals.
Animals with
body-cavity and
anus, and generally blood.     _a._ Vermalia (worm-like)     Rotatoria
Strongylaria Prosopygia Frontonia _b._ Molluscs     Cochlides Conchades
Teuthodes _c._ Articulates     Annelida Crustacea Tracheata _d._
Echinoderms     Monorchonia Pentorchonia _e._ Tunicates     Copelata
Ascidiæ Thalidiæ _f._ Vertebrates     I. Acrania-Lancelet (without
skull) II. Craniota (with skull) _a._ Cyclostomes (“round-mouthed”)
_b._ Fishes     Selachii Ganoids Teleosts Dipneusts _c._ Amphibia _d._
Reptiles _e._ Birds _f._ Mammal     Monotremes Marsupials Placentals:
    Rodents
    Edentates
    Ungulates
    Cetacea
    Sirenia
    Insectivora
    Cheiroptera
    Carnassia
    Primates

(This classification is given for the purpose of explaining Haeckel’s
use of terms in this volume. The general reader should bear in mind
that it differs very considerably from more recent schemes of
classification. He should compare the scheme framed by Professor E. Ray
Lankester.)



THE EVOLUTION OF MAN

Chapter I.
THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION


The field of natural phenomena into which I would introduce my readers
in the following chapters has a quite peculiar place in the broad realm
of scientific inquiry. There is no object of investigation that touches
man more closely, and the knowledge of which should be more acceptable
to him, than his own frame. But among all the various branches of the
natural history of mankind, or _anthropology,_ the story of his
development by natural means must excite the most lively interest. It
gives us the key of the great world-riddles at which the human mind has
been working for thousands of years. The problem of the nature of man,
or the question of man’s place in nature, and the cognate inquiries as
to the past, the earliest history, the present situation, and the
future of humanity—all these most important questions are directly and
intimately connected with that branch of study which we call the
science of the evolution of man, or, in one word, “Anthropogeny” (the
genesis of man). Yet it is an astonishing fact that the science of the
evolution of man does not even yet form part of the scheme of general
education. In fact, educated people even in our day are for the most
part quite ignorant of the important truths and remarkable phenomena
which anthropogeny teaches us.

As an illustration of this curious state of things, it may be pointed
out that most of what are considered to be “educated” people do not
know that every human being is developed from an egg, or ovum, and that
this egg is one simple cell, like any other plant or animal egg. They
are equally ignorant that in the course of the development of this
tiny, round egg-cell there is first formed a body that is totally
different from the human frame, and has not the remotest resemblance to
it. Most of them have never seen such a human embryo in the earlier
period of its development, and do not know that it is quite
indistinguishable from other animal embryos. At first the embryo is no
more than a round cluster of cells, then it becomes a simple hollow
sphere, the wall of which is composed of a layer of cells. Later it
approaches very closely, at one period, to the anatomic structure of
the lancelet, afterwards to that of a fish, and again to the typical
build of the amphibia and mammals. As it continues to develop, a form
appears which is like those we find at the lowest stage of mammal-life
(such as the duck-bills), then a form that resembles the marsupials,
and only at a late stage a form that has a resemblance to the ape;
until at last the definite human form emerges and closes the series of
transformations. These suggestive facts are, as I said, still almost
unknown to the general public—so completely unknown that, if one
casually mentions them, they are called in question or denied outright
as fairy-tales. Everybody knows that the butterfly emerges from the
pupa, and the pupa from a quite different thing called a larva, and the
larva from the butterfly’s egg. But few besides medical men are aware
that _man_, in the course of his individual formation, passes through a
series of transformations which are not less surprising and wonderful
than the familiar metamorphoses of the butterfly.

The mere description of these remarkable changes through which man
passes during his embryonic life should arouse considerable interest.
But the mind will experience a far keener satisfaction when we trace
these curious facts to their causes, and when we learn to behold in
them natural phenomena which are of the highest importance throughout
the whole field of human knowledge. They throw light first of all on
the “natural history of creation,” then on psychology, or “the science
of the soul,” and through this on the whole of philosophy. And as the
general results of every branch of inquiry are summed up in philosophy,
all the sciences come in turn to be touched and influenced more or less
by the study of the evolution of man.

But when I say that I propose to present here the most important
features of these phenomena and trace them to their causes, I take the
term, and I interpret my task, in a very much wider sense than is
usual. The lectures which have been delivered on this subject in the
universities during the last half-century are almost exclusively
adapted to medical men. Certainly, the medical man has the greatest
interest in studying the origin of the human body, with which he is
daily occupied. But I must not give here this special description of
the embryonic processes such as it has hitherto been given, as most of
my readers have not studied anatomy, and are not likely to be entrusted
with the care of the adult organism. I must content myself with giving
some parts of the subject only in general outline, and must not enter
upon all the marvellous, but very intricate and not easily described,
details that are found in the story of the development of the human
frame. To understand these fully a knowledge of anatomy is needed. I
will endeavour to be as plain as possible in dealing with this branch
of science. Indeed, a sufficient general idea of the course of the
embryonic development of man can be obtained without going too closely
into the anatomic details. I trust we may be able to arouse the same
interest in this delicate field of inquiry as has been excited already
in other branches of science; though we shall meet more obstacles here
than elsewhere.

The story of the evolution of man, as it has hitherto been expounded to
medical students, has usually been confined to embryology—more
correctly, _ontogeny_—or the science of the development of the
individual human organism. But this is really only the first part of
our task, the first half of the story of the evolution of man in that
wider sense in which we understand it here. We must add as the second
half—as another and not less important and interesting branch of the
science of the evolution of the human stem—_phylogeny_: this may be
described as the science of the evolution of the various animal forms
from which the human organism has been developed in the course of
countless ages. Everybody now knows of the great scientific activity
that was occasioned by the publication of Darwin’s _Origin of Species_
in 1859. The chief direct consequence of this publication was to
provoke a fresh inquiry into the origin of the human race, and this has
proved beyond question our gradual evolution from the lower species. We
give the name of “Phylogeny” to the science which describes this ascent
of man from the lower ranks of the animal world. The chief source that
it draws upon for facts is “Ontogeny,” or embryology, the science of
the development of the individual organism. Moreover, it derives a good
deal of support from _ paleontology,_ or the science of fossil remains,
and even more from comparative anatomy, or _morphology._

These two branches of our science—on the one side ontogeny or
embryology, and on the other phylogeny, or the science of
race-evolution—are most vitally connected. The one cannot be understood
without the other. It is only when the two branches fully co-operate
and supplement each other that “Biogeny” (or the science of the genesis
of life in the widest sense) attains to the rank of a philosophic
science. The connection between them is not external and superficial,
but profound, intrinsic, and causal. This is a discovery made by recent
research, and it is most clearly and correctly expressed in the
comprehensive law which I have called “the fundamental law of organic
evolution,” or “the fundamental law of biogeny.” This general law, to
which we shall find ourselves constantly recurring, and on the
recognition of which depends one’s whole insight into the story of
evolution, may be briefly expressed in the phrase: “The history of the
fœtus is a recapitulation of the history of the race”; or, in other
words, “Ontogeny is a recapitulation of phylogeny.” It may be more
fully stated as follows: The series of forms through which the
individual organism passes during its development from the ovum to the
complete bodily structure is a brief, condensed repetition of the long
series of forms which the animal ancestors of the said organism, or the
ancestral forms of the species, have passed through from the earliest
period of organic life down to the present day.

The causal character of the relation which connects embryology with
stem-history is due to the action of heredity and adaptation. When we
have rightly understood these, and recognised their great importance in
the formation of organisms, we can go a step further and say:
Phylogenesis is the mechanical cause of ontogenesis.[1] In other words,
the development of the stem, or race, is, in accordance with the laws
of heredity and adaptation, the cause of all the changes which appear
in a condensed form in the evolution of the fœtus.

 [1] The term “genesis,” which occurs throughout, means, of course,
 “birth” or origin. From this we get: Biogeny = the origin of life
 (_bios_); Anthropogeny = the origin of man (_anthropos_); Ontogeny =
 the origin of the individual (_on_); Phylogeny = the origin of the
 species (_phulon_); and so on. In each case the term may refer to the
 process itself, or to the science describing the process.—Translator.

The chain of manifold animal forms which represent the ancestry of each
higher organism, or even of man, according to the theory of descent,
always form a connected whole. We may designate this uninterrupted
series of forms with the letters of the alphabet: A, B, C, D, E, etc.,
to Z. In apparent contradiction to what I have said, the story of the
development of the individual, or the ontogeny of most organisms, only
offers to the observer a part of these forms; so that the defective
series of embryonic forms would run: A, B, D, F, H, K, M, etc.; or, in
other cases, B, D, H, L, M, N, etc. Here, then, as a rule, several of
the evolutionary forms of the original series have fallen out.
Moreover, we often find—to continue with our illustration from the
alphabet—one or other of the original letters of the ancestral series
represented by corresponding letters from a different alphabet. Thus,
instead of the Roman B and D, we often have the Greek Β and Δ. In this
case the text of the biogenetic law has been corrupted, just as it had
been abbreviated in the preceding case. But, in spite of all this, the
series of ancestral forms remains the same, and we are in a position to
discover its original complexion.

In reality, there is always a certain parallel between the two
evolutionary series. But it is obscured from the fact that in the
embryonic succession much is wanting that certainly existed in the
earlier ancestral succession. If the parallel of the two series were
complete, and if this great fundamental law affirming the causal
connection between ontogeny and phylogeny in the proper sense of the
word were directly demonstrable, we should only have to determine, by
means of the microscope and the dissecting knife, the series of forms
through which the fertilised ovum passes in its development; we should
then have before us a complete picture of the remarkable series of
forms which our animal ancestors have successively assumed from the
dawn of organic life down to the appearance of man. But such a
repetition of the ancestral history by the individual in its embryonic
life is very rarely complete. We do not often find our full alphabet.
In most cases the correspondence is very imperfect, being greatly
distorted and falsified by causes which we will consider later. We are
thus, for the most part, unable to determine in detail, from the study
of its embryology, all the different shapes which an organism’s
ancestors have assumed; we usually—and especially in the case of the
human fœtus—encounter many gaps. It is true that we can fill up most of
these gaps satisfactorily with the help of comparative anatomy, but we
cannot do so from direct embryological observation. Hence it is
important that we find a large number of lower animal forms to be still
represented in the course of man’s embryonic development. In these
cases we may draw our conclusions with the utmost security as to the
nature of the ancestral form from the features of the form which the
embryo momentarily assumes.

To give a few examples, we can infer from the fact that the human ovum
is a simple cell that the first ancestor of our species was a tiny
unicellular being, something like the amœba. In the same way, we know,
from the fact that the human fœtus consists, at the first, of two
simple cell-layers (the _ gastrula_), that the _gastræa_, a form with
two such layers, was certainly in the line of our ancestry. A later
human embryonic form (the _chordula_) points just as clearly to a
worm-like ancestor (the _prochordonia_), the nearest living relation of
which is found among the actual ascidiæ. To this succeeds a most
important embryonic stage (_acrania_), in which our headless fœtus
presents, in the main, the structure of the lancelet. But we can only
indirectly and approximately, with the aid of comparative anatomy and
ontogeny, conjecture what lower forms enter into the chain of our
ancestry between the gastræa and the chordula, and between this and the
lancelet. In the course of the historical development many intermediate
structures have gradually fallen out, which must certainly have been
represented in our ancestry. But, in spite of these many, and sometimes
very appreciable, gaps, there is no contradiction between the two
successions. In fact, it is the chief purpose of this work to prove the
real harmony and the original parallelism of the two. I hope to show,
on a substantial basis of facts, that we can draw most important
conclusions as to our genealogical tree from the actual and
easily-demonstrable series of embryonic changes. We shall then be in a
position to form a general idea of the wealth of animal forms which
have figured in the direct line of our ancestry in the lengthy history
of organic life.

In this evolutionary appreciation of the facts of embryology we must,
of course, take particular care to distinguish sharply and clearly
between the primitive, palingenetic (or ancestral) evolutionary
processes and those due to cenogenesis.[2] By _palingenetic_ processes,
or embryonic _recapitulations,_ we understand all those phenomena in
the development of the individual which are transmitted from one
generation to another by heredity, and which, on that account, allow us
to draw direct inferences as to corresponding structures in the
development of the species. On the other hand, we give the name of
_cenogenetic_ processes, or embryonic _variations,_ to all those
phenomena in the fœtal development that cannot be traced to inheritance
from earlier species, but are due to the adaptation of the fœtus, or
the infant-form, to certain conditions of its embryonic development.
These cenogenetic phenomena are foreign or later additions; they allow
us to draw no direct inference whatever as to corresponding processes
in our ancestral history, but rather hinder us from doing so.

 [2] Palingenesis = new birth, or re-incarnation (_palin_ = again,
 _genesis_ or _genea_ = development); hence its application to the
 phenomena which are recapitulated by heredity from earlier ancestral
 forms. Cenogenesis = foreign or negligible development (_kenos_ and _
 genea_); hence, those phenomena which come later in the story of life
 to disturb the inherited structure, by a fresh adaptation to
 environment.—Translator.

This careful discrimination between the primary or palingenetic
processes and the secondary or cenogenetic is of great importance for
the purposes of the scientific history of a species, which has to draw
conclusions from the available facts of embryology, comparative
anatomy, and paleontology, as to the processes in the formation of the
species in the remote past. It is of the same importance to the student
of evolution as the careful distinction between genuine and spurious
texts in the works of an ancient writer, or the purging of the real
text from interpolations and alterations, is for the student of
philology. It is true that this distinction has not yet been fully
appreciated by many scientists. For my part, I regard it as the first
condition for forming any just idea of the evolutionary process, and I
believe that we must, in accordance with it, divide embryology into two
sections—palingenesis, or the science of recapitulated forms; and
cenogenesis, or the science of supervening structures.

To give at once a few examples from the science of man’s origin in
illustration of this important distinction, I may instance the
following processes in the embryology of man, and of all the higher
vertebrates, as _palingenetic_: the formation of the two primary
germinal layers and of the primitive gut, the undivided structure of
the dorsal nerve-tube, the appearance of a simple axial rod between the
medullary tube and the gut, the temporary formation of the gill-clefts
and arches, the primitive kidneys, and so on.[3] All these, and many
other important structures, have clearly been transmitted by a steady
heredity from the early ancestors of the mammal, and are, therefore,
direct indications of the presence of similar structures in the history
of the stem. On the other hand, this is certainly not the case with the
following embryonic forms, which we must describe as cenogenetic
processes: the formation of the yelk-sac, the allantois, the placenta,
the amnion, the serolemma, and the chorion—or, generally speaking, the
various fœtal membranes and the corresponding changes in the blood
vessels. Further instances are: the dual structure of the heart cavity,
the temporary division of the plates of the primitive vertebræ and
lateral plates, the secondary closing of the ventral and intestinal
walls, the formation of the navel, and so on. All these and many other
phenomena are certainly not traceable to similar structures in any
earlier and completely-developed ancestral form, but have arisen simply
by adaptation to the peculiar conditions of embryonic life (within the
fœtal membranes). In view of these facts, we may now give the following
more precise expression to our chief law of biogeny: The evolution of
the fœtus (or _ontogenesis_) is a condensed and abbreviated
recapitulation of the evolution of the stem (or _ phylogenesis_); and
this recapitulation is the more complete in proportion as the original
development (or _palingenesis_) is preserved by a constant heredity; on
the other hand, it becomes less complete in proportion as a varying
adaptation to new conditions increases the disturbing factors in the
development (or cenogenesis).

 [3] All these, and the following structures, will be fully described
 in later chapters.—Translator.

The cenogenetic alterations or distortions of the original palingenetic
course of development take the form, as a rule, of a gradual
displacement of the phenomena, which is slowly effected by adaptation
to the changed conditions of embryonic existence during the course of
thousands of years. This displacement may take place as regards either
the position or the time of a phenomenon.

The great importance and strict regularity of the time-variations in
embryology have been carefully studied recently by Ernest Mehnert, in
his _Biomechanik_ (Jena, 1898). He contends that our biogenetic law has
not been impaired by the attacks of its opponents, and goes on to say:
“Scarcely any piece of knowledge has contributed so much to the advance
of embryology as this; its formulation is one of the most signal
services to general biology. It was not until this law passed into the
flesh and blood of investigators, and they had accustomed themselves to
see a reminiscence of ancestral history in embryonic structures, that
we witnessed the great progress which embryological research has made
in the last two decades.” The best proof of the correctness of this
opinion is that now the most fruitful work is done in all branches of
embryology with the aid of this biogenetic law, and that it enables
students to attain every year thousands of brilliant results that they
would never have reached without it.

It is only when one appreciates the cenogenetic processes in relation
to the palingenetic, and when one takes careful account of the changes
which the latter may suffer from the former, that the radical
importance of the biogenetic law is recognised, and it is felt to be
the most illuminating principle in the science of evolution. In this
task of discrimination it is the silver thread in relation to which we
can arrange all the phenomena of this realm of marvels—the “Ariadne
thread,” which alone enables us to find our way through this labyrinth
of forms. Hence the brothers Sarasin, the zoologists, could say with
perfect justice, in their study of the evolution of the _Ichthyophis,_
that “the great biogenetic law is just as important for the zoologist
in tracing long-extinct processes as spectrum analyses is for the
astronomer.”

Even at an earlier period, when a correct acquaintance with the
evolution of the human and animal frame was only just being
obtained—and that is scarcely eighty years ago!—the greatest
astonishment was felt at the remarkable similarity observed between the
embryonic forms, or stages of fœtal development, in very different
animals; attention was called even then to their close resemblance to
certain fully-developed animal forms belonging to some of the lower
groups. The older scientists (Oken, Treviranus, and others) knew
perfectly well that these lower forms in a sense illustrated and fixed,
in the hierarchy of the animal world, a temporary stage in the
evolution of higher forms. The famous anatomist Meckel spoke in 1821 of
a “similarity between the development of the embryo and the series of
animals.” Baer raised the question in 1828 how far, within the
vertebrate type, the embryonic forms of the higher animals assume the
permanent shapes of members of lower groups. But it was impossible
fully to understand and appreciate this remarkable resemblance at that
time. We owe our capacity to do this to the theory of descent; it is
this that puts in their true light the action of _heredity_ on the one
hand and _adaptation_ on the other. It explains to us the vital
importance of their constant reciprocal action in the production of
organic forms. Darwin was the first to teach us the great part that was
played in this by the ceaseless struggle for existence between living
things, and to show how, under the influence of this (by natural
selection), new species were produced and maintained solely by the
interaction of heredity and adaptation. It was thus Darwinism that
first opened our eyes to a true comprehension of the supremely
important relations between the two parts of the science of organic
evolution—Ontogeny and Phylogeny.

Heredity and adaptation are, in fact, the two constructive
physiological functions of living things; unless we understand these
properly we can make no headway in the study of evolution. Hence, until
the time of Darwin no one had a clear idea of the real nature and
causes of embryonic development. It was impossible to explain the
curious series of forms through which the human embryo passed; it was
quite unintelligible why this strange succession of animal-like forms
appeared in the series at all. It had previously been generally assumed
that the man was found complete in all his parts in the ovum, and that
the development consisted only in an unfolding of the various parts, a
simple process of growth. This is by no means the case. On the
contrary, the whole process of the development of the individual
presents to the observer a connected succession of different
animal-forms; and these forms display a great variety of external and
internal structure. But _why_ each individual human being should pass
through this series of forms in the course of his embryonic development
it was quite impossible to say until Lamarck and Darwin established the
theory of descent. Through this theory we have at last detected the
real causes, the _efficient causes,_ of the individual development; we
have learned that these _mechanical_ causes suffice of themselves to
effect the formation of the organism, and that there is no need of the
_final_ causes which were formerly assumed. It is true that in the
academic philosophies of our time these final causes still figure very
prominently; in the new philosophy of nature we can entirely replace
them by efficient causes. We shall see, in the course of our inquiry,
how the most wonderful and hitherto insoluble enigmas in the human and
animal frame have proved amenable to a mechanical explanation, by
causes acting without prevision, through Darwin’s reform of the science
of evolution. We have everywhere been able to substitute unconscious
causes, acting from necessity, for conscious, purposive causes.[4]

 [4] The monistic or mechanical philosophy of nature holds that only
 unconscious, necessary, efficient causes are at work in the whole
 field of nature, in organic life as well as in inorganic changes. On
 the other hand, the dualist or vitalist philosophy of nature affirms
 that unconscious forces are only at work in the inorganic world, and
 that we find conscious, purposive, or final causes in organic nature.

If the new science of evolution had done no more than this, every
thoughtful man would have to admit that it had accomplished an immense
advance in knowledge. It means that in the whole of philosophy that
tendency which we call monistic, in opposition to the dualistic, which
has hitherto prevailed, must be accepted.[5] At this point the science
of human evolution has a direct and profound bearing on the foundations
of philosophy. Modern anthropology has, by its astounding discoveries
during the second half of the nineteenth century, compelled us to take
a completely monistic view of life. Our bodily structure and its life,
our embryonic development and our evolution as a species, teach us that
the same laws of nature rule in the life of man as in the rest of the
universe. For this reason, if for no others, it is desirable, nay,
indispensable, that every man who wishes to form a serious and
philosophic view of life, and, above all, the expert philosopher,
should acquaint himself with the chief facts of this branch of science.

 [5] Monism is neither purely materialistic nor purely spiritualistic,
 but a reconciliation of these two principles, since it regards the
 whole of nature as one, and sees only efficient causes at work in it.
 Dualism, on the contrary, holds that nature and spirit, matter and
 force, the world and God, inorganic and organic nature, are separate
 and independent existences. Cf. _The Riddle of the Universe,_ chap.
 xii.


The facts of embryology have so great and obvious a significance in
this connection that even in recent years dualist and teleological
philosophers have tried to rid themselves of them by simply denying
them. This was done, for instance, as regards the fact that man is
developed from an egg, and that this egg or ovum is a simple cell, as
in the case of other animals. When I had explained this pregnant fact
and its significance in my _History of Creation,_ it was described in
many of the theological journals as a dishonest invention of my own.
The fact that the embryos of man and the dog are, at a certain stage of
their development, almost indistinguishable was also denied. When we
examine the human embryo in the third or fourth week of its
development, we find it to be quite different in shape and structure
from the full-grown human being, but almost identical with that of the
ape, the dog, the rabbit, and
other mammals, at the same stage of ontogeny. We find a bean-shaped
body of very simple construction, with a tail below and a pair of fins
at the sides, something like those of a fish, but very different from
the limbs of man and the mammals. Nearly the whole front half of the
body is taken up by a shapeless head without face, at the sides of
which we find gill-clefts and arches as in the fish. At this stage of
its development the human embryo does not differ in any essential
detail from that of the ape, dog, horse, ox, etc., at a corresponding
period. This important fact can easily be verified at any moment by a
comparison of the embryos of man, the dog, rabbit, etc. Nevertheless,
the theologians and dualist philosophers pronounced it to be a
materialistic invention; even scientists, to whom the facts should be
known, have sought to deny them.

There could not be a clearer proof of the profound importance of these
embryological facts in favour of the monistic philosophy than is
afforded by these efforts of its opponents to get rid of them by
silence or denial. The truth is that these facts are most inconvenient
for them, and are quite irreconcilable with their views. We must be all
the more pressing on our side to put them in their proper light. I
fully agree with Huxley when he says, in his _Man’s Place in Nature_:
“Though these facts are ignored by several well-known popular leaders,
they are easy to prove, and are accepted by all scientific men; on the
other hand, their importance is so great that those who have once
mastered them will, in my opinion, find few other biological
discoveries to astonish them.”

We shall make it our chief task to study the evolution of man’s bodily
frame and its various organs in their external form and internal
structures. But I may observe at once that this is accompanied step by
step with a study of the evolution of their functions. These two
branches of inquiry are inseparably united in the whole of
anthropology, just as in zoology (of which the former is only a
section) or general biology. Everywhere the peculiar form of the
organism and its structures, internal and external, is directly related
to the special physiological functions which the organism or organ has
to execute. This intimate connection of structure and function, or of
the instrument and the work done by it, is seen in the science of
evolution and all its parts. Hence the story of the evolution of
structures, which is our immediate concern, is also the history of the
development of functions; and this holds good of the human organism as
of any other.

At the same time, I must admit that our knowledge of the evolution of
functions is very far from being as complete as our acquaintance with
the evolution of structures. One might say, in fact, that the whole
science of evolution has almost confined itself to the study of
structures; the evolution of _ functions_ hardly exists even in name.
That is the fault of the physiologists, who have as yet concerned
themselves very little about evolution. It is only in recent times that
physiologists like W. Engelmann, W. Preyer, M. Verworn, and a few
others, have attacked the evolution of functions.

It will be the task of some future physiologist to engage in the study
of the evolution of functions with the same zeal and success as has
been done for the evolution of structures in morphogeny (the science of
the genesis of forms). Let me illustrate the close connection of the
two by a couple of examples. The heart in the human embryo has at first
a very simple construction, such as we find in permanent form among the
ascidiæ and other low organisms; with this is associated a very simple
system of circulation of the blood. Now, when we find that with the
full-grown heart there comes a totally different and much more
intricate circulation, our inquiry into the development of the heart
becomes at once, not only an anatomical, but also a physiological,
study. Thus it is clear that the ontogeny of the heart can only be
understood in the light of its phylogeny (or development in the past),
both as regards function and structure. The same holds true of all the
other organs and their functions. For instance, the science of the
evolution of the alimentary canal, the lungs, or the sexual organs,
gives us at the same time, through the exact comparative investigation
of structure-development, most important information with regard to the
evolution of the functions of these organs.

This significant connection is very clearly seen in the evolution of
the nervous system. This system is in the economy of the human body the
medium of sensation, will, and even thought, the highest of the psychic
functions; in a word, of all the various functions which constitute the
proper object of psychology. Modern anatomy and physiology have proved
that these psychic functions are immediately dependent on the fine
structure and the composition of the central nervous system, or the
internal texture of the brain and spinal cord. In these we find the
elaborate cell-machinery, of which the psychic or soul-life is the
physiological function. It is so intricate that most men still look
upon the mind as something supernatural that cannot be explained on
mechanical principles.

But embryological research into the gradual appearance and the
formation of this important system of organs yields the most astounding
and significant results. The first sketch of a central nervous system
in the human embryo presents the same very simple type as in the other
vertebrates. A spinal tube is formed in the external skin of the back,
and from this first comes a simple spinal cord without brain, such as
we find to be the permanent psychic organ in the lowest type of
vertebrate, the amphioxus. Not until a later stage is a brain formed at
the anterior end of this cord, and then it is a brain of the most
rudimentary kind, such as we find permanently among the lower fishes.
This simple brain develops step by step, successively assuming forms
which correspond to those of the amphibia, the reptiles, the
duck-bills, and the lemurs. Only in the last stage does it reach the
highly organised form which distinguishes the apes from the other
vertebrates, and which attains its full development in man.

Comparative physiology discovers a precisely similar growth. The
function of the brain, the psychic activity, rises step by step with
the advancing development of its structure.

Thus we are enabled, by this story of the evolution of the nervous
system, to understand at length _the natural development of the human
mind_ and its gradual unfolding. It is only with the aid of embryology
that we can grasp how these highest and most striking faculties of the
animal organism have been historically evolved. In other words, a
knowledge of the evolution of the spinal cord and brain in the human
embryo leads us directly to a comprehension of the historic development
(or phylogeny) of the human mind, that highest of all faculties, which
we regard as something so marvellous and supernatural in the adult man.
This is certainly one of the greatest and most pregnant results of
evolutionary science. Happily our embryological knowledge of man’s
central nervous system is now so adequate, and agrees so thoroughly
with the complementary results of comparative anatomy and physiology,
that we are thus enabled to obtain a clear insight into one of the
highest problems of philosophy, the phylogeny of the soul, or the
ancestral history of the mind of man. Our chief support in this comes
from the embryological study of it, or the ontogeny of the soul. This
important section of psychology owes its origin especially to W.
Preyer, in his interesting works, such as _The Mind of the Child. The
Biography of a Baby_ (1900), of Milicent Washburn Shinn, also deserves
mention. [See also Preyer’s _Mental Development in the Child_
(translation), and Sully’s _Studies of Childhood_ and _Children’s
Ways._]

In this way we follow the only path along which we may hope to reach
the solution of this difficult problem.

Thirty-six years have now elapsed since, in my _General Morphology,_ I
established phylogeny as an independent science and showed its intimate
causal connection with ontogeny; thirty years have passed since I gave
in my gastræa-theory the proof of the justice of this, and completed it
with the theory of germinal layers. When we look back on this period we
may ask, What has been accomplished during it by the fundamental law of
biogeny? If we are impartial, we must reply that it has proved its
fertility in hundreds of sound results, and that by its aid we have
acquired a vast fund of knowledge which we should never have obtained
without it.

There has been no dearth of attacks—often violent attacks—on my
conception of an intimate causal connection between ontogenesis and
phylogenesis; but no other satisfactory explanation of these important
phenomena has yet been offered to us. I say this especially with regard
to Wilhelm His’s theory of a “mechanical evolution,” which questions
the truth of phylogeny generally, and would explain the complicated
embryonic processes without going beyond by simple physical
changes—such as the bending and folding of leaves by electricity, the
origin of cavities through unequal strain of the tissues, the formation
of processes by uneven growth, and so on. But the fact is that these
embryological phenomena themselves demand explanation in turn, and this
can only be found, as a rule, in the corresponding changes in the long
ancestral series, or in the physiological functions of heredity and
adaptation.



Chapter II.
THE OLDER EMBRYOLOGY


It is in many ways useful, on entering upon the study of any science,
to cast a glance at its historical development. The saying that
“everything is best understood in its growth” has a distinct
application to science. While we follow its gradual development we get
a clearer insight into its aims and objects. Moreover, we shall see
that the present condition of the science of human evolution, with all
its characteristics, can only be rightly understood when we examine its
historical growth. This task will, however, not detain us long. The
study of man’s evolution is one of the latest branches of natural
science, whether you consider the embryological or the phylogenetic
section of it.

Apart from the few germs of our science which we find in classical
antiquity, and which we shall notice presently, we may say that it
takes its definite rise, as a science, in the year 1759, when one of
the greatest German scientists, Caspar Friedrich Wolff, published his
_Theoria generationis._ That was the foundation-stone of the science of
animal embryology. It was not until fifty years later, in 1809, that
Jean Lamarck published his _Philosophie Zoologique_—the first effort to
provide a base for the theory of evolution; and it was another
half-century before Darwin’s work appeared (in 1859), which we may
regard as the first scientific attainment of this aim. But before we go
further into this solid establishment of evolution, we must cast a
brief glance at that famous philosopher and scientist of antiquity, who
stood alone in this, as in many other branches of science, for more
than 2000 years: the “father of Natural History,” Aristotle.

The extant scientific works of Aristotle deal with many different sides
of biological research; the most comprehensive of them is his famous
_History of Animals._ But not less interesting is the smaller work, On
the _Generation of Animals (Peri zoon geneseos)._ This work treats
especially of embryonic development, and it is of great interest as
being the earliest of its kind and the only one that has come down to
us in any completeness from classical antiquity.

Aristotle studied embryological questions in various classes of
animals, and among the lower groups he learned many most remarkable
facts which we only rediscovered between 1830 and 1860. It is certain,
for instance, that he was acquainted with the very peculiar mode of
propagation of the cuttlefishes, or cephalopods, in which a yelk-sac
hangs out of the mouth of the fœtus. He knew, also, that embryos come
from the eggs of the bee even when they have not been fertilised. This
“parthenogenesis” (or virgin-birth) of the bees has only been
established in our time by the distinguished zoologist of Munich,
Siebold. He discovered that male bees come from the unfertilised, and
female bees only from the fertilised, eggs. Aristotle further states
that some kinds of fishes (of the genus _serranus_) are hermaphrodites,
each individual having both male and female organs and being able to
fertilise itself; this, also, has been recently confirmed. He knew that
the embryo of many fishes of the shark family is attached to the
mother’s body by a sort of placenta, or nutritive organ very rich in
blood; apart from these, such an arrangement is only found among the
higher mammals and man. This placenta of the shark was looked upon as
legendary for a long time, until Johannes Müller proved it to be a fact
in 1839. Thus a number of remarkable discoveries were found in
Aristotle’s embryological work, proving a very good acquaintance of the
great scientist—possibly helped by his predecessors—with the facts of
ontogeny, and a great advance upon succeeding generations in this
respect.

In the case of most of these discoveries he did not merely describe the
fact, but added a number of observations on its significance. Some of
these theoretical remarks are of particular interest, because they show
a correct appreciation of the nature of the embryonic processes. He
conceives the development of the individual as a new formation, in the
course of which the various parts of the body take shape successively.
When the human or animal frame is developed in the mother’s body, or
separately in an egg, the heart—which he regards as the starting-point
and centre of the organism—must appear first. Once the heart is formed
the other organs arise, the internal ones before the external, the
upper (those above the diaphragm) before the lower (or those beneath
the diaphragm). The brain is formed at an early stage, and the eyes
grow out of it. These observations are quite correct. And, if we try to
form some idea from these data of Aristotle’s general conception of the
embryonic process, we find a dim prevision of the theory which Wolff
showed 2000 years afterwards to be the correct view. It is significant,
for instance, that Aristotle denied the eternity of the individual in
any respect. He said that the species or genus, the group of similar
individuals, might be eternal, but the individual itself is temporary.
It comes into being in the act of procreation, and passes away at
death.

During the 2000 years after Aristotle no progress whatever was made in
general zoology, or in embryology in particular. People were content to
read, copy, translate, and comment on Aristotle. Scarcely a single
independent effort at research was made in the whole of the period.
During the Middle Ages the spread of strong religious beliefs put
formidable obstacles in the way of independent scientific
investigation. There was no question of resuming the advance of
biology. Even when human anatomy began to stir itself once more in the
sixteenth century, and independent research was resumed into the
structure of the developed body, anatomists did not dare to extend
their inquiries to the unformed body, the embryo, and its development.
There were many reasons for the prevailing horror of such studies. It
is natural enough, when we remember that a Bull of Boniface VIII
excommunicated every man who ventured to dissect a human corpse. If the
dissection of a developed body were a crime to be thus punished, how
much more dreadful must it have seemed to deal with the embryonic body
still enclosed in the womb, which the Creator himself had decently
veiled from the curiosity of the scientist! The Christian Church, then
putting many thousands to death for unbelief, had a shrewd presentiment
of the menace that science contained against its authority. It was
powerful enough to see that its rival did not grow too quickly.

It was not until the Reformation broke the power of the Church, and a
refreshing breath of the spirit dissolved the icy chains that bound
science, that anatomy and embryology, and all the other branches of
research, could begin to advance once more. However, embryology lagged
far behind anatomy. The first works on embryology appear at the
beginning of the sixteenth century. The Italian anatomist, Fabricius ab
Aquapendente, a professor at Padua, opened the advance. In his two
books (_De formato fœtu,_ 1600, and _De formatione fœtus,_ 1604) he
published the older illustrations and descriptions of the embryos of
man and other mammals, and of the hen. Similar imperfect illustrations
were given by Spigelius (_De formato fœtu,_ 1631), and by Needham
(1667) and his more famous compatriot, Harvey (1652), who discovered
the circulation of the blood in the animal body and formulated the
important principle, _Omne vivum ex vivo_ (all life comes from
pre-existing life). The Dutch scientist, Swammerdam, published in his
_Bible of Nature_ the earliest observations on the embryology of the
frog and the division of its egg-yelk. But the most important
embryological studies in the sixteenth century were those of the famous
Italian, Marcello Malpighi, of Bologna, who led the way both in zoology
and botany. His treatises, _De formatione pulli_ and _De ovo incubato_
(1687), contain the first consistent description of the development of
the chick in the fertilised egg.

Here I ought to say a word about the important part played by the chick
in the growth of our science. The development of the chick, like that
of the young of all other birds, agrees in all its main features with
that of the other chief vertebrates, and even of man. The three highest
classes of vertebrates—mammals, birds, and reptiles (lizards, serpents,
tortoises, etc.)—have from the beginning of their embryonic development
so striking a resemblance in all the chief points of structure, and
especially in their first forms, that for a long time it is impossible
to distinguish between them. We have known now for some time that we
need only examine the embryo of a bird, which is the easiest to get at,
in order to learn the typical mode of development of a mammal (and
therefore of man). As soon as scientists began to study the human
embryo, or the mammal-embryo generally, in its earlier stages about the
middle and end of the seventeenth century, this important fact was very
quickly discovered. It is both theoretically and practically of great
value. As regards the _theory_ of evolution, we can draw the most
weighty inferences from this similarity between the embryos of widely
different classes of animals. But for the practical purposes of
embryological research the discovery is invaluable, because we can fill
up the gaps in our imperfect knowledge of the embryology of the mammals
from the more thoroughly studied embryology of the bird. Hens’ eggs are
easily to be had in any quantity, and the development of the chick may
be followed step by step in artificial incubation. The development of
the mammal is much more difficult to follow, because here the embryo is
not detached and enclosed in a large egg, but the tiny ovum remains in
the womb until the growth is completed. Hence, it is very difficult to
keep up sustained observation of the various stages in any great
extent, quite apart from such extrinsic considerations as the cost, the
technical difficulties, and many other obstacles which we encounter
when we would make an extensive study of the fertilised mammal. The
chicken has, therefore, always been the chief object of study in this
connection. The excellent incubators we now have enable us to observe
it in any quantity and at any stage of development, and so follow the
whole course of its formation step by step.

By the end of the seventeenth century Malpighi had advanced as far as
it was possible to do with the imperfect microscope of his time in the
embryological study of the chick. Further progress was arrested until
the instrument and the technical methods should be improved. The
vertebrate embryos are so small and delicate in their earlier stages
that you cannot go very far into the study of them without a good
microscope and other technical aid. But this substantial improvement of
the microscope and the other apparatus did not take place until the
beginning of the nineteenth century.

Embryology made scarcely any advance in the first half of the
eighteenth century, when the systematic natural history of plants and
animals received so great an impulse through the publication of Linné’s
famous _Systema Naturæ._ Not until 1759 did the genius arise who was to
give it an entirely new character, Caspar Friedrich Wolff. Until then
embryology had been occupied almost exclusively in unfortunate and
misleading efforts to build up theories on the imperfect empirical
material then available.

The theory which then prevailed, and remained in favour throughout
nearly the whole of the eighteenth century, was commonly called at that
time “the evolution theory”; it is better to describe it as “the
preformation theory.”[6] Its chief point is this: There is no new
formation of structures in the embryonic development of any organism,
animal or plant, or even of man; there is only a growth, or unfolding,
of parts which have been constructed or _pre-formed_ from all eternity,
though on a very small scale and closely packed together. Hence, every
living germ contains all the organs and parts of the body, in the form
and arrangement they will present later, already within it, and thus
the whole embryological process is merely an _evolution_ in the literal
sense of the word, or an _unfolding,_ of parts that were pre-formed and
folded up in it. So, for instance, we find in the hen’s egg not merely
a simple cell, that divides and subdivides and forms germinal layers,
and at last, after all kinds of variation and cleavage and
reconstruction, brings forth the body of the chick; but there is in
every egg from the first a complete chicken, with all its parts made
and neatly packed. These parts are so small or so transparent that the
microscope cannot detect them. In the hatching, these parts merely grow
larger, and spread out in the normal way.

 [6] This theory is usually known as the “evolution theory” in Germany,
 in contradistinction to the “epigenesis theory.” But as it is the
 latter that is called the “evolution theory” in England, France, and
 Italy, and “evolution” and “epigenesis” are taken to be synonymous, it
 seems better to call the first the “pre-formation theory.”


When this theory is consistently developed it becomes a “scatulation
theory.”[7] According to its teaching, there was made in the beginning
one couple or one individual of each species of animal or plant; but
this one individual contained the germs of all the other individuals of
the same species who should ever come to life. As the age of the earth
was generally believed at that time to be fixed by the Bible at 5000 or
6000 years, it seemed possible to calculate how many individuals of
each species had lived in the period, and so had been packed inside the
first being that was created. The theory was consistently extended to
man, and it was affirmed that our common parent Eve had had stored in
her ovary the germs of all the children of men.

 [7] “Packing theory” would be the literal translation. Scatula is the
 Latin for a case or box.—Translator.


The theory at first took the form of a belief that it was the _females_
who were thus encased in the first being. One couple of each species
was created, but the female contained in her ovary all the future
individuals of the species, of either sex. However, this had to be
altered when the Dutch microscopist, Leeuwenhoek, discovered the male
spermatozoa in 1690, and showed that an immense number of these
extremely fine and mobile thread-like beings exist in the male sperm
(this will be explained in Chapter VII). This astonishing discovery was
further advanced when it was proved that these living bodies, swimming
about in the seminal fluid, were real animalcules, and, in fact, were
the pre-formed germs of the future generation. When the male and female
procreative elements came together at conception, these thread-like
spermatozoa (“seed-animals”) were supposed to penetrate into the
fertile body of the ovum and begin to develop there, as the plant seed
does in the fruitful earth. Hence, every spermatozoon was regarded as a
_homunculus,_ a tiny complete man; all the parts were believed to be
pre-formed in it, and merely grew larger when it reached its proper
medium in the female ovum. This theory, also, was consistently
developed in the sense that in each of these thread-like bodies the
whole of its posterity was supposed to be present in the minutest form.
Adam’s sexual glands were thought to have contained the germs of the
whole of humanity.

This “theory of male scatulation” found itself at once in keen
opposition to the prevailing “female” theory. The two rival theories at
once opened a very lively campaign, and the physiologists of the
eighteenth century were divided into two great camps—the Animalculists
and the Ovulists—which fought vigorously. The animalculists held that
the spermatozoa were the true germs, and appealed to the lively
movements and the structure of these bodies. The opposing party of the
Ovulists, who clung to the older “evolution theory,” affirmed that the
ovum is the real germ, and that the spermatozoa merely stimulate it at
conception to begin its growth; all the future generations are stored
in the ovum. This view was held by the great majority of the biologists
of the eighteenth century, in spite of the fact that Wolff proved it in
1759 to be without foundation. It owed its prestige chiefly to the
circumstance that the most weighty authorities in the biology and
philosophy of the day decided in favour of it, especially Haller,
Bonnet, and Leibnitz.

Albrecht Haller, professor at Göttingen, who is often called the father
of physiology, was a man of wide and varied learning, but he does not
occupy a very high position in regard to insight into natural
phenomena. He made a vigorous defence of the “evolutionary theory” in
his famous work, _Elementa physiologiae,_ affirming: “There is no such
thing as formation (_nulla est epigenesis_). No part of the animal
frame is made before another; all were made together.” He thus denied
that there was any evolution in the proper sense of the word, and even
went so far as to say that the beard existed in the new-born child and
the antlers in the hornless fawn; all the parts were there in advance,
and were merely hidden from the eye of man for the time being. Haller
even calculated the number of human beings that God must have created
on the sixth day and stored away in Eve’s ovary. He put the number at
200,000 millions, assuming the age of the world to be 6000 years, the
average age of a human being to be thirty years, and the population of
the world at that time to be 1000 millions. And the famous Haller
maintained all this nonsense, in spite of its ridiculous consequences,
even after Wolff had discovered the real course of embryonic
development and established it by direct observation!

Among the philosophers of the time the distinguished Leibnitz was the
chief defender of the “preformation theory,” and by his authority and
literary prestige won many adherents to it. Supported by his system of
monads, according to which body and soul are united in inseparable
association and by their union form the individual, or the “monad,”
Leibnitz consistently extended the “scatulation theory” to the soul,
and held that this was no more evolved than the body. He says, for
instance, in his _Théodicée_: “I mean that these souls, which one day
are to be the souls of men, are present in the seed, like those of
other species; in such wise that they existed in our ancestors as far
back as Adam, or from the beginning of the world, in the forms of
organised bodies.”

The theory seemed to receive considerable support from the observations
of one of its most zealous supporters, Bonnet. In 1745 he discovered,
in the plant-louse, a case of parthenogenesis, or virgin-birth, an
interesting form of reproduction that has lately been found by Siebold
and others among various classes of the articulata, especially
crustacea and insects. Among these and other animals of certain lower
species the female may reproduce for several generations without having
been fertilised by the male. These ova that do not need fertilisation
are called “false ova,” pseudova or spores. Bonnet saw that a female
plant-louse, which he had kept in cloistral isolation, and rigidly
removed from contact with males, had on the eleventh day (after forming
a new skin for the fourth time) a living daughter, and during the next
twenty days ninety-four other daughters; and that all of them went on
to reproduce in the same way without any contact with males. It seemed
as if this furnished an irrefutable proof of the truth of the
scatulation theory, as it was held by the Ovulists; it is not
surprising to find that the theory then secured general acceptance.

This was the condition of things when suddenly, in 1759, Caspar
Friedrich Wolff appeared, and dealt a fatal blow at the whole
preformation theory with his new theory of epigenesis. Wolff, the son
of a Berlin tailor, was born in 1733, and went through his scientific
and medical studies, first at Berlin under the famous anatomist Meckel,
and afterwards at Halle. Here he secured his doctorate in his
twenty-sixth year, and in his academic dissertation (November 28th,
1759), the _Theoria generationis,_ expounded the new theory of a real
development on a basis of epigenesis. This treatise is, in spite of its
smallness and its obscure phraseology, one of the most valuable in the
whole range of biological literature. It is equally distinguished for
the mass of new and careful observations it contains, and the
far-reaching and pregnant ideas which the author everywhere extracts
from his observations and builds into a luminous and accurate theory of
generation. Nevertheless, it met with no success at the time. Although
scientific studies were then assiduously cultivated owing to the
impulse given by Linné—although botanists and zoologists were no longer
counted by dozens, but by hundreds, hardly any notice was taken of
Wolff’s theory. Even when he established the truth of epigenesis by the
most rigorous observations, and demolished the airy structure of the
preformation theory, the “exact” scientist Haller proved one of the
most strenuous supporters of the old theory, and rejected Wolff’s
correct view with a dictatorial “There is no such thing as evolution.”
He even went on to say that religion was menaced by the new theory! It
is not surprising that the whole of the physiologists of the second
half of the eighteenth century submitted to the ruling of this
physiological pontiff, and attacked the theory of epigenesis as a
dangerous innovation. It was not until more than fifty years afterwards
that Wolff’s work was appreciated. Only when Meckel translated into
German in 1812 another valuable work of Wolff’s on _The Formation of
the Alimentary Canal_ (written in 1768), and called attention to its
great importance, did people begin to think of him once more; yet this
obscure writer had evinced a profounder insight into the nature of the
living organism than any other scientist of the eighteenth century.

Wolff’s idea led to an appreciable advance over the whole field of
biology. There is such a vast number of new and important observations
and pregnant thoughts in his writings that we have only gradually
learned to appreciate them rightly in the course of the nineteenth
century. He opened up the true path for research in many directions. In
the first place, his theory of epigenesis gave us our first real
insight into the nature of embryonic development. He showed
convincingly that the development of every organism consists of a
series of _new formations,_ and that there is no trace whatever of the
complete form either in the ovum or the spermatozoon. On the contrary,
these are quite simple bodies, with a very different purport. The
embryo which is developed from them is also quite different, in its
internal arrangement and outer configuration, from the complete
organism. There is no trace whatever of preformation or in-folding of
organs. To-day we can scarcely call epigenesis a _theory,_ because we
are convinced it is a fact, and can demonstrate it at any moment with
the aid of the microscope.

Wolff furnished the conclusive empirical proof of his theory in his
classic dissertation on _The Formation of the Alimentary Canal_ (1768).
In its complete state the alimentary canal of the hen is a long and
complex tube, with which the lungs, liver, salivary glands, and many
other small glands, are connected. Wolff showed that in the early
stages of the embryonic chick there is no trace whatever of this
complicated tube with all its dependencies, but instead of it only a
flat, leaf-shaped body; that, in fact, the whole embryo has at first
the appearance of a flat, oval-shaped leaf. When we remember how
difficult the exact observation of so fine and delicate a structure as
the early leaf-shaped body of the chick must have been with the poor
microscopes then in use, we must admire the rare faculty for
observation which enabled Wolff to make the most important discoveries
in this most difficult part of embryology. By this laborious research
he reached the correct opinion that the embryonic body of all the
higher animals, such as the birds, is for some time merely a flat,
thin, leaf-shaped disk—consisting at first of one layer, but afterwards
of several. The lowest of these layers is the alimentary canal, and
Wolff followed its development from its commencement to its completion.
He showed how this leaf-shaped structure first turns into a groove,
then the margins of this groove fold together and form a closed canal,
and at length the two external openings of the tube (the mouth and
anus) appear.

Moreover, the important fact that the other systems of organs are
developed in the same way, from tubes formed out of simple layers, did
not escape Wolff. The nerveless system, muscular system, and vascular
(blood-vessel) system, with all the organs appertaining thereto, are,
like the alimentary system, developed out of simple leaf-shaped
structures. Hence, Wolff came to the view by 1768 which Pander
developed in the _Theory of Germinal Layers_ fifty years afterwards.
His principles are not literally correct; but he comes as near to the
truth in them as was possible at that time, and could be expected of
him.

Our admiration of this gifted genius increases when we find that he was
also the precursor of Goethe in regard to the metamorphosis of plants
and of the famous cellular theory. Wolff had, as Huxley showed, a clear
presentiment of this cardinal theory, since he recognised small
microscopic globules as the elementary parts out of which the germinal
layers arose.

Finally, I must invite special attention to the _mechanical_ character
of the profound philosophic reflections which Wolff always added to his
remarkable observations. He was a great monistic philosopher, in the
best meaning of the word. It is unfortunate that his philosophic
discoveries were ignored as completely as his observations for more
than half a century. We must be all the more careful to emphasise the
fact of their clear monistic tendency.



Chapter III.
MODERN EMBRYOLOGY


We may distinguish three chief periods in the growth of our science of
human embryology. The first has been considered in the preceding
chapter; it embraces the whole of the preparatory period of research,
and extends from Aristotle to Caspar Friedrich Wolff, or to the year
1759, in which the epoch-making _Theoria generationis_ was published.
The second period, with which we have now to deal, lasts about a
century—that is to say, until the appearance of Darwin’s _Origin of
Species,_ which brought about a change in the very foundations of
biology, and, in particular, of embryology. The third period begins
with Darwin. When we say that the second period lasted a full century,
we must remember that Wolff’s work had remained almost unnoticed during
half the time—namely, until the year 1812. During the whole of these
fifty-three years not a single book that appeared followed up the path
that Wolff had opened, or extended his theory of embryonic development.
We merely find his views—perfectly correct views, based on extensive
observations of fact—mentioned here and there as erroneous; their
opponents, who adhered to the dominant theory of preformation, did not
even deign to reply to them. This unjust treatment was chiefly due to
the extraordinary authority of Albrecht von Haller; it is one of the
most astonishing instances of a great authority, as such, preventing
for a long time the recognition of established facts.

The general ignorance of Wolff’s work was so great that at the
beginning of the nineteenth century two scientists of Jena, Oken (1806)
and Kieser (1810), began independent research into the development of
the alimentary canal of the chick, and hit upon the right clue to the
embryonic puzzle, without knowing a word about Wolff’s important
treatise on the same subject. They were treading in his very footsteps
without suspecting it. This can be easily proved from the fact that
they did not travel as far as Wolff. It was not until Meckel translated
into German Wolff’s book on the alimentary system, and pointed out its
great importance, that the eyes of anatomists and physiologists were
suddenly opened. At once a number of biologists instituted fresh
embryological inquiries, and began to confirm Wolff’s theory of
epigenesis.

This resuscitation of embryology and development of the
epigenesis-theory was chiefly connected with the university of
Würtzburg. One of the professors there at that time was Döllinger, an
eminent biologist, and father of the famous Catholic historian who
later distinguished himself by his opposition to the new dogma of papal
infallibility. Döllinger was both a profound thinker and an accurate
observer. He took the keenest interest in embryology, and worked at it
a good deal. However, he is not himself responsible for any important
result in this field. In 1816 a young medical doctor, whom we may at
once designate as Wolff’s chief successor, Karl Ernst von Baer, came to
Würtzburg. Baer’s conversations with Döllinger on embryology led to a
fresh series of most extensive investigations. Döllinger had expressed
a wish that some young scientist should begin again under his guidance
an independent inquiry into the development of the chick during the
hatching of the egg. As neither he nor Baer had money enough to pay for
an incubator and the proper control of the experiments, and for a
competent artist to illustrate the various stages observed, the lead of
the enterprise was given to Christian Pander, a wealthy friend of
Baer’s who had been induced by Baer to come to Würtzburg. An able
engraver, Dalton, was engaged to do the copper-plates. In a short time
the embryology of the chick, in which Baer was taking the greatest
indirect interest, was so far advanced that Pander was able to sketch
the main features of it on the ground of Wolff’s theory in the
dissertation he published in 1817. He clearly enunciated the theory of
germinal layers which Wolff
had anticipated, and established the truth of Wolff’s idea of a
development of the complicated systems of organs out of simple
leaf-shaped primitive structures. According to Pander, the leaf-shaped
object in the hen’s egg divides, before the incubation has proceeded
twelve hours, into two different layers, an external _serous_ layer and
an internal _mucous_ layer; between the two there develops later a
third layer, the _vascular_ (blood-vessel) layer.[8]

 [8] The technical terms which are bound to creep into this chapter
 will be fully understood later on.—Translator.


Karl Ernst von Baer, who had set afoot Pander’s investigation, and had
shown the liveliest interest in it after Pander’s departure from
Würtzburg, began his own much more comprehensive research in 1819. He
published the mature result nine years afterwards in his famous work,
_Animal Embryology: Observation and Reflection_ (not translated). This
classic work still remains a model of careful observation united to
profound philosophic speculation. The first part appeared in 1828, the
second in 1837. The book proved to be the foundation on which the whole
science of embryology has built down to our own day. It so far
surpassed its predecessors, and Pander in particular, that it has
become, after Wolff’s work, the chief base of modern embryology.

Baer was one of the greatest scientists of the nineteenth century, and
exercised considerable influence on other branches of biology as well.
He built up the theory of germinal layers, as a whole and in detail, so
clearly and solidly that it has been the starting-point of
embryological research ever since. He taught that in all the
vertebrates first two and then four of these germinal layers are
formed; and that the earliest rudimentary organs of the body arise by
the conversion of these layers into tubes. He described the first
appearance of the vertebrate embryo, as it may be seen in the globular
yelk of the fertilised egg, as an oval disk which first divides into
two layers. From the upper or _animal_ layer are developed all the
organs which accomplish the phenomena of animal life—the functions of
sensation and motion, and the covering of the body. From the lower or
_vegetative_ layer come the organs which effect the vegetative life of
the organism—nutrition, digestion, blood-formation, respiration,
secretion, reproduction, etc.

Each of these original layers divides, according to Baer, into two
thinner and superimposed layers or plates. He calls the two plates of
the animal layer, the skin-stratum and muscle-stratum. From the upper
of these plates, the _skin-stratum,_ the external skin, or outer
covering of the body, the central nervous system, and the sense-organs,
are formed. From the lower, or _muscle-stratum,_ the muscles, or fleshy
parts and the bony skeleton—in a word, the motor organs—are evolved. In
the same way, Baer said, the lower or vegetative layer splits into two
plates, which he calls the vascular-stratum and the mucous-stratum.
From the outer of the two (the _vascular_) the heart, blood-vessels,
spleen, and the other vascular glands, the kidneys, and sexual glands,
are formed. From the fourth or _mucous_ layer, in fine, we get the
internal and digestive lining of the alimentary canal and all its
dependencies, the liver, lungs, salivary glands, etc. Baer had, in the
main, correctly judged the significance of these four secondary
embryonic layers, and he followed the conversion of them into the
tube-shaped primitive organs with great perspicacity. He first solved
the difficult problem of the transformation of this four-fold, flat,
leaf-shaped, embryonic disk into the complete vertebrate body, through
the conversion of the layers or plates into tubes. The flat leaves bend
themselves in obedience to certain laws of growth; the borders of the
curling plates approach nearer and nearer; until at last they come into
actual contact. Thus out of the flat gut-plate is formed a hollow
gut-tube, out of the flat spinal plate a hollow nerve-tube, from the
skin-plate a skin-tube, and so on.

Among the many great services which Baer rendered to embryology,
especially vertebrate embryology, we must not forget his discovery of
the human ovum. Earlier scientists had, as a rule, of course, assumed
that man developed out of an egg, like the other animals. In fact, the
preformation theory held that the germs of the whole of humanity were
stored already in Eve’s ova. But the real ovum escaped detection until
the year 1827. This ovum is extremely small, being a tiny round vesicle
about the 1/120 of an inch in diameter; it can be seen under very
favourable circumstances with the naked eye as a tiny particle, but is
otherwise quite invisible. This particle is formed in the ovary inside
a much larger
globule, which takes the name of the Graafian follicle, from its
discoverer, Graaf, and had previously been regarded as the true ovum.
However, in 1827 Baer proved that it was not the real ovum, which is
much smaller, and is contained within the follicle. (Compare the end of
Chapter XXIX.)

Baer was also the first to observe what is known as the _segmentation
sphere_ of the vertebrate; that is to say, the round vesicle which
first develops out of the impregnated ovum, and the thin wall of which
is made up of a single layer of regular, polygonal (many-cornered)
cells (see the illustration in Chapter XII). Another discovery of his
that was of great importance in constructing the vertebrate stem and
the characteristic organisation of this extensive group (to which man
belongs) was the detection of the axial rod, or the _chorda dorsalis._
There is a long, round, cylindrical rod of cartilage which runs down
the longer axis of the vertebrate embryo; it appears at an early stage,
and is the first sketch of the spinal column, the solid skeletal axis
of the vertebrate. In the lowest of the vertebrates, the amphioxus, the
internal skeleton consists only of this cord throughout life. But even
in the case of man and all the higher vertebrates it is round this cord
that the spinal column and the brain are afterwards formed.

However, important as these and many other discoveries of Baer’s were
in vertebrate embryology, his researches were even more influential,
from the circumstance that he was the first to employ the _comparative_
method in studying the development of the animal frame. Baer occupied
himself chiefly with the embryology of vertebrates (especially the
birds and fishes). But he by no means confined his attention to these,
gradually taking the various groups of the invertebrates into his
sphere of study. As the general result of his comparative embryological
research, Baer distinguished four different modes of development and
four corresponding groups in the animal world. These chief groups or
types are: 1, the vertebrata; 2, the articulata; 3, the mollusca; and
4, all the lower groups which were then wrongly comprehended under the
general name of the radiata. Georges Cuvier had been the first to
formulate this distinction, in 1812. He showed that these groups
present specific differences in their whole internal structure, and the
connection and disposal of their systems of organs; and that, on the
other hand, all the animals of the same type—say, the
vertebrates—essentially agreed in their inner structure, in spite of
the greatest superficial differences. But Baer proved that these four
groups are also quite differently developed from the ovum; and that the
series of embryonic forms is the same throughout for animals of the
same type, but different in the case of other animals. Up to that time
the chief aim in the classification of the animal kingdom was to
arrange all the animals from lowest to highest, from the infusorium to
man, in one long and continuous series. The erroneous idea prevailed
nearly everywhere that there was one uninterrupted chain of evolution
from the lowest animal to the highest. Cuvier and Baer proved that this
view was false, and that we must distinguish four totally different
types of animals, on the ground of anatomic structure and embryonic
development.

Baer’s epoch-making works aroused an extraordinary and widespread
interest in embryological research. Immediately afterwards we find a
great number of observers at work in the newly opened field, enlarging
it in a very short time with great energy by their various discoveries
in detail. Next to Baer’s comes the admirable work of Heinrich Rathke,
of Königsberg (died 1860); he made an extensive study of the
embryology, not only of the invertebrates (crustaceans, insects,
molluscs), but also, and particularly, of the vertebrates (fishes,
tortoises, serpents, crocodiles, etc.). We owe the first comprehensive
studies of mammal embryology to the careful research of Wilhelm
Bischoff, of Munich; his embryology of the rabbit (1840), the dog
(1842), the guinea-pig (1852), and the doe (1854), still form classical
studies. About the same time a great impetus was given to the
embryology of the invertebrates. The way was opened through this
obscure province by the studies of the famous Berlin zoologist,
Johannes Müller, on the echinoderms. He was followed by Albert
Kölliker, of Würtzburg, writing on the cuttlefish (or the cephalopods),
Siebold and Huxley on worms and zoophytes, Fritz Muller (Desterro) on
the crustacea, Weismann on insects, and so on. The number of workers in
this field has greatly increased of late, and a quantity of new and
astonishing discoveries have been made. One notices, in several of
these recent works on
embryology, that their authors are too little acquainted with
comparative anatomy and classification. Paleontology is, unfortunately,
altogether neglected by many of these new workers, although this
interesting science furnishes most important facts for phylogeny, and
thus often proves of very great service in ontogeny.

A very important advance was made in our science in 1839, when the
cellular theory was established, and a new field of inquiry bearing on
embryology was suddenly opened. When the famous botanist, M. Schleiden,
of Jena, showed in 1838, with the aid of the microscope, that every
plant was made up of innumerable elementary parts, which we call
_cells,_ a pupil of Johannes Müller at Berlin, Theodor Schwann, applied
the discovery at once to the animal organism. He showed that in the
animal body as well, when we examine its tissues in the microscope, we
find these cells everywhere to be the elementary units. All the
different tissues of the organism, especially the very dissimilar
tissues of the nerves, muscles, bones, external skin, mucous lining,
etc., are originally formed out of cells; and this is also true of all
the tissues of the plant. These cells are separate living beings; they
are the citizens of the State which the entire multicellular organism
seems to be. This important discovery was bound to be of service to
embryology, as it raised a number of new questions. What is the
relation of the cells to the germinal layers? Are the germinal layers
composed of cells, and what is their relation to the cells of the
tissues that form later? How does the ovum stand in the cellular
theory? Is the ovum itself a cell, or is it composed of cells? These
important questions were now imposed on the embryologist by the
cellular theory.

The most notable effort to answer these questions—which were attacked
on all sides by different students—is contained in the famous work,
_Inquiries into the Development of the Vertebrates_ (not translated) of
Robert Remak, of Berlin (1851). This gifted scientist succeeded in
mastering, by a complete reform of the science, the great difficulties
which the cellular theory had at first put in the way of embryology. A
Berlin anatomist, Carl Boguslaus Reichert, had already attempted to
explain the origin of the tissues. But this attempt was bound to
miscarry, since its not very clear-headed author lacked a sound
acquaintance with embryology and the cell theory, and even with the
structure and development of the tissue in particular. Remak at length
brought order into the dreadful confusion that Reichert had caused; he
gave a perfectly simple explanation of the origin of the tissues. In
his opinion the animal ovum is always _a simple cell_ : the germinal
layers which develop out of it are always composed of cells; and these
cells that constitute the germinal layers arise simply from the
continuous and repeated cleaving (segmentation) of the original
solitary cell. It first divides into two and then into four cells; out
of these four cells are born eight, then sixteen, thirty-two, and so
on. Thus, in the embryonic development of every animal and plant there
is formed first of all out of the simple egg cell, by a repeated
subdivision, a cluster of cells, as Kölliker had already stated in
connection with the cephalopods in 1844. The cells of this group spread
themselves out flat and form leaves or plates; each of these leaves is
formed exclusively out of cells. The cells of different layers assume
different shapes, increase, and differentiate; and in the end there is
a further cleavage (differentiation) and division of work of the cells
within the layers, and from these all the different tissues of the body
proceed.

These are the simple foundations of _histogeny,_ or the science that
treats of the development of the tissues ( _hista_), as it was
established by Remak and Kölliker. Remak, in determining more closely
the part which the different germinal layers play in the formation of
the various tissues and organs, and in applying the theory of evolution
to the cells and the tissues they compose, raised the theory of
germinal layers, at least as far as it regards the vertebrates, to a
high degree of perfection.

Remak showed that three layers are formed out of the two germinal
layers which compose the first simple leaf-shaped structure of the
vertebrate body (or the “germinal disk”), as the lower layer splits
into two plates. These three layers have a very definite relation to
the various tissues. First of all, the cells which form the outer skin
of the body (the epidermis), with its various dependencies (hairs,
nails, etc.)—that is to say, the entire outer envelope of the body—are
developed out of the outer or upper layer; but there are also developed
in a curious way out of the same layer the cells which form the central
nervous system, the brain and the spinal cord. In the second place, the
inner or lower germinal layer gives rise only to the cells which form
the epithelium (the whole inner lining) of the alimentary canal and all
that depends on it (the lungs, liver, pancreas, etc.), or the tissues
that receive and prepare the nourishment of the body. Finally, the
middle layer gives rise to all the other tissues of the body, the
muscles, blood, bones, cartilage, etc. Remak further proved that this
middle layer, which he calls “the motor-germinative layer,” proceeds to
subdivide into two secondary layers. Thus we find once more the four
layers which Baer had indicated. Remak calls the outer secondary leaf
of the middle layer (Baer’s “muscular layer”) the “skin layer” (it
would be better to say, skin-fibre layer); it forms the outer wall of
the body (the true skin, the muscles, etc.). To the inner secondary
leaf (Baer’s “vascular layer”) he gave the name of the
“alimentary-fibre layer”; this forms the outer envelope of the
alimentary canal, with the mesentery, the heart, the blood-vessels,
etc.

On this firm foundation provided by Remak for _histogeny,_ or the
science of the formation of the tissues, our knowledge has been
gradually built up and enlarged in detail. There have been several
attempts to restrict and even destroy Remak’s principles. The two
anatomists, Reichert (of Berlin) and Wilhelm His (of Leipzic),
especially, have endeavoured in their works to introduce a new
conception of the embryonic development of the vertebrate, according to
which the two primary germinal layers would not be the sole sources of
formation. But these efforts were so seriously marred by ignorance of
comparative anatomy, an imperfect acquaintance with ontogenesis, and a
complete neglect of phylogenesis, that they could not have more than a
passing success. We can only explain how these curious attacks of
Reichert and His came to be regarded for a time as advances by the
general lack of discrimination and of grasp of the true object of
embryology.

Wilhelm His published, in 1868, his extensive Researches into the
_Earliest Form of the Vertebrate Body,_[9] one of the curiosities of
embryological literature. The author imagines that he can build a
“mechanical theory of embryonic development” by merely giving an exact
description of the embryology of the chick, without any regard to
comparative anatomy and phylogeny, and thus falls into an error that is
almost without parallel in the history of biological literature. As the
final result of his laborious investigations, His tells us “that a
comparatively simple law of growth is the one essential thing in the
first development. Every formation, whether it consist in cleavage of
layers, or folding, or complete division, is a consequence of this
fundamental law.” Unfortunately, he does not explain what this “law of
growth” is; just as other opponents of the theory of selection, who
would put in its place a great “law of evolution,” omit to tell us
anything about the nature of this. Nevertheless, it is quite clear from
His’s works that he imagines constructive Nature to be a sort of
skilful tailor. The ingenious operator succeeds in bringing into
existence, by “evolution,” all the various forms of living things by
cutting up in different ways the germinal layers, bending and folding,
tugging and splitting, and so on.

 [9] None of His’s works have been translated into English.


His’s embryological theories excited a good deal of interest at the
time of publication, and have evoked a fair amount of literature in the
last few decades. He professed to explain the most complicated parts of
organic construction (such as the development of the brain) in the
simplest way on mechanical principles, and to derive them immediately
from simple physical processes (such as unequal distribution of strain
in an elastic plate). It is quite true that a mechanical or monistic
explanation (or a reduction of natural processes) is the ideal of
modern science, and this ideal would be realised if we could succeed in
expressing these formative processes in mathematical formulæ. His has,
therefore, inserted plenty of numbers and measurements in his
embryological works, and given them an air of “exact” scholarship by
putting in a quantity of mathematical tables. Unfortunately, they are
of no value, and do not help us in the least in forming an “exact”
acquaintance with the embryonic phenomena. Indeed, they wander from the
true path altogether by neglecting the phylogenetic method; this, he
thinks, is “a mere by-path,” and is “not necessary at all for the
explanation of the facts of embryology,” which are the direct
consequence of physiological principles. What His takes to be a simple
physical process—for instance, the folding of the germinal layers (in
the formation of the medullary tube, alimentary tube, etc.)—is, as a
matter of fact, the direct result of the growth of the various cells
which form those organic structures; but these growth-motions have
themselves been transmitted by heredity from parents and ancestors, and
are only the hereditary repetition of countless phylogenetic changes
which have taken place for thousands of years in the race-history of
the said ancestors. Each of these historical changes was, of course,
originally due to adaptation; it was, in other words, physiological,
and reducible to mechanical causes. But we have, naturally, no means of
observing them now. It is only by the hypotheses of the science of
evolution that we can form an approximate idea of the organic links in
this historic chain.

All the best recent research in animal embryology has led to the
confirmation and development of Baer and Remak’s theory of the germinal
layers. One of the most important advances in this direction of late
was the discovery that the two primary layers out of which is built the
body of all vertebrates (including man) are also present in all the
invertebrates, with the sole exception of the lowest group, the
unicellular protozoa. Huxley had detected them in the medusa in 1849.
He showed that the two layers of cells from which the body of this
zoophyte is developed correspond, both morphologically and
physiologically, to the two original germinal layers of the vertebrate.
The outer layer, from which come the external skin and the muscles, was
then called by Allman (1853) the “ectoderm” (outer layer, or skin); the
inner layer, which forms the alimentary and reproductory organs, was
called the “entoderm” (= inner layer). In 1867 and the following years
the discovery of the germinal layers was extended to other groups of
the invertebrates. In particular, the indefatigable Russian zoologist,
Kowalevsky, found them in all the most diverse sections of the
invertebrates—the worms, tunicates, echinoderms, molluscs, articulates,
etc.

In my monograph on the sponges (1872) I proved that these two primary
germinal layers are also found in that group, and that they may be
traced from it right up to man, through all the various classes, in
identical form. This “homology of the two primary germinal layers”
extends through the whole of the metazoa, or tissue-forming animals;
that is to say, through the whole animal kingdom, with the one
exception of its lowest section, the unicellular beings, or protozoa.
These lowly organised animals do not form germinal layers, and
therefore do not succeed in forming true tissue. Their whole body
consists of a single cell (as is the case with the amœbæ and
infusoria), or of a loose aggregation of only slightly differentiated
cells, though it may not even reach the full structure of a single cell
(as with the monera). But in all other animals the ovum first grows
into two primary layers, the outer or _animal_ layer (the ectoderm,
epiblast, or ectoblast), and the inner or _vegetal_ layer (the
entoderm, hypoblast, or endoblast); and from these the tissues and
organs are formed. The first and oldest organ of all these metazoa is
the primitive gut (or progaster) and its opening, the primitive mouth
(prostoma). The typical embryonic form of the metazoa, as it is
presented for a time by this simple structure of the two-layered body,
is called the _gastrula_ ; it is to be conceived as the hereditary
reproduction of some primitive common ancestor of the metazoa, which we
call the _gastræa._ This applies to the sponges and other zoophyta, and
to the worms, the mollusca, echinoderma, articulata, and vertebrata.
All these animals may be comprised under the general heading of “gut
animals,” or metazoa, in contradistinction to the gutless protozoa.

I have pointed out in my Study of the _Gastræa Theory_ [not translated]
(1873) the important consequences of this conception in the morphology
and classification of the animal world. I also divided the realm of
metazoa into two great groups, the lower and higher metazoa. In the
first are comprised the _cœlenterata_ (also called zoophytes, or
plant-animals). In the lower forms of this group the body consists
throughout life merely of the primary germinal layers, with the cells
sometimes more and sometimes less differentiated. But with the higher
forms of the cœlentarata (the corals, higher medusæ, ctenophoræ, and
platodes) a middle layer, or _mesoderm,_ often of considerable size, is
developed between the other two layers; but blood and an internal
cavity are still lacking.

To the second great group of the metazoa I gave the name of the
_cœlomaria,_ or _bilaterata_ (or the bilateral higher forms). They all
have a cavity within the body (cœloma), and most of them have blood and
blood-vessels. In this are comprised the six higher stems of the animal
kingdom, the annulata and their descendants, the mollusca, echinoderma,
articulata, tunicata, and vertebrata. In all these bilateral organisms
the two-sided body is formed out of four secondary germinal layers, of
which the inner two construct the wall of the alimentary canal, and the
outer two the wall of the body. Between the two pairs of layers lies
the cavity (cœloma).

Although I laid special stress on the great morphological importance of
this cavity in my _Study of the Gastræa Theory,_ and endeavoured to
prove the significance of the four secondary germinal layers in the
organisation of the cœlomaria, I was unable to deal satisfactorily with
the difficult question of the mode of their origin. This was done eight
years afterwards by the brothers Oscar and Richard Hertwig in their
careful and extensive comparative studies. In their masterly _Cœlum
Theory: An Attempt to Explain the Middle Germinal Layer_ [not
translated] (1881) they showed that in most of the metazoa, especially
in all the vertebrates, the body-cavity arises in the same way, by the
outgrowth of two sacs from the inner layer. These two cœlom-pouches
proceed from the rudimentary mouth of the gastrula, between the two
primary layers. The inner plate of the two-layered cœlom-pouch (the
visceral layer) joins itself to the entoderm; the outer plate (parietal
layer) unites with the ectoderm. Thus are formed the double-layered
gut-wall within and the double-layered body-wall without; and between
the two is formed the cavity of the cœlom, by the blending of the right
and left cœlom-sacs. We shall see this more fully in Chapter X.

The many new points of view and fresh ideas suggested by my gastræa
theory and Hertwig’s cœlom theory led to the publication of a number of
writings on the theory of germinal layers. Most of them set out to
oppose it at first, but in the end the majority supported it. Of late
years both theories are accepted in their essential features by nearly
every competent man of science, and light and order have been
introduced into this once dark and contradictory field of research. A
further cause of congratulation for this solution of the great
embryological controversy is that it brought with it a recognition of
the need for phylogenetic study and explanation.

Interest and practice in embryological research have been remarkably
stimulated during the past thirty years by this appreciation of
phylogenetic methods. Hundreds of assiduous and able observers are now
engaged in the development of comparative embryology and its
establishment on a basis of evolution, whereas they numbered only a few
dozen not many decades ago. It would take too long to enumerate even
the most important of the countless valuable works which have enriched
embryological literature since that time. References to them will be
found in the latest manuals of embryology of Kölliker, Balfour,
Hertwig, Kollman, Korschelt, and Heider.

Kölliker’s _Entwickelungsgeschichte des Menschen und der höherer
Thiere,_ the first edition of which appeared forty-two years ago, had
the rare merit at that time of gathering into presentable form the
scattered attainments of the science, and expounding them in some sort
of unity on the basis of the cellular theory and the theory of germinal
layers. Unfortunately, the distinguished Würtzburg anatomist, to whom
comparative anatomy, histology, and ontogeny owe so much, is opposed to
the theory of descent generally and to Darwinism in particular. All the
other manuals I have mentioned take a decided stand on evolution.
Francis Balfour has carefully collected and presented with
discrimination, in his _Manual of Comparative Embryology_ (1880), the
very scattered and extensive literature of the subject; he has also
widened the basis of the gastræa theory by a comparative description of
the rise of the organs from the germinal layers in all the chief groups
of the animal kingdom, and has given a most thorough empirical support
to the principles I have formulated. A comparison of his work with the
excellent _Text-book of the Embryology of the Vertebrates_ (1890)
[translation 1895] of Korschelt and Heider shows what astonishing
progress has been made in the science in the course of ten years. I
would especially recommend the manuals of Julius Kollmann and Oscar
Hertwig to those readers who are stimulated to further study by these
chapters on human embryology. Kollmann’s work is commendable for its
clear treatment of the subject and very fine original illustrations;
its author adheres firmly to the biogenetic law, and uses it throughout
with considerable profit. That is not the case in Oscar Hertwig’s
recent _Text-book of the Embryology of Man and the Mammals_
[translations 1892 and 1899] (seventh edition 1902). This able
anatomist has of late often been quoted as an opponent of the
biogenetic law, although he himself had demonstrated its great value
thirty years ago. His recent vacillation is partly due to the timidity
which our “exact” scientists have with regard to hypotheses; though it
is impossible to make any headway in the explanation of facts without
them. However, the purely descriptive part of embryology in Hertwig’s
_Text-book_ is very thorough and reliable.

A new branch of embryological research has been studied very
assiduously in the last decade of the nineteenth century—namely,
“experimental embryology.” The great importance which has been attached
to the application of physical experiments to the living organism for
the last hundred years, and the valuable results that it has given to
physiology in the study of the vital phenomena, have led to its
extension to embryology. I was the first to make experiments of this
kind during a stay of four months on the Canary Island, Lanzerote, in
1866. I there made a thorough investigation of the almost unknown
embryology of the siphonophoræ. I cut a number of the embryos of these
animals (which develop freely in the water, and pass through a very
curious transformation), at an early stage, into several pieces, and
found that a fresh organism (more or less complete, according to the
size of the piece) was developed from each particle. More recently some
of my pupils have made similar experiments with the embryos of
vertebrates (especially the frog) and some of the invertebrates.
Wilhelm Roux, in particular, has made extensive experiments, and based
on them a special “mechanical embryology,” which has given rise to a
good deal of discussion and controversy. Roux has published a special
journal for these subjects since 1895, the _Archiv für
Entwickelungsmechanik._ The contributions to it are very varied in
value. Many of them are valuable papers on the physiology and pathology
of the embryo. Pathological experiments—the placing of the embryo in
abnormal conditions—have yielded many interesting results; just as the
physiology of the normal body has for a long time derived assistance
from the pathology of the diseased organism. Other of these
mechanical-embryological articles return to the erroneous methods of
His, and are only misleading. This must be said of the many
contributions of mechanical embryology which take up a position of
hostility to the theory of descent and its chief embryological
foundation—the biogenetic law. This law, however, when rightly
understood, is not opposed to, but is the best and most solid support
of, a sound mechanical embryology. Impartial reflection and a due
attention to paleontology and comparative anatomy should convince these
one-sided mechanicists that the facts they have discovered—and, indeed,
the whole embryological process—cannot be fully understood without the
theory of descent and the biogenetic law.



Chapter IV.
THE OLDER PHYLOGENY


The embryology of man and the animals, the history of which we have
reviewed in the last two chapters, was mainly a descriptive science
forty years ago. The earlier investigations in this province were
chiefly directed to the discovery, by careful observation, of the
wonderful facts of the embryonic development of the animal body from
the ovum. Forty years ago no one dared attack the question of the
_causes_ of these phenomena. For fully a century, from the year 1759,
when Wolff’s solid _Theoria generationis_ appeared, until 1859, when
Darwin published his famous Origin of Species, the real causes of the
embryonic processes were quite unknown. No one thought of seeking the
agencies that effected this marvellous succession of structures. The
task was thought to be so difficult as almost to pass beyond the limits
of human thought. It was reserved for Charles Darwin to initiate us
into the knowledge of these causes. This compels us to recognise in
this great genius, who wrought a complete revolution in the whole field
of biology, a founder at the same time of a new period in embryology.
It is true that Darwin occupied himself very little with direct
embryological research, and even in his chief work he only touches
incidentally on the embryonic phenomena; but by his reform of the
theory of descent and the founding of the theory of selection he has
given us the means of attaining to a real knowledge of the causes of
embryonic formation. That is, in my opinion, the chief feature in
Darwin’s incalculable influence on the whole science of evolution.

When we turn our attention to this latest period of embryological
research, we pass into the second division of organic
evolution—stem-evolution, or phylogeny. I have already indicated in
Chapter I the important and intimate causal connection between these
two sections of the science of evolution—between the evolution of the
individual and that of his ancestors. We have formulated this
connection in the biogenetic law; the shorter evolution, that of the
individual, or _ontogenesis,_ is a rapid and summary repetition, a
condensed recapitulation, of the larger evolution, or that of the
species. In this principle we express all the essential points relating
to the causes of evolution; and we shall seek throughout this work to
confirm this principle and lend it the support of facts. When we look
to its _causal_ significance, perhaps it would be better to formulate
the biogenetic law thus: “The evolution of the species and the stem (
_phylon_) shows us, in the physiological functions of heredity and
adaptation, the conditioning causes on which the evolution of the
individual depends”; or, more briefly: “Phylogenesis is the mechanical
cause of ontogenesis.”

But before we examine the great achievement by which Darwin revealed
the causes of evolution to us, we must glance at the efforts of earlier
scientists to attain this object. Our historical inquiry into these
will be even shorter than that into the work done in the field of
ontogeny. We have very few names to consider here. At the head of them
we find the great French naturalist, Jean Lamarck, who first
established evolution as a scientific theory in 1809. Even before his
time, however, the chief philosopher, Kant, and the chief poet, Goethe,
of Germany had occupied themselves with the subject. But their efforts
passed almost without recognition in the eighteenth century. A
“philosophy of nature” did not arise until the beginning of the
nineteenth century. In the whole of the time before this no one had
ventured to raise seriously the question of the origin of species,
which is the culminating point of phylogeny. On all sides it was
regarded as an insoluble enigma.

The whole science of the evolution of man and the other animals is
intimately connected with the question of the nature of species, or
with the problem of the origin of the various animals which we group
together under the name of species. Thus the definition of the species
becomes important. It is well known that this definition was given by
Linné, who, in his famous _Systema Naturæ_ (1735), was the first to
classify and name the various groups of animals and plants, and drew up
an orderly scheme of the species then known. Since that time “species”
has been the most important and indispensable idea in descriptive
natural history, in zoological and botanical classification; although
there have been endless controversies as to its real meaning.

What, then, is this “organic species”? Linné himself appealed directly
to the Mosaic narrative; he believed that, as it is stated in
_Genesis,_ one pair of each species of animals and plants was created
in the beginning, and that all the individuals of each species are the
descendants of these created couples. As for the hermaphrodites
(organisms that have male and female organs in one being), he thought
it sufficed to assume the creation of one sole individual, since this
would be fully competent to propagate its species. Further developing
these mystic ideas, Linné went on to borrow from _Genesis_ the account
of the deluge and of Noah’s ark as a ground for a science of the
geographical and topographical distribution of organisms. He accepted
the story that all the plants, animals, and men on the earth were swept
away in a universal deluge, except the couples preserved with Noah in
the ark, and ultimately landed on Mount Ararat. This mountain seemed to
Linné particularly suitable for the landing, as it reaches a height of
more than 16,000 feet, and thus provides in its higher zones the
several climates demanded by the various species of animals and plants:
the animals that were accustomed to a cold climate could remain at the
summit; those used to a warm climate could descend to the foot; and
those requiring a temperate climate could remain half-way down. From
this point the re-population of the earth with animals and plants could
proceed.

It was impossible to have any scientific notion of the method of
evolution in Linné’s time, as one of the chief sources of information,
paleontology, was still wholly unknown. This science of the fossil
remains of extinct animals and plants is very closely bound up with the
whole question of evolution. It is impossible to explain the origin of
living organisms without appealing to it. But this science did not rise
until a much later date. The real founder of scientific paleontology
was Georges Cuvier, the most distinguished zoologist who, after Linné,
worked at the classification of the animal world, and effected a
complete revolution in systematic zoology at the beginning of the
nineteenth century. In regard to the nature of the species he
associated himself with Linné and the Mosaic story of creation, though
this was more difficult for him with his acquaintance with fossil
remains. He clearly showed that a number of quite different animal
populations have lived on the earth; and he claimed that we must
distinguish a number of stages in the history of our planet, each of
which was characterised by a special population of animals and plants.
These successive populations were, he said, quite independent of each
other, and therefore the supernatural creative act, which was demanded
as the origin of the animals and plants by the dominant creed, must
have been repeated several times. In this way a whole series of
different creative periods must have succeeded each other; and in
connection with these he had to assume that stupendous revolutions or
cataclysms—something like the legendary deluge—must have taken place
repeatedly. Cuvier was all the more interested in these catastrophes or
cataclysms as geology was just beginning to assert itself, and great
progress was being made in our knowledge of the structure and formation
of the earth’s crust. The various strata of the crust were being
carefully examined, especially by the famous geologist Werner and his
school, and the fossils found in them were being classified; and these
researches also seemed to point to a variety of creative periods. In
each period the earth’s crust, composed of the various strata, seemed
to be differently constituted, just like the population of animals and
plants that then lived on it. Cuvier combined this notion with the
results of his own paleontological and zoological research; and in his
effort to get a consistent view of the whole process of the earth’s
history he came to form the theory which is known as “the catastrophic
theory,” or the theory of terrestrial revolutions. According to this
theory, there have been a series of mighty cataclysms on the earth, and
these have suddenly destroyed the whole animal and plant population
then living on it; after each cataclysm there was a fresh creation of
living things throughout the earth. As this creation could not be
explained by natural laws, it was necessary to appeal to an
intervention on the part of the Creator. This catastrophic theory,
which Cuvier described in a special work, was soon generally accepted,
and retained its position in biology for half a century.

However, Cuvier’s theory was completely overthrown sixty years ago by
the geologists, led by Charles Lyell, the most distinguished worker in
this field of science. Lyell proved in his famous _Principles of
Geology_ (1830) that the theory was false, in so far as it concerned
the crust of the earth; that it was totally unnecessary to bring in
supernatural agencies or general catastrophes in order to explain the
structure and formation of the mountains; and that we can explain them
by the familiar agencies which are at work to-day in altering and
reconstructing the surface of the earth. These causes are—the action of
the atmosphere and water in its various forms (snow, ice, fog, rain,
the wear of the river, and the stormy ocean), and the volcanic action
which is exerted by the molten central
mass. Lyell convincingly proved that these natural causes are quite
adequate to explain every feature in the build and formation of the
crust. Hence Cuvier’s theory of cataclysms was very soon driven out of
the province of geology, though it remained for another thirty years in
undisputed authority in biology. All the zoologists and botanists who
gave any thought to the question of the origin of organisms adhered to
Cuvier’s erroneous idea of revolutions and new creations.

In order to illustrate the complete stagnancy of biology from 1830 to
1859 on the question of the origin of the various species of animals
and plants, I may say, from my own experience, that during the whole of
my university studies I never heard a single word said about this most
important problem of the science. I was fortunate enough at that time
(1852–1857) to have the most distinguished masters for every branch of
biological science. Not one of them ever mentioned this question of the
origin of species. Not a word was ever said about the earlier efforts
to understand the formation of living things, nor about Lamarck’s
_Philosophie Zoologique_ which had made a fresh attack on the problem
in 1809. Hence it is easy to understand the enormous opposition that
Darwin encountered when he took up the question for the first time. His
views seemed to float in the air, without a single previous effort to
support them. The whole question of the formation of living things was
considered by biologists, until 1859, as pertaining to the province of
religion and transcendentalism; even in speculative philosophy, in
which the question had been approached from various sides, no one had
ventured to give it serious treatment. This was due to the dualistic
system of Immanuel Kant, who taught a natural system of evolution as
far as the inorganic world was concerned; but, on the whole, adopted a
supernaturalist system as regards the origin of living things. He even
went so far as to say: “It is quite certain that we cannot even
satisfactorily understand, much less explain, the nature of an organism
and its internal forces on purely mechanical principles; it is so
certain, indeed, that we may confidently say: ‘It is absurd for a man
to imagine even that some day a Newton will arise who will explain the
origin of a single blade of grass by natural laws not controlled by
design’—such a hope is entirely forbidden us.” In these words Kant
definitely adopts the dualistic and teleological point of view for
biological science.

Nevertheless, Kant deserted this point of view at times, particularly
in several remarkable passages which I have dealt with at length in my
_Natural History of Creation_ (chap. v), where he expresses himself in
the opposite, or monistic, sense. In fact, these passages would justify
one, as I showed, in claiming his support for the theory of evolution.
However, these monistic passages are only stray gleams of light; as a
rule, Kant adheres in biology to the obscure dualistic ideas, according
to which the forces at work in inorganic nature are quite different
from those of the organic world. This dualistic system prevails in
academic philosophy to-day—most of our philosophers still regarding
these two provinces as totally distinct. They put, on the one side, the
inorganic or “lifeless” world, in which there are at work only
mechanical laws, acting necessarily and without design; and, on the
other, the province of organic nature, in which none of the phenomena
can be properly understood, either as regards their inner nature or
their origin, except in the light of preconceived design, carried out
by final or purposive causes.

The prevalence of this unfortunate dualistic prejudice prevented the
problem of the origin of species, and the connected question of the
origin of man, from being regarded by the bulk of people as a
scientific question at all until 1859. Nevertheless, a few
distinguished students, free from the current prejudice, began, at the
commencement of the nineteenth century, to make a serious attack on the
problem. The merit of this attaches particularly to what is known as
“the older school of natural philosophy,” which has been so much
misrepresented, and which included Jean Lamarck, Buffon, Geoffroy St.
Hilaire, and Blainville in France; Wolfgang Goethe, Reinhold
Treviranus, Schelling, and Lorentz Oken in Germany [and Erasmus Darwin
in England].

The gifted natural philosopher who treated this difficult question with
the greatest sagacity and comprehensiveness was Jean Lamarck. He was
born at Bazentin, in Picardy, on August 1st, 1744; he was the son of a
clergyman, and was destined for the Church. But he turned to seek glory
in the army, and eventually devoted himself to science.

His _Philosophie Zoologique_ was the
first scientific attempt to sketch the real course of the origin of
species, the first “natural history of creation” of plants, animals,
and men. But, as in the case of Wolff’s book, this remarkably able work
had no influence whatever; neither one nor the other could obtain any
recognition from their prejudiced contemporaries. No man of science was
stimulated to take an interest in the work, and to develop the germs it
contained of the most important biological truths. The most
distinguished botanists and zoologists entirely rejected it, and did
not even deign to reply to it. Cuvier, who lived and worked in the same
city, has not thought fit to devote a single syllable to this great
achievement in his memoir on progress in the sciences, in which the
pettiest observations found a place. In short, Lamarck’s _Philosophie
Zoologique_ shared the fate of Wolff’s theory of development, and was
for half a century ignored and neglected. The German scientists,
especially Oken and Goethe, who were occupied with similar speculations
at the same time, seem to have known nothing about Lamarck’s work. If
they had known it, they would have been greatly helped by it, and might
have carried the theory of evolution much farther than they found it
possible to do.

To give an idea of the great importance of the _Philosophie
Zoologique,_ I will briefly explain Lamarck’s leading thought. He held
that there was no essential difference between living and lifeless
beings. Nature is one united and connected system of phenomena; and the
forces which fashion the lifeless bodies are the only ones at work in
the kingdom of living things. We have, therefore, to use the same
method of investigation and explanation in both provinces. Life is only
a physical phenomenon. All the plants and animals, with man at their
head, are to be explained, in structure and life, by mechanical or
efficient causes, without any appeal to final causes, just as in the
case of minerals and other inorganic bodies. This applies equally to
the origin of the various species. We must not assume any original
creation, or repeated creations (as in Cuvier’s theory), to explain
this, but a natural, continuous, and necessary evolution. The whole
evolutionary process has been uninterrupted. All the different kinds of
animals and plants which we see to-day, or that have ever lived, have
descended in a natural way from earlier and different species; all come
from one common stock, or from a few common ancestors. These remote
ancestors must have been quite simple organisms of the lowest type,
arising by spontaneous generation from inorganic matter. The succeeding
species have been constantly modified by adaptation to their varying
environment (especially by use and habit), and have transmitted their
modifications to their successors by heredity.

Lamarck was the first to formulate as a scientific theory the natural
origin of living things, including man, and to push the theory to its
extreme conclusions—the rise of the earliest organisms by spontaneous
generation (or abiogenesis) and the descent of man from the nearest
related mammal, the ape. He sought to explain this last point, which is
of especial interest to us here, by the same agencies which he found at
work in the natural origin of the plant and animal species. He
considered use and habit (adaptation) on the one hand, and heredity on
the other, to be the chief of these agencies. The most important
modifications of the organs of plants and animals are due, in his
opinion, to the function of these very organs, or to the use or disuse
of them. To give a few examples, the woodpecker and the humming-bird
have got their peculiarly long tongues from the habit of extracting
their food with their tongues from deep and narrow folds or canals; the
frog has developed the web between his toes by his own swimming; the
giraffe has lengthened his neck by stretching up to the higher branches
of trees, and so on. It is quite certain that this use or disuse of
organs is a most important factor in organic development, but it is not
sufficient to explain the origin of species.

To adaptation we must add heredity as the second and not less important
agency, as Lamarck perfectly recognised. He said that the modification
of the organs in any one individual by use or disuse was slight, but
that it was increased by accumulation in passing by heredity from
generation to generation. But he missed altogether the principle which
Darwin afterwards found to be the chief factor in the theory of
transformation—namely, the principle of natural selection in the
struggle for existence. It was partly owing to his failure to detect
this supremely important element, and partly to the poor condition of
all biological science at the time, that Lamarck did not
succeed in establishing more firmly his theory of the common descent of
man and the other animals.

Independently of Lamarck, the older German school of natural
philosophy, especially Reinhold Treviranus, in his _Biologie_ (1802),
and Lorentz Oken, in his _Naturphilosophie_ (1809), turned its
attention to the problem of evolution about the end of the eighteenth
and beginning of the nineteenth century. I have described its work in
my _History of Creation_ (chap. iv). Here I can only deal with the
brilliant genius whose evolutionary ideas are of special interest—the
greatest of German poets, Wolfgang Goethe. With his keen eye for the
beauties of nature, and his profound insight into its life, Goethe was
early attracted to the study of various natural sciences. It was the
favourite occupation of his leisure hours throughout life. He gave
particular and protracted attention to the theory of colours. But the
most valuable of his scientific studies are those which relate to that
“living, glorious, precious thing,” the organism. He made profound
research into the science of structures or morphology (morphæ = forms).
Here, with the aid of comparative anatomy, he obtained the most
brilliant results, and went far in advance of his time. I may mention,
in particular, his vertebral theory of the skull, his discovery of the
pineal gland in man, his system of the metamorphosis of plants, etc.
These morphological studies led Goethe on to research into the
formation and modification of organic structures which we must count as
the first germ of the science of evolution. He approaches so near to
the theory of descent that we must regard him, after Lamarck, as one of
its earliest founders. It is true that he never formulated a complete
scientific theory of evolution, but we find a number of remarkable
suggestions of it in his splendid miscellaneous essays on morphology.
Some of them are really among the very basic ideas of the science of
evolution. He says, for instance (1807): “When we compare plants and
animals in their most rudimentary forms, it is almost impossible to
distinguish between them. But we may say that the plants and animals,
beginning with an almost inseparable closeness, gradually advance along
two divergent lines, until the plant at last grows in the solid,
enduring tree and the animal attains in man to the highest degree of
mobility and freedom.” That Goethe was not merely speaking in a
poetical, but in a literal genealogical, sense of this close affinity
of organic forms is clear from other remarkable passages in which he
treats of their variety in outward form and unity in internal
structure. He believes that every living thing has arisen by the
interaction of two opposing formative forces or impulses. The internal
or “centripetal” force, the type or “impulse to specification,” seeks
to maintain the constancy of the specific forms in the succession of
generations: this is _heredity._ The external or “centrifugal” force,
the element of variation or “impulse to metamorphosis,” is continually
modifying the species by changing their environment: this is
_adaptation._ In these significant conceptions Goethe approaches very
close to a recognition of the two great mechanical factors which we now
assign as the chief causes of the formation of species.

However, in order to appreciate Goethe’s views on morphology, one must
associate his decidedly monistic conception of nature with his
pantheistic philosophy. The warm and keen interest with which he
followed, in his last years, the controversies of contemporary French
scientists, and especially the struggle between Cuvier and Geoffroy St.
Hilaire (see chap. iv of _The History of Creation_), is very
characteristic. It is also necessary to be familiar with his style and
general tenour of thought in order to appreciate rightly the many
allusions to evolution found in his writings. Otherwise, one is apt to
make serious errors.

He approached so close, at the end of the eighteenth century, to the
principles of the science of evolution that he may well be described as
the first forerunner of Darwin, although he did not go so far as to
formulate evolution as a scientific system, as Lamarck did.



Chapter V.
THE MODERN SCIENCE OF EVOLUTION


We owe so much of the progress of scientific knowledge to Darwin’s
_Origin of Species_ that its influence is almost without parallel in
the history of science. The literature of Darwinism grows from day to
day, not only on the side of academic zoology and botany, the sciences
which were chiefly affected by Darwin’s theory, but in a far wider
circle, so that we find Darwinism discussed in popular literature with
a vigour and zest that are given to no other scientific conception.
This remarkable success is due chiefly to two circumstances. In the
first place, all the sciences, and especially biology, have made
astounding progress in the last half-century, and have furnished a very
vast quantity of proofs of the theory of evolution. In striking
contrast to the failure of Lamarck and the older scientists to attract
attention to their effort to explain the origin of living things and of
man, we have this second and successful effort of Darwin, which was
able to gather to its support a large number of established facts.
Availing himself of the progress already made, he had very different
scientific proofs to allege than Lamarck, or St. Hilaire, or Goethe, or
Treviranus had had. But, in the second place, we must acknowledge that
Darwin had the special distinction of approaching the subject from an
entirely new side, and of basing the theory of descent on a consistent
system, which now goes by the name of Darwinism.

Lamarck had unsuccessfully attempted to explain the modification of
organisms that descend from a common form chiefly by the action of
habit and the use of organs, though with the aid of heredity. But
Darwin’s success was complete when he independently sought to give a
mechanical explanation, on a quite new ground, of this modification of
plant and animal structures by adaptation and heredity. He was impelled
to his theory of selection on the following grounds. He compared the
origin of the various kinds of animals and plants which we modify
artificially—by the action of artificial selection in horticulture and
among domestic animals—with the origin of the species of animals and
plants in their natural state. He then found that the agencies which we
employ in the modification of forms by artificial selection are also at
work in Nature. The chief of these agencies he held to be “the struggle
for life.” The gist of this peculiarly Darwinian idea is given in this
formula: The struggle for existence produces new species without
premeditated design in the life of Nature, in the same way that the
will of man consciously selects new races in artificial conditions. The
gardener or the farmer selects new forms as he wills for his own
profit, by ingeniously using the agency of heredity and adaptation for
the modification of structures; so, in the natural state, the struggle
for life is always unconsciously modifying the various species of
living things. This struggle for life, or competition of organisms in
securing the means of subsistence, acts without any conscious design,
but it is none the less effective in modifying structures. As heredity
and adaptation enter into the closest reciprocal action under its
influence, new structures, or alterations of structure, are produced;
and these are purposive in the sense that they serve the organism when
formed, but they were produced without any pre-conceived aim.

This simple idea is the central thought of Darwinism, or the theory of
selection. Darwin conceived this idea at an early date, and then, for
more than twenty years, worked at the collection of empirical evidence
in support of it before he published his theory. His grandfather,
Erasmus Darwin, was an able scientist of the older school of natural
philosophy, who published a number of natural-philosophic works about
the end of the eighteenth century. The most important of them is his
_Zoonomia,_ published in 1794, in which he expounds views similar to
those of Goethe and Lamarck, without really knowing anything of the
work of these
contemporaries. However, in the writings of the grandfather the plastic
imagination rather outran the judgment, while in Charles Darwin the two
were better balanced.

Darwin did not publish any account of his theory until 1858, when
Alfred Russel Wallace, who had independently reached the same theory of
selection, published his own work. In the following year appeared the
_Origin of Species,_ in which he develops it at length and supports it
with a mass of proof. Wallace had reached the same conclusion, but he
had not so clear a perception as Darwin of the effectiveness of natural
selection in forming species, and did not develop the theory so fully.
Nevertheless, Wallace’s writings, especially those on mimicry, etc.,
and an admirable work on _The Geographical Distribution of Animals,_
contain many fine original contributions to the theory of selection.
Unfortunately, this gifted scientist has since devoted himself to
spiritism.[10]

 [10] Darwin and Wallace arrived at the theory quite independently.
 _Vide_ Wallace’s _Contributions to the Theory of Natural Selection_
 (1870) and _Darwinism_ (1891).


Darwin’s _Origin of Species_ had an extraordinary influence, though not
at first on the experts of the science. It took zoologists and
botanists several years to recover from the astonishment into which
they had been thrown through the revolutionary idea of the work. But
its influence on the special sciences with which we zoologists and
botanists are concerned has increased from year to year; it has
introduced a most healthy fermentation in every branch of biology,
especially in comparative anatomy and ontogeny, and in zoological and
botanical classification. In this way it has brought about almost a
revolution in the prevailing views.

However, the point which chiefly concerns us here—the extension of the
theory to man—was not touched at all in Darwin’s first work in 1859. It
was believed for several years that he had no thought of applying his
principles to man, but that he shared the current idea of man holding a
special position in the universe. Not only ignorant laymen (especially
several theologians), but also a number of men of science, said very
naively that Darwinism in itself was not to be opposed; that it was
quite right to use it to explain the origin of the various species of
plants and animals, but that it was totally inapplicable to man.

In the meantime, however, it seemed to a good many thoughtful people,
laymen as well as scientists, that this was wrong; that the descent of
man from some other animal species, and immediately from some ape-like
mammal, followed logically and necessarily from Darwin’s reformed
theory of evolution. Many of the acuter opponents of the theory saw at
once the justice of this position, and, as this consequence was
intolerable, they wanted to get rid of the whole theory.

The first scientific application of the Darwinian theory to man was
made by Huxley, the greatest zoologist in England. This able and
learned scientist, to whom zoology owes much of its progress, published
in 1863 a small work entitled _Evidence as to Man’s Place in Nature._
In the extremely important and interesting lectures which made up this
work he proved clearly that the descent of man from the ape followed
necessarily from the theory of descent. If that theory is true, we are
bound to conceive the animals which most closely resemble man as those
from which humanity has been gradually evolved. About the same time
Carl Vogt published a larger work on the same subject. We must also
mention Gustav Jaeger and Friedrich Rolle among the zoologists who
accepted and taught the theory of evolution immediately after the
publication of Darwin’s book, and maintained that the descent of man
from the lower animals logically followed from it. The latter
published, in 1866, a work on the origin and position of man.

About the same time I attempted, in the second volume of my _General
Morphology_ (1866), to apply the theory of evolution to the whole
organic kingdom, including man.[11] I endeavoured to sketch the
probable ancestral trees of the various classes of the animal world,
the protists, and the plants, as it seemed necessary to do on Darwinian
principles, and as we can actually do now with a high degree of
confidence. If the theory of descent, which Lamarck first clearly
formulated and Darwin thoroughly established, is true, we should be
able to draw up a natural classification of plants and animals in the
light of their genealogy, and to conceive the large and small divisions
of
the system as the branches and twigs of an ancestral tree. The eight
genealogical tables which I inserted in the second volume of the
_General Morphology_ are the first sketches of their kind. In Chapter
27, particularly, I trace the chief stages in man’s ancestry, as far as
it is possible to follow it through the vertebrate stem. I tried
especially to determine, as well as one could at that time, the
position of man in the classification of the mammals and its
genealogical significance. I have greatly improved this attempt, and
treated it in a more popular form, in chaps. xxvi–xxviii of my _History
of Creation_ (1868).[12]

 [11] Huxley spoke of this “as one of the greatest scientific works
 ever published.”—Translator.


 [12] Of which Darwin said that the _Descent of Man_ would probably
 never have been written if he had seen it earlier.—Translator.


It was not until 1871, twelve years after the appearance of _The Origin
of Species,_ that Darwin published the famous work which made the
much-contested application of his theory to man, and crowned the
splendid structure of his system. This important work was _The Descent
of Man, and Selection in Relation to Sex._ In this Darwin expressly
drew the conclusion, with rigorous logic, that man also must have been
developed out of lower species, and described the important part played
by sexual selection in the elevation of man and the other higher
animals. He showed that the careful selection which the sexes exercise
on each other in regard to sexual relations and procreation, and the
æsthetic feeling which the higher animals develop through this, are of
the utmost importance in the progressive development of forms and the
differentiation of the sexes. The males choosing the handsomest females
in one class of animals, and the females choosing only the
finest-looking males in another, the special features and the sexual
characteristics are increasingly accentuated. In fact, some of the
higher animals develop in this connection a finer taste and judgment
than man himself. But, even as regards man, it is to this sexual
selection that we owe the family-life, which is the chief foundation of
civilisation. The rise of the human race is due for the most part to
the advanced sexual selection which our ancestors exercised in choosing
their mates.

Darwin accepted in the main the general outlines of man’s ancestral
tree, as I gave it in the _General Morphology_ and the _History of
Creation,_ and admitted that his studies led him to the same
conclusion. That he did not at once apply the theory to man in his
first work was a commendable piece of discretion; such a sequel was
bound to excite the strongest opposition to the whole theory. The first
thing to do was to establish it as regards the animal and plant worlds.
The subsequent extension to man was bound to be made sooner or later.

It is important to understand this very clearly. If all living things
come from a common root, man must be included in the general scheme of
evolution. On the other hand, if the various species were separately
created, man, too, must have been created, and not evolved. We have to
choose between these two alternatives. This cannot be too frequently or
too strongly emphasised. _Either_ all the species of animals and plants
are of supernatural origin—created, not evolved—and in that case man
also is the outcome of a creative act, as religion teaches, _or_ the
different species have been evolved from a few common, simple ancestral
forms, and in that case man is the highest fruit of the tree of
evolution.

We may state this briefly in the following principle—_The descent of
man from the lower animals is a special deduction which inevitably
follows from the general inductive law of the whole theory of
evolution._ In this principle we have a clear and plain statement of
the matter. Evolution is in reality nothing but a great induction,
which we are compelled to make by the comparative study of the most
important facts of morphology and physiology. But we must draw our
conclusion according to the laws of induction, and not attempt to
determine scientific truths by direct measurement and mathematical
calculation. In the study of living things we can scarcely ever
directly and fully, and with mathematical accuracy, determine the
nature of phenomena, as is done in the simpler study of the inorganic
world—in chemistry, physics, mineralogy, and astronomy. In the latter,
especially, we can always use the simplest and absolutely safest
method—that of mathematical determination. But in biology this is quite
impossible for various reasons; one very obvious reason being that most
of the facts of the science are very complicated and much too intricate
to allow a direct mathematical analysis. The greater part of the
phenomena that biology deals with are
complicated _historical processes,_ which are related to a far-reaching
past, and as a rule can only be approximately estimated. Hence we have
to proceed by _induction_—that is to say, to draw general conclusions,
stage by stage, and with proportionate confidence, from the
accumulation of detailed observations. These inductive conclusions
cannot command absolute confidence, like mathematical axioms; but they
approach the truth, and gain increasing probability, in proportion as
we extend the basis of observed facts on which we build. The importance
of these inductive laws is not diminished from the circumstance that
they are looked upon merely as temporary acquisitions of science, and
may be improved to any extent in the progress of scientific knowledge.
The same may be said of the attainments of many other sciences, such as
geology or archeology. However much they may be altered and improved in
detail in the course of time, these inductive truths may retain their
substance unchanged.

Now, when we say that the theory of evolution in the sense of Lamarck
and Darwin is an inductive law—in fact, the greatest of all biological
inductions—we rely, in the first place, on the facts of paleontology.
This science gives us some direct acquaintance with the historical
phenomena of the changes of species. From the situations in which we
find the fossils in the various strata of the earth we gather
confidently, in the first place, that the living population of the
earth has been gradually developed, as clearly as the earth’s crust
itself; and that, in the second place, several different populations
have succeeded each other in the various geological periods. Modern
geology teaches that the formation of the earth has been gradual, and
unbroken by any violent revolutions. And when we compare together the
various kinds of animals and plants which succeed each other in the
history of our planet, we find, in the first place, a constant and
gradual increase in the number of species from the earliest times until
the present day; and, in the second place, we notice that the forms in
each great group of animals and plants also constantly improve as the
ages advance. Thus, of the vertebrates there are at first only the
lower fishes; then come the higher fishes, and later the amphibia.
Still later appear the three higher classes of vertebrates—the
reptiles, birds, and mammals, for the first time; only the lowest and
least perfect forms of the mammals are found at first; and it is only
at a very late period that placental mammals appear, and man belongs to
the latest and youngest branch of these. Thus perfection of form
increases as well as variety from the earliest to the latest stage.
That is a fact of the greatest importance. It can only be explained by
the theory of evolution, with which it is in perfect harmony. If the
different groups of plants and animals do really descend from each
other, we must expect to find this increase in their number and
perfection under the influence of natural selection, just as the
succession of fossils actually discloses it to us.

Comparative anatomy furnishes a second series of facts which are of
great importance for the forming of our inductive law. This branch of
morphology compares the adult structures of living things, and seeks in
the great variety of organic forms the stable and simple law of
organisation, or the common type or structure. Since Cuvier founded
this science at the beginning of the nineteenth century it has been a
favourite study of the most distinguished scientists. Even before
Cuvier’s time Goethe had been greatly stimulated by it, and induced to
take up the study of morphology. Comparative osteology, or the
philosophic study and comparison of the bony skeleton of the
vertebrates—one of its most interesting sections—especially fascinated
him, and led him to form the theory of the skull which I mentioned
before. Comparative anatomy shows that the internal structure of the
animals of each stem and the plants of each class is the same in its
essential features, however much they differ in external appearance.
Thus man has so great a resemblance in the chief features of his
internal organisation to the other mammals that no comparative
anatomist has ever doubted that he belongs to this class. The whole
internal structure of the human body, the arrangement of its various
systems of organs, the distribution of the bones, muscles,
blood-vessels, etc., and the whole structure of these organs in the
larger and the finer scale, agree so closely with those of the other
mammals (such as the apes, rodents, ungulates, cetacea, marsupials,
etc.) that their external differences are of no account whatever. We
learn further from comparative anatomy that the chief features of
animal structure
are so similar in the various classes (fifty to sixty in number
altogether) that they may all be comprised in from eight to twelve
great groups. But even in these groups, the stem-forms or animal types,
certain organs (especially the alimentary canal) can be proved to have
been originally the same for all. We can only explain by the theory of
evolution this essential unity in internal structure of all these
animal forms that differ so much in outward appearance. This wonderful
fact can only be really understood and explained when we regard the
internal resemblance as an inheritance from common-stem forms, and the
external differences as the effect of adaptation to different
environments.

In recognising this, comparative anatomy has itself advanced to a
higher stage. Gegenbaur, the most distinguished of recent students of
this science, says that with the theory of evolution a new period began
in comparative anatomy, and that the theory in turn found a touch stone
in the science. “Up to now there is no fact in comparative anatomy that
is inconsistent with the theory of evolution; indeed, they all lead to
it. In this way the theory receives back from the science all the
service it rendered to its method.” Until then students had marvelled
at the wonderful resemblance of living things in their inner structure
without being able to explain it. We are now in a position to explain
the causes of this, by showing that this remarkable agreement is the
necessary consequence of the inheriting of common stem-forms; while the
striking difference in outward appearance is a result of adaptation to
changes of environment. Heredity and adaptation alone furnish the true
explanation.

But one special part of comparative anatomy is of supreme interest and
of the utmost philosophic importance in this connection. This is the
science of rudimentary or useless organs; I have given it the name of
“dysteleology” in view of its philosophic consequences. Nearly every
organism (apart from the very lowest), and especially every
highly-developed animal or plant, including man, has one or more organs
which are of no use to the body itself, and have no share in its
functions or vital aims. Thus we all have, in various parts of our
frame, muscles which we never use, as, for instance, in the shell of
the ear and adjoining parts. In most of the mammals, especially those
with pointed ears, these internal and external ear-muscles are of great
service in altering the shell of the ear, so as to catch the waves of
sound as much as possible. But in the case of man and other short-eared
mammals these muscles are useless, though they are still present. Our
ancestors having long abandoned the use of them, we cannot work them at
all to-day. In the inner corner of the eye we have a small
crescent-shaped fold of skin; this is the last relic of a third inner
eye-lid, called the nictitating (winking) membrane. This membrane is
highly developed and of great service in some of our distant relations,
such as fishes of the shark type and several other vertebrates; in us
it is shrunken and useless. In the intestines we have a process that is
not only quite useless, but may be very harmful—the vermiform
appendage. This small intestinal appendage is often the cause of a
fatal illness. If a cherry-stone or other hard body is unfortunately
squeezed through its narrow aperture during digestion, a violent
inflammation is set up, and often proves fatal. This appendix has no
use whatever now in our frame; it is a dangerous relic of an organ that
was much larger and was of great service in our vegetarian ancestors.
It is still large and important in many vegetarian animals, such as
apes and rodents.

There are similar rudimentary organs in all parts of our body, and in
all the higher animals. They are among the most interesting phenomena
to which comparative anatomy introduces us; partly because they furnish
one of the clearest proofs of evolution, and partly because they most
strikingly refute the teleology of certain philosophers. The theory of
evolution enables us to give a very simple explanation of these
phenomena.

We have to look on them as organs which have fallen into disuse in the
course of many generations. With the decrease in the use of its
function, the organ itself shrivels up gradually, and finally
disappears. There is no other way of explaining rudimentary organs.
Hence they are also of great interest in philosophy; they show clearly
that the _monistic_ or mechanical view of the organism is the only
correct one, and that the _dualistic_ or teleological conception is
wrong. The ancient legend of the direct creation of man according to a
pre-conceived plan and the empty phrases about
“design” in the organism are completely shattered by them. It would be
difficult to conceive a more thorough refutation of teleology than is
furnished by the fact that all the higher animals have these
rudimentary organs.

The theory of evolution finds its broadest inductive foundation in the
natural classification of living things, which arranges all the various
forms in larger and smaller groups, according to their degree of
affinity. These groupings or categories of classification—the
varieties, species, genera, families, orders, classes, etc.—show such
constant features of coordination and subordination that we are bound
to look on them as _genealogical,_ and represent the whole system in
the form of a branching tree. This is the genealogical tree of the
variously related groups; their likeness in form is the expression of a
real affinity. As it is impossible to explain in any other way the
natural tree-like form of the system of organisms, we must regard it at
once as a weighty proof of the truth of evolution. The careful
construction of these genealogical trees is, therefore, not an
amusement, but the chief task of modern classification.

Among the chief phenomena that bear witness to the inductive law of
evolution we have the geographical distribution of the various species
of animals and plants over the surface of the earth, and their
topographical distribution on the summits of mountains and in the
depths of the ocean. The scientific study of these features—the
“science of distribution,” or chorology (_chora_ = a place)—has been
pursued with lively interest since the discoveries made by Alexander
von Humboldt. Until Darwin’s time the work was confined to the
determination of the facts of the science, and chiefly aimed at
settling the spheres of distribution of the existing large and small
groups of living things. It was impossible at that time to explain the
causes of this remarkable distribution, or the reasons why one group is
found only in one locality and another in a different place, and why
there is this manifold distribution at all. Here, again, the theory of
evolution has given us the solution of the problem. It furnishes the
only possible explanation when it teaches that the various species and
groups of species descend from common stem-forms, whose ever-branching
offspring have gradually spread themselves by migration over the earth.
For each group of species we must admit a “centre of production,” or
common home; this is the original habitat in which the ancestral form
was developed, and from which its descendants spread out in every
direction. Several of these descendants became in their turn the
stem-forms for new groups of species, and these also scattered
themselves by active and passive migration, and so on. As each
migrating organism found a different environment in its new home, and
adapted itself to it, it was modified, and gave rise to new forms.

This very important branch of science that deals with active and
passive migration was founded by Darwin, with the aid of the theory of
evolution; and at the same time he advanced the true explanation of the
remarkable relation or similarity of the living population in any
locality to the fossil forms found in it. Moritz Wagner very ably
developed his idea under the title of “the theory of migration.” In my
opinion, this famous traveller has rather over-estimated the value of
his theory of migration when he takes it to be an indispensable
condition of the formation of new species and opposes the theory of
selection. The two theories are not opposed in their main features.
Migration (by which the stem-form of a new species is isolated) is
really only a special case of selection. The striking and interesting
facts of chorology can be explained only by the theory of evolution,
and therefore we must count them among the most important of its
inductive bases.

The same must be said of all the remarkable phenomena which we perceive
in the economy of the living organism. The many and various relations
of plants and animals to each other and to their environment, which are
treated in _bionomy_ (from _nomos,_ law or norm, and _bios,_ life), the
interesting facts of parasitism, domesticity, care of the young, social
habits, etc., can only be explained by the action of heredity and
adaptation. Formerly people saw only the guidance of a beneficent
Providence in these phenomena; to-day we discover in them admirable
proofs of the theory of evolution. It is impossible to understand them
except in the light of this theory and the struggle for life.

Finally, we must, in my opinion, count among the chief inductive bases
of the
theory of evolution the fœtal development of the individual organism,
the whole science of embryology or ontogeny. But as the later chapters
will deal with this in detail, I need say nothing further here. I shall
endeavour in the following pages to show, step by step, how the whole
of the embryonic phenomena form a massive chain of proof for the theory
of evolution; for they can be explained in no other way. In thus
appealing to the close causal connection between ontogenesis and
phylogenesis, and taking our stand throughout on the biogenetic law, we
shall be able to prove, stage by stage, from the facts of embryology,
the evolution of man from the lower animals.

The general adoption of the theory of evolution has definitely closed
the controversy as to the nature or definition of the species. The word
has no _absolute_ meaning whatever, but is only a group-name, or
category of classification, with a purely relative value. In 1857, it
is true, a famous and gifted, but inaccurate and dogmatic, scientist,
Louis Agassiz, attempted to give an absolute value to these “categories
of classification.” He did this in his _Essay on Classification,_ in
which he turns upside down the phenomena of organic nature, and,
instead of tracing them to their natural causes, examines them through
a theological prism. The true species (_bona species_) was, he said, an
“incarnate idea of the Creator.” Unfortunately, this pretty phrase has
no more scientific value than all the other attempts to save the
absolute or intrinsic value of the species.

The dogma of the fixity and creation of species lost its last great
champion when Agassiz died in 1873. The opposite theory, that all the
different species descend from common stem-forms, encounters no serious
difficulty to-day. All the endless research into the nature of the
species, and the possibility of several species descending from a
common ancestor, has been closed to-day by the removal of the sharp
limits that had been set up between species and varieties on the one
hand, and species and genera on the other. I gave an analytic proof of
this in my monograph on the sponges (1872), having made a very close
study of variability in this small but highly instructive group, and
shown the impossibility of making any dogmatic distinction of species.
According as the classifier takes his ideas of genus, species, and
variety in a broader or in a narrower sense, he will find in the small
group of the sponges either one genus with three species, or three
genera with 238 species, or 113 genera with 591 species. Moreover, all
these forms are so connected by intermediate forms that we can
convincingly prove the descent of all the sponges from a common
stem-form, the olynthus.

Here, I think, I have given an analytic solution of the problem of the
origin of species, and so met the demand of certain opponents of
evolution for an actual instance of descent from a stem-form. Those who
are not satisfied with the synthetic proofs of the theory of evolution
which are provided by comparative anatomy, embryology, paleontology,
dysteleology, chorology, and classification, may try to refute the
analytic proof given in my treatise on the sponge, the outcome of five
years of assiduous study. I repeat: It is now impossible to oppose
evolution on the ground that we have no convincing example of the
descent of all the species of a group from a common ancestor. The
monograph on the sponges furnishes such a proof, and, in my opinion, an
indisputable proof. Any man of science who will follow the protracted
steps of my inquiry and test my assertions will find that in the case
of the sponges we can follow the actual evolution of species in a
concrete case. And if this is so, if we can show the origin of all the
species from a common form in one single class, we have the solution of
the problem of man’s origin, because we are in a position to prove
clearly his descent from the lower animals.

At the same time, we can now reply to the often-repeated assertion,
even heard from scientists of our own day, that the descent of man from
the lower animals, and proximately from the apes, still needs to be
“proved with certainty.” These “certain proofs” have been available for
a long time; one has only to open one’s eyes to see them. It is a
mistake to seek them in the discovery of intermediate forms between man
and the ape, or the conversion of an ape into a human being by skilful
education. The proofs lie in the great mass of empirical material we
have already collected. They are furnished in the strongest form by the
data of comparative anatomy and embryology, completed by paleontology.
It is not a question now of detecting new proofs of the evolution of
man, but of examining
and understanding the proofs we already have.

I was almost alone thirty-six years ago when I made the first attempt,
in my _General Morphology,_ to put organic science on a mechanical
foundation through Darwin’s theory of descent. The association of
ontogeny and phylogeny and the proof of the intimate causal connection
between these two sections of the science of evolution, which I
expounded in my work, met with the most spirited opposition on nearly
all sides. The next ten years were a terrible “struggle for life” for
the new theory. But for the last twenty-five years the tables have been
turned. The phylogenetic method has met with so general a reception,
and found so prolific a use in every branch of biology, that it seems
superfluous to treat any further here of its validity and results. The
proof of it lies in the whole morphological literature of the last
three decades. But no other science has been so profoundly modified in
its leading thoughts by this adoption, and been forced to yield such
far-reaching consequences, as that science which I am now seeking to
establish—monistic anthropogeny.

This statement may seem to be rather audacious, since the very next
branch of biology, anthropology in the stricter sense, makes very
little use of these results of anthropogeny, and sometimes expressly
opposes them.[13] This applies especially to the attitude which has
characterised the German Anthropological Society (the _Deutsche
Gesellschaft fur Anthropologie_) for some thirty years. Its powerful
president, the famous pathologist, Rudolph Virchow, is chiefly
responsible for this. Until his death (September 5th, 1902) he never
ceased to reject the theory of descent as unproven, and to ridicule its
chief consequence—the descent of man from a series of mammal
ancestors—as a fantastic dream. I need only recall his well-known
expression at the Anthropological Congress at Vienna in 1894, that “it
would be just as well to say man came from the sheep or the elephant as
from the ape.”

 [13] This does not apply to English anthropologists, who are almost
 all evolutionists.


Virchow’s assistant, the secretary of the German Anthropological
Society, Professor Johannes Ranke of Munich, has also indefatigably
opposed transformism: he has succeeded in writing a work in two volumes
(_Der Mensch_), in which all the facts relating to his organisation are
explained in a sense hostile to evolution. This work has had a wide
circulation, owing to its admirable illustrations and its able
treatment of the most interesting facts of anatomy and
physiology—exclusive of the sexual organs! But, as it has done a great
deal to spread erroneous views among the general public, I have
included a criticism of it in my _History of Creation,_ as well as met
Virchow’s attacks on anthropogeny.

Neither Virchow, nor Ranke, nor any other “exact” anthropologist, has
attempted to give any other natural explanation of the origin of man.
They have either set completely aside this “question of questions” as a
transcendental problem, or they have appealed to religion for its
solution. We have to show that this rejection of the rational
explanation is totally without justification. The fund of knowledge
which has accumulated in the progress of biology in the nineteenth
century is quite adequate to furnish a rational explanation, and to
establish the theory of the evolution of man on the solid facts of his
embryology.



Chapter VI.
THE OVUM AND THE AMŒBA


In order to understand clearly the course of human embryology, we must
select the more important of its wonderful and manifold processes for
fuller explanation, and then proceed from these to the innumerable
features of less importance. The most important feature in this sense,
and the best starting-point for ontogenetic study, is the fact that man
is developed from an ovum, and that this ovum is a simple cell. The
human ovum does not materially differ in form and composition from that
of the other mammals, whereas there is a distinct difference between
the fertilised ovum of the mammal and that of any other animal.


Fig.1 The human ovum Fig. 1—The human ovum. The globular mass of yelk
(_b_) is enclosed by a transparent membrane (the ovolemma or zona
pellucida [_a_]), and contains a noncentral nucleus (the germinal
vesicle, _c_). Cf. Fig. 14.

This fact is so important that few should be unaware of its extreme
significance; yet it was quite unknown in the first quarter of the
nineteenth century. As we have seen, the human and mammal ovum was not
discovered until 1827, when Carl Ernst von Baer detected it. Up to that
time the larger vesicles, in which the real and much smaller ovum is
contained, had been wrongly regarded as ova. The important circumstance
that this mammal ovum is a simple cell, like the ovum of other animals,
could not, of course, be recognised until the cell theory was
established. This was not done, by Schleiden for the plant and Schwann
for the animal, until 1838. As we have seen, this cell theory is of the
greatest service in explaining the human frame and its embryonic
development. Hence we must say a few words about the actual condition
of the theory and the significance of the views it has suggested.

In order properly to appreciate the cellular theory, the most important
element in our science, it is necessary to understand in the first
place that the cell is a _unified organism,_ a self-contained living
being. When we anatomically dissect the fully-formed animal or plant
into its various organs, and then examine the finer structure of these
organs with the microscope, we are surprised to find that all these
different parts are ultimately made up of the same structural element
or unit. This common unit of structure is the cell. It does not matter
whether we thus dissect a leaf, flower, or fruit, or a bone, muscle,
gland, or bit of skin, etc.; we find in every case the same ultimate
constituent, which has been called the cell since Schleiden’s
discovery. There are many opinions as to its real nature, but the
essential point in our view of the cell is to look upon it as a
self-contained or independent living unit. It is, in the words of
Brucke, “an elementary organism.” We may define it most precisely as
the ultimate organic unit, and, as the cells are the sole active
principles in every vital function, we may call them the “plastids,” or
“formative elements.” This unity is found in both the anatomic
structure and the physiological function. In the case of the protists,
the entire organism usually consists of a single independent cell
throughout life. But in the tissue-forming animals and plants, which
are the great majority, the organism begins its career as a simple
cell, and then grows into a cell-community, or, more correctly, an
organised cell-state. Our own body is not really the simple unity that
it is generally supposed to be. On the contrary, it is a very elaborate
social system of countless microscopic organisms, a colony or
commonwealth, made up of innumerable independent units, or very
different tissue-cells.

In reality, the term “cell,” which existed long before the cell theory
was formulated, is not happily chosen. Schleiden, who first brought it
into scientific use in the sense of the cell theory, gave this name to
the elementary organisms because, when you find them in the dissected
plant, they generally have the appearance of chambers, like the cells
in a bee-hive, with firm walls and a fluid or pulpy content. But some
cells, especially young ones, are entirely without the enveloping
membrane, or stiff wall. Hence we now generally describe the cell as a
living, viscous particle of protoplasm, enclosing a firmer nucleus in
its albuminoid body. There may be an enclosing membrane, as there
actually is in the case of most of the plants; but it may be wholly
lacking, as is the case with most of the animals. There is no membrane
at all in the first stage. The young cells are usually round, but they
vary much in shape later on. Illustrations of this will be found in the
cells of the various parts of the body shown in Figs. 3–7.

Hence the essential point in the modern idea of the cell is that it is
made up of two different active constituents—an inner and an outer
part. The smaller and inner part is the nucleus (or _caryon_ or
_cytoblastus,_ Fig. 1_c_ and Fig. 2_k_). The outer and larger part,
which encloses the other, is the body of the cell (_celleus, cytos,_ or
_cytosoma_). The soft living substance of which the two are composed
has a peculiar chemical composition, and belongs to the group of the
albuminoid plasma-substances (“formative matter”), or protoplasm. The
essential and indispensable element of the nucleus is called nuclein
(or caryoplasm); that of the cell body is called plastin (or
cytoplasm). In the most rudimentary cases both substances seem to be
quite simple and homogeneous, without any visible structure. But, as a
rule, when we examine them under a high power of the microscope, we
find a certain structure in the protoplasm. The chief and most common
form of this is the fibrous or net-like “thready structure” (Frommann)
and the frothy “honeycomb structure” (Bütschli).


Fig.2 Stem-cell of one of the echinoderms Fig. 2—Stem-cell of one of
the echinoderms (cytula, or “first segmentation-cell” = fertilised
ovum), after _Hertwig. k_ is the nucleus or caryon.


The shape or outer form of the cell is infinitely varied, in accordance
with its endless power of adapting itself to the most diverse
activities or environments. In its simplest form the cell is globular
(Fig. 2). This normal round form is especially found in cells of the
simplest construction, and those that are developed in a free fluid
without any external pressure. In such cases the nucleus also is not
infrequently round, and located in the centre of the cell-body (Fig.
2_k_). In other cases, the cells have no definite shape; they are
constantly changing their form owing to their automatic movements. This
is the case with the amœbæ (Fig. 15 and 16) and the amœboid travelling
cells (Fig. 11), and also with very young ova (Fig. 13).However, as a
rule, the cell assumes a definite form in the course of its career. In
the tissues of the multicellular organism, in which a number of similar
cells are bound together in virtue of certain laws of heredity, the
shape is determined partly by the form of their connection and partly
by their special functions. Thus, for instance, we find in the mucous
lining of our tongue very thin and delicate flat cells of roundish
shape (Fig. 3). In the outer skin we find similar, but harder, covering
cells, joined together by saw-like edges (Fig. 4). In the liver and
other glands there are thicker and softer cells, linked together in
rows (Fig. 5).

The last-named tissues (Figs. 3–5) belong to the simplest and most
primitive type, the group of the “covering-tissues,” or epithelia. In
these “primary tissues” (to which the germinal layers belong) simple
cells of the same kind are arranged in layers. The arrangement and
shape are more complicated in the “secondary tissues,” which are
gradually developed out of the primary, as in the tissues of the
muscles, nerves, bones, etc. In the bones, for instance, which belong
to the group of supporting or connecting organs,
the cells (Fig. 6) are star-shaped, and are joined together by numbers
of net-like interlacing processes; so, also, in the tissues of the
teeth (Fig. 7), and in other forms of supporting-tissue, in which a
soft or hard substance (intercellular matter, or base) is inserted
between the cells.


Fig.3 Three epithelial cells. Fig. 4 Five spiny or grooved cells. Fig.
5 Ten liver-cells. Fig. 3—Three epithelial cells from the mucous lining
of the tongue.
Fig. 4—Five spiny or grooved cells, with edges joined, from the outer
skin (epidermis): one of them (_b_) is isolated.
Fig. 5—Ten liver-cells: one of them (_b_) has two nuclei.


The cells also differ very much in size. The great majority of them are
invisible to the naked eye, and can be seen only through the microscope
(being as a rule between 1/2500 and 1/250 inch in diameter). There are
many of the smaller plastids—such as the famous bacteria—which only
come into view with a very high magnifying power. On the other hand,
many cells attain a considerable size, and run occasionally to several
inches in diameter, as do certain kinds of rhizopods among the
unicellular protists (such as the radiolaria and thalamophora). Among
the tissue-cells of the animal body many of the muscular fibres and
nerve fibres are more than four inches, and sometimes more than a yard,
in length. Among the largest cells are the yelk-filled ova; as, for
instance, the yellow “yolk” in the hen’s egg, which we shall describe
later (Fig. 15).

Cells also vary considerably in structure. In this connection we must
first distinguish between the active and passive components of the
cell. It is only the former, or _active_ parts of the cell, that really
live, and effect that marvellous world of phenomena to which we give
the name of “organic life.” The first of these is the inner nucleus
(_caryoplasm_), and the second the body of the cell (_cytoplasm_). The
_ passive_ portions come third; these are subsequently formed from the
others, and I have given them the name of “plasma-products.” They are
partly external (cell-membranes and intercellular matter) and partly
internal (cell-sap and cell-contents).

The nucleus (or caryon), which is usually of a simple roundish form, is
quite structureless at first (especially in very young cells), and
composed of homogeneous nuclear matter or caryoplasm (Fig. 2_k_). But,
as a rule, it forms a sort of vesicle later on, in which we can
distinguish a more solid _nuclear base (caryobasis)_ and a softer or
fluid _nuclear sap (caryolymph)._ In a mesh of the nuclear network (or
it may be on the inner side of the nuclear envelope) there is, as a
rule, a dark, very opaque, solid body, called the _nucleolus._ Many of
the nuclei contain several of these nucleoli (as, for instance, the
germinal vesicle of the ova of fishes and amphibia). Recently a very
small, but particularly important, part of the nucleus has been
distinguished as the _central body_ (centrosoma)—a tiny particle that
is originally found in the nucleus itself, but is usually outside it,
in the cytoplasm; as a rule, fine threads stream out from it in the
cytoplasm. From the position of the central body with regard to the
other parts it seems probable that it has a high physiological
importance as a centre of movement; but it is lacking in many cells.

The cell-body also consists originally, and in its simplest form, of a
homogeneous viscid plasmic matter. But, as a rule,
only the smaller part of it is formed of the living active
cell-substance (protoplasm); the greater part consists of dead, passive
plasma-products (metaplasm). It is useful to distinguish between the
inner and outer of these. External plasma-products (which are thrust
out from the protoplasm as solid “structural matter”) are the
cell-membranes and the intercellular matter. The _internal_
plasma-products are either the fluid cell-sap or hard structures. As a
rule, in mature and differentiated cells these various parts are so
arranged that the protoplasm (like the caryoplasm in the round nucleus)
forms a sort of skeleton or framework. The spaces of this network are
filled partly with the fluid cell-sap and partly by hard structural
products.


Fig.6 Nine star-shaped bone cells. Fig. 6—Nine star-shaped bone-cells,
with interlaced branches.


The simple round ovum, which we take as the starting-point of our study
(Figs. 1 and 2), has in many cases the vague, indifferent features of
the typical primitive cell. As a contrast to it, and as an instance of
a very highly differentiated plastid, we may consider for a moment a
large nerve-cell, or ganglionic cell, from the brain. The ovum stands
potentially for the entire organism—in other words, it has the faculty
of building up out of itself the whole multicellular body. It is the
common parent of all the countless generations of cells which form the
different tissues of the body; it unites all their powers in itself,
though only potentially or in germ. In complete contrast to this, the
neural cell in the brain (Fig. 9) develops along one rigid line. It
cannot, like the ovum, beget endless generations of cells, of which
some will become skin-cells, others muscle-cells, and others again
bone-cells. But, on the other hand, the nerve-cell has become fitted to
discharge the highest functions of life; it has the powers of
sensation, will, and thought. It is a real soul-cell, or an elementary
organ of the psychic activity. It has, therefore, a most elaborate and
delicate structure.


Fig.7 Eleven star-shaped cells. Fig. 7—Eleven star-shaped cells from
the enamel of a tooth, joined together by their branchlets.

 Numbers of extremely fine threads, like the electric wires at a large
 telegraphic centre, cross and recross in the delicate protoplasm of
 the nerve cell, and pass out in the branching processes which proceed
 from it and put it in communication with other nerve-cells or
 nerve-fibres (_a, b_). We can only partly follow their intricate paths
 in the fine matter of the body of the cell.

Here we have a most elaborate apparatus, the delicate structure of
which we are just beginning to appreciate through our most powerful
microscopes, but whose significance is rather a matter of
conjecture than knowledge. Its intricate structure corresponds to the
very complicated functions of the mind. Nevertheless, this elementary
organ of psychic activity—of which there are thousands in our brain—is
nothing but a single cell. Our whole mental life is only the joint
result of the combined activity of all these nerve-cells, or
soul-cells. In the centre of each cell there is a large transparent
nucleus, containing a small and dark nuclear body. Here, as elsewhere,
it is the nucleus that determines the individuality of the cell; it
proves that the whole structure, in spite of its intricate composition,
amounts to only a single cell.


Fig.8 Unfertilised ovum of an echinoderm. Fig. 8—Unfertilised ovum of
an echinoderm (from _Hertwig_). The vesicular nucleus (or “germinal
vesicle”) is globular, half the size of the round ovum, and encloses a
nuclear framework, in the central knot of which there is a dark
nucleolus (the “germinal spot”).


In contrast with this very elaborate and very strictly differentiated
psychic cell (Fig. 9), we have our ovum (Figs. 1 and 2), which has
hardly any structure at all. But even in the case of the ovum we must
infer from its properties that its protoplasmic body has a very
complicated chemical composition and a fine molecular structure which
escapes our observation. This presumed molecular structure of the plasm
is now generally admitted; but it has never been seen, and, indeed,
lies far beyond the range of microscopic vision. It must not be
confused—as is often done—with the structure of the plasm (the fibrous
network, groups of granules, honey-comb, etc.) which does come within
the range of the microscope.

But when we speak of the cells as the elementary organisms, or
structural units, or “ultimate individualities,” we must bear in mind a
certain restriction of the phrases. I mean, that the cells are not, as
is often supposed, the very lowest stage of organic individuality.
There are yet more elementary organisms to which I must refer
occasionally. These are what we call the “cytodes” (_cytos_ = cell),
certain living, independent beings, consisting only of a particle of _
plasson_—an albuminoid substance, which is not yet differentiated into
caryoplasm and cytoplasm, but combines the properties of both. Those
remarkable beings called the _ monera_—especially the chromacea and
bacteria—are specimens of these simple cytodes. (Compare Chapter XIX.)
To be quite accurate, then, we must say: the elementary organism, or
the ultimate individual, is found in two different stages. The first
and lower stage is the cytode, which consists merely of a particle of
plasson, or quite simple plasm. The second and higher stage is the
cell, which is already divided or differentiated into nuclear matter
and cellular matter. We comprise both kinds—the cytodes and the
cells—under the name of _plastids_ (“formative particles”), because
they are the real builders of the organism. However, these cytodes are
not found, as a rule, in the higher animals and plants; here we have
only real cells with a nucleus. Hence, in these tissue-forming
organisms (both plant and animal) the organic unit always consists of
two chemically and anatomically different parts—the outer cell-body and
the inner nucleus.

In order to convince oneself that this cell is really an independent
organism, we have only to observe the development and vital phenomena
of one of them. We see then that it performs all the essential
functions of life—both vegetal and animal—which we find in the entire
organism. Each of these tiny beings grows and nourishes itself
independently. It takes its food from the surrounding fluid; sometimes,
even, the naked cells take in solid particles at certain points of
their surface—in other words, “eat” them—without needing any special
mouth and stomach for the purpose (cf. Fig. 19).

Further, each cell is able to reproduce itself. This multiplication, in
most cases, takes the form of a simple cleavage, sometimes direct,
sometimes indirect; the simple direct (or “amitotic”) division is less
common, and is found, for instance, in the blood cells (Fig. 10). In
these the nucleus first divides into two equal parts by constriction.
The indirect (or “mitotic”)

cleavage is much more frequent; in this the caryoplasm of the nucleus
and the cytoplasm of the cell-body act upon each other in a peculiar
way, with a partial dissolution (_caryolysis_), the formation of knots
and loops (_mitosis_), and a movement of the halved plasma-particles
towards two mutually repulsive poles of attraction (_caryokinesis,_
Fig. 11.)


Fig.9 A large branching nerve-cell Fig. 9—A large branching nerve-cell,
or “soul-cell”, from the brain of an electric fish (_Torpedo_). In the
middle of the cell is the large transparent round _nucleus,_ one
_nucleolus,_ and, within the latter again, a _nucleolinus._ The
protoplasm of the cell is split into innumerable fine threads (or
fibrils), which are embedded in intercellular matter, and are prolonged
into the branching processes of the cell (_b_). One branch (_a_) passes
into a nerve-fibre. (From _Max Schultze._)


Fig.10 Blood-cells, multiplying by direct division Fig. 10—Blood-cells,
multiplying by direct division, from the blood of the embryo of a stag.
Originally, each blood-cell has a nucleus and is round (_a_). When it
is going to multiply, the nucleus divides into two (_b, c, d_). Then
the protoplasmic body is constricted between the two nuclei, and these
move away from each other (_e_). Finally, the constriction is complete,
and the cell splits into two daughter-cells (_f_). (From _Frey._)


The intricate physiological processes which accompany this “mitosis”
have been very closely studied of late years. The inquiry has led to
the detection of certain laws of evolution which are of extreme
importance in connection with heredity. As a rule, two very different
parts of the nucleus play an important part in these changes. They are:
the _chromatin,_ or coloured nuclear substance, which has a peculiar
property of tingeing itself deeply with certain colouring matters
(carmine, hæmatoxylin, etc.), and the _achromin_ (or _linin,_ or _
achromatin_), a colourless nuclear substance that lacks this property.
The latter generally forms in the dividing cell a sort of spindle, at
the poles of which there is a very small particle, also colourless,
called the “central body” (_centrosoma_). This acts as the centre or
focus in a “sphere of attraction” for the granules of protoplasm in the
surrounding cell-body, and assumes a star-like appearance (the
cell-star, or _monaster_). The two central bodies, standing opposed to
each other at the poles of the nuclear spindle, form “the double-star”
(or _amphiaster,_ Fig. 11, B C). The chromatin often forms a long,
irregularly-wound thread—“the coil” (_spirema,_ Fig. A). At the
commencement of the cleavage it gathers at the equator of the cell,
between the stellar poles, and forms a crown of U-shaped loops
(generally four or eight, or some other definite number). The loops
split lengthwise into two halves (B), and these back away from each
other towards the poles of the spindle (C). Here each group forms a
crown once more, and this, with the corresponding half of the divided
spindle, forms a fresh nucleus (D). Then the protoplasm of the
cell-body begins to contract in the middle, and gather about the new
daughter-nuclei, and at last the two daughter-cells become independent
beings.

Between this common mitosis, or _indirect_ cell-division—which is the
normal cleavage-process in most cells of the higher animals and
plants—and the simple _ direct_ division (Fig. 10) we find every grade
of segmentation; in some circumstances even one kind of division may be
converted into another.

The plastid is also endowed with the functions of movement and
sensation. The single cell can move and creep about, when it has space
for free movement and is not prevented by a hard envelope; it then
thrusts out at its surface processes like fingers, and quickly
withdraws them again, and thus changes its shape (Fig. 12). Finally,
the young cell is sensitive, or more or less responsive to stimuli; it
makes certain movements on the application of chemical and mechanical
irritation. Hence we can ascribe to the individual cell all the chief
functions which we comprehend under the general heading of
“life”—sensation, movement, nutrition, and reproduction. All these
properties of the multicellular and highly developed animal are also
found in the single animal-cell, at least in its younger stages. There
is no longer any doubt about this, and so we may regard it as a solid
and important base of our physiological conception of the elementary
organism.

Without going any further here into these very interesting phenomena of
the life of the cell, we will pass on to consider the application of
the cell theory to the ovum. Here comparative research yields the
important result that _every ovum is at first a simple cell._ I say
this is very important, because our whole science of embryology now
resolves itself into the problem: “How does the multicellular
organism arise from the unicellular?” Every organic individual is at
first a simple cell, and as such an elementary organism, or a unit of
individuality. This cell produces a cluster of cells by segmentation,
and from these develops the multicellular organism, or individual of
higher rank.


Fig. 11 Indirect or mitotic cell-division. A. Mother-cell (Knot,
spirema) 1. Nuclear threads (chromosomata) (coloured nuclear matter,
chromatin) 2. Nuclear membrane 3. Nuclear sap 4. Cytosoma 5. Protoplasm
of the cell-body

 B. Mother-star, the loops beginning to split lengthways (nuclear
 membrane gone) 1. Star-like appearance in cytoplasm 2. Centrosoma
 (sphere of attraction) 3. Nuclear spindle (achromin, colourless
 matter) 4. Nuclear loops (chromatin, coloured matter)

 C. The two daughter-stars, produced by the breaking of the loops of
 the mother-star (moving away) 1. Upper daughter-crown 2. Connecting
 threads of the two crowns (achromin) 3. Lower daughter-crown 4.
 Double-star (amphiaster)


D. The two daughter-cells, produced by the complete division of the two
nuclear halves (cytosomata still connected at the equator)
(Double-knot, Dispirema) 1. Upper daughter-nucleus 2. Equatorial
constriction of the cell-body 3. Lower daughter-nucleus.
Fig. 11—Indirect or mitotic cell-division (with caryolysis and
caryokinesis) from the skin of the larva of a salamander. (From
_Rabl._).


When we examine a little closer the original features of the ovum, we
notice the extremely significant fact that in its first stage the ovum
is just the same simple and indefinite structure in the case of man and
all the animals (Fig. 13). We are unable to detect any material
difference between them, either in outer shape or internal
constitution. Later, though the ova remain unicellular, they differ in
size and shape, enclose various kinds of yelk-particles, have different
envelopes, and so on. But when we examine them at their birth, in the
ovary of the female animal, we find them to be always of the same form
in the first stages of their life. In the beginning each ovum is a very
simple, roundish, naked, mobile cell, without a membrane; it consists
merely of a particle of cytoplasm enclosing a nucleus (Fig. 13).
Special names have been given to these parts of the ovum; the cell-body
is called the _yelk_ (_vitellus_), and the cell-nucleus the _germinal
vesicle._ As a rule, the
nucleus of the ovum is soft, and looks like a small pimple or vesicle.
Inside it, as in many other cells, there is a nuclear skeleton or frame
and a third, hard nuclear body (the _ nucleolus_). In the ovum this is
called the _germinal spot._ Finally, we find in many ova (but not in
all) a still further point within the germinal spot, a “nucleolin,”
which goes by the name of the germinal point. The latter parts
(germinal spot and germinal point) have, apparently, a minor
importance, in comparison with the other two (the yelk and germinal
vesicle). In the yelk we must distinguish the active _formative yelk_
(or protoplasm = first plasm) from the passive _ nutritive yelk_ (or
deutoplasm = second plasm).


Fig.12 Mobile cells from the inflamed eye of a frog. Fig. 12—Mobile
cells from the inflamed eye of a frog (from the watery fluid of the
eye, the _humor aqueus_). The naked cells creep freely about, by (like
the amœba or rhizopods) protruding fine processes from the uncovered
protoplasmic body. These bodies vary continually in number, shape, and
size. The nucleus of these amœboid lymph-cells (“travelling cells,” or
planocytes) is invisible, because concealed by the numbers of fine
granules which are scattered in the protoplasm. (From _Frey._)


In many of the lower animals (such as sponges, polyps, and medusæ) the
naked ova retain their original simple appearance until impregnation.
But in most animals they at once begin to change; the change consists
partly in the formation of connections with the yelk, which serve to
nourish the ovum, and partly of external membranes for their protection
(the ovolemma, or prochorion). A membrane of this sort is formed in all
the mammals in the course of the embryonic process. The little globule
is surrounded by a thick capsule of glass-like transparency, the _ zona
pellucida,_ or _ovolemma pellucidum_ (Fig. 14). When we examine it
closely under the microscope, we see very fine radial streaks in it,
piercing the _ zona,_ which are really very narrow canals. The human
ovum, whether fertilised or not, cannot be distinguished from that of
most of the other mammals. It is nearly the same everywhere in form,
size, and composition. When it is fully formed, it has a diameter of
(on an average) about 1/120 of an inch. When the mammal ovum has been
carefully isolated, and held against the light on a glass-plate, it may
be seen as a fine point even with the naked eye. The ova of most of the
higher mammals are about the same size. The diameter of the ovum is
almost always between 1/250 to 1/125 inch. It has always the same
globular shape; the same characteristic membrane; the same transparent
germinal vesicle with its dark germinal spot. Even when we use the most
powerful microscope with its highest power, we can detect no material
difference between the ova of man, the ape, the dog, and so on. I do
not mean to say that there are no differences between the ova of these
different mammals. On the contrary, we are bound to assume that there
are such, at least as regards chemical composition. Even the ova of
different men must differ from each other; otherwise we should not have
a different individual from each ovum. It is true that our crude and
imperfect apparatus cannot detect these subtle individual differences,
which are probably in the molecular structure. However, such a striking
resemblance of their ova in form, so great as to seem to be a complete
similarity, is a strong proof of the common parentage of man and the
other mammals. From the common germ-form we infer a common stem-form.
On the other hand, there are striking peculiarities by which we can
easily distinguish the fertilised ovum of the mammal from the
fertilised ovum of the birds, amphibia, fishes, and other vertebrates
(see the close of Chap. XXIX).

The fertilised bird-ovum (Fig. 15) is notably different. It is true
that in its earliest stage (Fig. 13 E) this ovum also is very like that
of the mammal (Fig. 13 F). But afterwards, while still within the
oviduct, it takes up a quantity of nourishment and works this into the
familiar large yellow yelk. When we examine a very young ovum in the
hen’s oviduct, we
find it to be a simple, small, naked, amœboid cell, just like the young
ova of other animals (Fig. 13). But it then grows to the size we are
familiar with in the round yelk of the egg. The nucleus of the ovum, or
the germinal vesicle, is thus pressed right to the surface of the
globular ovum, and is embedded there in a small quantity of transparent
matter, the so-called white yelk. This forms a round white spot, which
is known as the “tread” (_cicatricula_) (Fig. 15 _b_). From the tread a
thin column of the white yelk penetrates through the yellow yelk to the
centre of the globular cell, where it swells into a small, central
globule (wrongly called the yelk-cavity, or _latebra,_ Fig. 15 _d′_).
The yellow yelk-matter which surrounds this white yelk has the
appearance in the egg (when boiled hard) of concentric layers (_c_).
The yellow yelk is also enclosed in a delicate structureless membrane
(the _membrana vitellina, a_).


Fig.13 Ova of various animals, executing amœboid movements. Fig. 13—Ova
of various animals, executing amœboid movements, magnified. All the ova
are naked cells of varying shape. In the dark fine-grained protoplasm
(yelk) is a large vesicular nucleus (the germinal vesicle), and in this
is seen a nuclear body (the germinal spot), in which again we often see
a germinal point. Figs. _A1–A4_ represent the ovum of a sponge
(_Leuculmis echinus_) in four successive movements. _B1–B8_ are the
ovum of a parasitic crab (_Chondracanthus cornutus_), in eight
successive movements. (From _Edward von Beneden._) _C1–C5_ show the
ovum of the cat in various stages of movement (from _ Pflüger_); Fig.
_D_ the ovum of a trout; _E_ the ovum of a chicken; _F_ a human ovum.


As the large yellow ovum of the bird
attains a diameter of several inches in the bigger birds, and encloses
round yelk-particles, there was formerly a reluctance to consider it as
a simple cell. This was a mistake. Every animal that has only one
cell-nucleus, every amœba, every gregarina, every infusorium, is
unicellular, and remains unicellular whatever variety of matter it
feeds on. So the ovum remains a simple cell, however much yellow yelk
it afterwards accumulates within its protoplasm. It is, of course,
different, with the bird’s egg when it has been fertilised. The ovum
then consists of as many cells as there are nuclei in the tread. Hence,
in the fertilised egg which we eat daily, the yellow yelk is already a
multicellular body. Its tread is composed of several cells, and is now
commonly called the _germinal disc._ We shall return to this
_discogastrula_ in Chap. IX.


Fig.14 The human ovum. Fig. 14—The human ovum, taken from the female
ovary, magnified. The whole ovum is a simple round cell. The chief part
of the globular mass is formed by the nuclear yelk (_deutoplasm_),
which is evenly distributed in the active protoplasm, and consists of
numbers of fine yelk-granules. In the upper part of the yelk is the
transparent round germinal vesicle, which corresponds to the _
nucleus._ This encloses a darker granule, the germinal spot, which
shows a _nucleolus._ The globular yelk is surrounded by the thick
transparent germinal membrane (_ovolemma,_ or _ zona pellucida_). This
is traversed by numbers of lines as fine as hairs, which are directed
radially towards the centre of the ovum. These are called the
pore-canals; it is through these that the moving spermatozoa penetrate
into the yelk at impregnation.


When the mature bird-ovum has left the ovary and been fertilised in the
oviduct, it covers itself with various membranes which are secreted
from the wall of the oviduct. First, the large clear albuminous layer
is deposited around the yellow yelk; afterwards, the hard external
shell, with a fine inner skin. All these gradually forming envelopes
and processes are of no importance in the formation of the embryo; they
serve merely for the protection of the original simple ovum. We
sometimes find extraordinarily large eggs with strong envelopes in the
case of other animals, such as fishes of the shark type. Here, also,
the ovum is originally of the same character as it is in the mammal; it
is a perfectly simple and naked cell. But, as in the case of the bird,
a considerable quantity of nutritive yelk is accumulated inside the
original yelk as food for the developing embryo; and various coverings
are formed round the egg. The ovum of many other animals has the same
internal and external features. They have, however, only a
physiological, not a morphological, importance; they have no direct
influence on the formation of the fœtus. They are partly consumed as
food by the embryo, and partly serve as protective envelopes. Hence we
may leave them out of consideration altogether here, and restrict
ourselves to material points—_to the substantial identity of the
original ovum in man and the rest of the animals_ (Fig. 13).

Now, let us for the first time make use of our biogenetic law; and
directly apply this fundamental law of evolution to the human ovum. We
reach a very simple, but very important, conclusion. _ From_
_the fact that the human ovum and that of all other animals consists of
a single cell, it follows immediately, according to the biogenetic law,
that all the animals, including man, descend from a unicellular
organism._ If our biogenetic law is true, if the embryonic development
is a summary or condensed recapitulation of the stem-history—and there
can be no doubt about it—we are bound to conclude, from the fact that
all the ova are at first simple cells, that all the multicellular
organisms originally sprang from a unicellular being. And as the
original ovum in man and all the other animals has the same simple and
indefinite appearance, we may assume with some probability that this
unicellular stem-form was the common ancestor of the whole animal
world, including man. However, this last hypothesis does not seem to me
as inevitable and as absolutely certain as our first conclusion.


Fig.15 A fertilised ovum from the oviduct of a hen Fig. 15—A fertilised
ovum from the oviduct of a hen. The yellow yelk (_c_) consists of
several concentric layers (_d_), and is enclosed in a thin
yelk-membrane (_a_). The nucleus or germinal vesicle is seen above in
the cicatrix or “tread” (_b_). From that point the white yelk
penetrates to the central yelk-cavity (_d′_). The two kinds of yelk do
not differ very much.

This inference from the unicellular embryonic form to the unicellular
ancestor is so simple, but so important, that we cannot sufficiently
emphasise it. We must, therefore, turn next to the question whether
there are to-day any unicellular organisms, from the features of which
we may draw some approximate conclusion as to the unicellular ancestors
of the multicellular organisms. The answer is: Most certainly there
are. There are assuredly still unicellular organisms which are, in
their whole nature, really nothing more than permanent ova. There are
independent unicellular organisms of the simplest character which
develop no further, but reproduce themselves as such, without any
further growth. We know to-day of a great number of these little
beings, such as the gregarinæ, flagellata, acineta, infusoria, etc.
However, there is one of them that has an especial interest for us,
because it at once suggests itself when we raise our question, and it
must be regarded as the unicellular being that approaches nearest to
the real ancestral form. This organism is the _Amœba._


Fig.16 A creeping amœba. Fig. 16—A creeping amœba (highly magnified).
The whole organism is a simple naked cell, and moves about by means of
the changing arms which it thrusts out of and withdraws into its
protoplasmic body. Inside it is the roundish nucleus with its
nucleolus.

For a long time now we have comprised under the general name of amœbæ a
number of microscopic unicellular organisms, which are very widely
distributed, especially in fresh-water, but also in the ocean; in fact,
they have lately been discovered in damp soil. There are also parasitic
amœbæ which live inside other animals. When we place one of these amœbæ
in a drop of water under the microscope and examine it with a high
power, it generally appears as a roundish particle of a very irregular
and varying shape (Figs. 16 and 17). In its soft, slimy, semi-fluid
substance, which consists of protoplasm, we see only the solid globular
particle it contains, the nucleus. This unicellular body moves about
continually, creeping in every direction on the glass on which we are
examining it. The movement is effected by the shapeless body thrusting
out finger-like processes at various parts of its surface; and these
are slowly but continually changing, and drawing the rest of the body
after them. After a time, perhaps, the action changes. The amœba
suddenly stands still, withdraws its projections, and assumes a
globular shape. In a little while, however, the round body begins to
expand again, thrusts out arms in another
direction, and moves on once more. These changeable processes are
called “false feet,” or pseudopodia, because they act physiologically
as feet, yet are not special organs in the anatomic sense. They
disappear as quickly as they come, and are nothing more than temporary
projections of the semi-fluid and structureless body.


Fig.17 Division of a unicellular amœba. Fig. 17—Division of a
unicellular amœba (_Amœba polypodia_) in six stages. (From _F. E.
Schultze._) the dark spot is the nucleus, the lighter spot a
contractile vacuole in the protoplasm. The latter reforms in one of the
daughter-cells.)


If you touch one of these creeping amœbæ with a needle, or put a drop
of acid in the water, the whole body at once contracts in consequence
of this mechanical or physical stimulus. As a rule, the body then
resumes its globular shape. In certain circumstances—for instance, if
the impurity of the water lasts some time—the amœba begins to develop a
covering. It exudes a membrane or capsule, which immediately hardens,
and assumes the appearance of a round cell with a protective membrane.
The amœba either takes its food directly by imbibition of matter
floating in the water, or by pressing into its protoplasmic body solid
particles with which it comes in contact. The latter process may be
observed at any moment by forcing it to eat. If finely ground colouring
matter, such as carmine or indigo, is put into the water, you can see
the body of the amœba pressing these coloured particles into itself,
the substance of the cell closing round them. The amœba can take in
food in this way at any point on its surface, without having any
special organs for intussusception and digestion, or a real mouth or
gut.

The amœba grows by thus taking in food and dissolving the particles
eaten in its protoplasm. When it reaches a certain size by this
continual feeding, it begins to reproduce. This is done by the simple
process of cleavage (Fig. 17). First, the nucleus divides into two
parts. Then the protoplasm is separated between the two new nuclei, and
the whole cell splits into two daughter-cells, the protoplasm gathering
about each of the nuclei. The thin bridge of protoplasm which at first
connects the daughter-cells soon breaks. Here we have the simple form
of direct cleavage of the nuclei. Without mitosis, or formation of
threads, the homogeneous nucleus divides into two halves. These move
away from each other, and become centres of attraction for the
enveloping matter, the protoplasm. The same direct cleavage of the
nuclei is also witnessed in the reproduction of many other protists,
while other unicellular organisms show the indirect division of the
cell.

Hence, although the amœba is nothing but a simple cell, it is evidently
able to accomplish all the functions of the multicellular organism. It
moves, feels, nourishes itself, and reproduces. Some kinds of these
amœbæ can be seen with the naked eye, but most of them are
microscopically small. It is for the following reasons that we regard
the amœbæ as the unicellular organisms which have
special phylogenetic (or evolutionary) relations to the ovum. In many
of the lower animals the ovum retains its original naked form until
fertilisation, develops no membranes, and is then often
indistinguishable from the ordinary amœba. Like the amœbæ, these naked
ova may thrust out processes, and move about as travelling cells. In
the sponges these mobile ova move about freely in the maternal body
like independent amœbæ (Fig. 17). They had been observed by earlier
scientists, but described as foreign bodies—namely, parasitic amœbæ,
living parasitically on the body of the sponge. Later, however, it was
discovered that they were not parasites, but the ova of the sponge. We
also find this remarkable phenomenon among other animals, such as the
graceful, bell-shaped zoophytes, which we call polyps and medusæ. Their
ova remain naked cells, which thrust out amœboid projections, nourish
themselves, and move about. When they have been fertilised, the
multicellular organism is formed from them by repeated segmentation.

It is, therefore, no audacious hypothesis, but a perfectly sound
conclusion, to regard the amœba as the particular unicellular organism
which offers us an approximate illustration of the ancient common
unicellular ancestor of all the metazoa, or multicellular animals. The
simple naked amœba has a less definite and more original character than
any other cell. Moreover, there is the fact that recent research has
discovered such amœba-like cells everywhere in the mature body of the
multicellular animals. They are found, for instance, in the human
blood, side by side with the red corpuscles, as colourless blood-cells;
and it is the same with all the vertebrates. They are also found in
many of the invertebrates—for instance, in the blood of the snail. I
showed, in 1859, that these colourless blood-cells can, like the
independent amœbæ, take up solid particles, or “eat” (whence they are
called _phagocytes_ = “eating-cells,” Fig. 19). Lately, it has been
discovered that many different cells may, if they have room enough,
execute the same movements, creeping about and eating. They behave just
like amœbæ (Fig. 12). It has also been shown that these
“travelling-cells,” or _planocytes,_ play an important part in man’s
physiology and pathology (as means of transport for food, infectious
matter, bacteria, etc.).

The power of the naked cell to execute these characteristic amœba-like
movements comes from the contractility (or automatic mobility) of its
protoplasm. This seems to be a universal property of young cells. When
they are not enclosed by a firm membrane, or confined in a “cellular
prison,” they can always accomplish these amœboid movements. This is
true of the naked ova as well as of any other naked cells, of the
“travelling-cells,” of various kinds in connective tissue, lymph-cells,
mucus-cells, etc.


Fig.18. Ovum of a sponge. Fig. 18—Ovum of a sponge (_Olynthus_). The
ovum creeps about in a body of the sponge by thrusting out
ever-changing processes. It is indistinguishable from the common
amœba.)

We have now, by our study of the ovum and the comparison of it with the
amœba, provided a perfectly sound and most valuable foundation for both
the embryology and the evolution of man. We have learned that the human
ovum is a simple cell, that this ovum is not materially different from
that of other mammals, and that we may infer from it the existence of a
primitive unicellular ancestral form, with a substantial resemblance to
the amœba.

The statement that the earliest progenitors of the human race were
simple cells of this kind, and led an independent unicellular life like
the amœba, has not only been ridiculed as the dream of a natural
philosopher, but also been violently censured in theological journals
as “shameful and immoral.” But, as I observed in my essay _On the
Origin and Ancestral Tree of the Human Race_ in 1870, this offended
piety must equally protest against the “shameful and immoral” fact that
each human individual is developed from a simple ovum, and that this
human ovum is indistinguishable from those of the other mammals, and in
its earliest stage is like a naked amœba.
We can show this to be a fact any day with the microscope, and it is
little use to close one’s eyes to “immoral” facts of this kind. It is
as indisputable as the momentous conclusions we draw from it and as the
vertebrate character of man (see Chap. XI).


Fig.19 Blood-cells that eat, or phagocytes, from a naked sea-snail.
Fig. 19—Blood-cells that eat, or phagocytes, from a naked sea-snail
(_Thetis_), greatly magnified. I was the first to observe in the
blood-cells of this snail the important fact that “the blood-cells of
the invertebrates are unprotected pieces of plasm, and take in food, by
means of their peculiar movements, like the amœbæ.” I had (in Naples,
on May 10th, 1859) injected into the blood-vessels of one of these
snails an infusion of water and ground indigo, and was greatly
astonished to find the blood-cells themselves more or less filled with
the particles of indigo after a few hours. After repeated injections I
succeeded in “observing the very entrance of the coloured particles in
the blood-cells, which took place just in the same way as with the
amœba.” I have given further particulars about this in my _Monograph on
the Radiolaria._


We now see very clearly how extremely important the cell theory has
been for our whole conception of organic nature. “Man’s place in
nature” is settled beyond question by it. Apart from the cell theory,
man is an insoluble enigma to us. Hence philosophers, and especially
physiologists, should be thoroughly conversant with it. The soul of man
can only be really understood in the light of the cell-soul, and we
have the simplest form of this in the amœba. Only those who are
acquainted with the simple psychic functions of the unicellular
organisms and their gradual evolution in the series of lower animals
can understand how the elaborate mind of the higher vertebrates, and
especially of man, was gradually evolved from them. The academic
psychologists who lack this zoological equipment are unable to do so.

This naturalistic and realistic conception is a stumbling-block to our
modern idealistic metaphysicians and their theological colleagues.
Fenced about with their transcendental and dualistic prejudices, they
attack not only the monistic system we establish on our scientific
knowledge, but even the plainest facts which go to form its foundation.
An instructive instance of this was seen a few years ago, in the
academic discourse delivered by a distinguished theologian, Willibald
Beyschlag, at Halle, January 12th, 1900, on the occasion of the
centenary festival. The theologian protested violently against the
“materialistic dustmen of the scientific world who offer our people the
diploma of a descent from the ape, and would prove to them that the
genius of a Shakespeare or a Goethe is merely a distillation from a
drop of primitive mucus.” Another well-known theologian protested
against “the horrible idea that the greatest of men, Luther and Christ,
were descended from a mere globule of protoplasm.” Nevertheless, not a
single informed and impartial scientist doubts the fact that these
greatest men were, like all other men—and all other
vertebrates—developed from an impregnated ovum, and that this simple
nucleated globule of protoplasm has the same chemical constitution in
all the mammals.



Chapter VII.
CONCEPTION


The recognition of the fact that every man begins his individual
existence as a simple cell is the solid foundation of all research into
the genesis of man. From this fact we are forced, in virtue of our
biogenetic law, to draw the weighty phylogenetic conclusion that the
earliest ancestors of the human race were also unicellular organisms;
and among these protozoa we may single out the vague form of the amœba
as particularly important (cf. Chapter VI). That these unicellular
ancestral forms did once exist follows directly from the phenomena
which we perceive every day in the fertilised ovum. The development of
the multicellular organism from the ovum, and the formation of the
germinal layers and the tissues, follow the same laws in man and all
the higher animals. It will, therefore, be our next task to consider
more closely the impregnated ovum and the process of conception which
produces it.

The process of impregnation or sexual conception is one of those
phenomena that people love to conceal behind the mystic veil of
supernatural power. We shall soon see, however, that it is a purely
mechanical process, and can be reduced to familiar physiological
functions. Moreover, this process of conception is of the same type,
and is effected by the same organs, in man as in all the other mammals.
The pairing of the male and female has in both cases for its main
purpose the introduction of the ripe matter of the male seed or sperm
into the female body, in the sexual canals of which it encounters the
ovum. Conception then ensues by the blending of the two.

We must observe, first, that this important process is by no means so
widely distributed in the animal and plant world as is commonly
supposed. There is a very large number of lower organisms which
propagate unsexually, or by monogamy; these are especially the sexless
monera (chromacea, bacteria, etc.) but also many other protists, such
as the amœbæ, foraminifera, radiolaria, myxomycetæ, etc. In these the
multiplication of individuals takes place by unsexual reproduction,
which takes the form of cleavage, budding, or spore-formation. The
copulation of two coalescing cells, which in these cases often precedes
the reproduction, cannot be regarded as a sexual act unless the two
copulating plastids differ in size or structure. On the other hand,
sexual reproduction is the general rule with all the higher organisms,
both animal and plant; very rarely do we find asexual reproduction
among them. There are, in particular, no cases of parthenogenesis
(virginal conception) among the vertebrates.

Sexual reproduction offers an infinite variety of interesting forms in
the different classes of animals and plants, especially as regards the
mode of conception, and the conveyance of the spermatozoon to the ovum.
These features are of great importance not only as regards conception
itself, but for the development of the organic form, and especially for
the differentiation of the sexes. There is a particularly curious
correlation of plants and animals in this respect. The splendid studies
of Charles Darwin and Hermann Müller on the fertilisation of flowers by
insects have given us very interesting particulars of this.[14] This
reciprocal service has given rise to a most intricate sexual apparatus.
Equally elaborate structures have been developed in man and the higher
animals, serving partly for the isolation of the sexual products on
each side, partly for bringing them together in conception. But,
however interesting these phenomena are in themselves, we cannot go
into them here, as they have only a minor importance—if any at all—in
the real process of conception. We must, however, try to get a very
clear idea of this process and the meaning of sexual reproduction.

 [14] See Darwin’s work, _On the Various Contrivances by which Orchids
 are Fertilised_ (1862).


In every act of conception we have, as I said, to consider two
different kinds of cells—a female and a male cell. The female cell of
the animal organism is always called the ovum (or _ovulum,_ egg, or
egg-cell); the male cells are known as the sperm or seed-cells, or the
spermatozoa (also spermium and zoospermium). The ripe ovum is, on the
whole, one of the largest cells we know. It attains colossal dimensions
when it absorbs great quantities of nutritive yelk, as is the case with
birds and reptiles and many of the fishes. In the great majority of the
animals the ripe ovum is rich in yelk and much larger than the other
cells. On the other hand, the next cell which we have to consider in
the process of conception, the male sperm-cell or spermatozoon, is one
of the smallest cells in the animal body. Conception usually consists
in the bringing into contact with the ovum of a slimy fluid secreted by
the male, and this may take place either inside or out of the female
body. This fluid is called sperm, or the male seed. Sperm, like saliva
or blood, is not a simple fluid, but a thick agglomeration of
innumerable cells, swimming about in a comparatively small quantity of
fluid. It is not the fluid, but the independent male cells that swim in
it, that cause conception.


Fig.20 Spermia or spermatozoa of various mammals. Fig. 20—Spermia or
spermatozoa of various mammals. The pear-shaped flattened nucleus is
seen from the front in _I_ and sideways in _II. k_ is the nucleus, _m_
its middle part (protoplasm), _ s_ the mobile, serpent-like tail (or
whip); _M_ four human spermatozoa, _A_ four spermatozoa from the ape;
_K_ from the rabbit; _H_ from the mouse; _C_ from the dog; _ S_ from
the pig.


The spermatozoa of the great majority of animals have two
characteristic features. Firstly, they are extraordinarily small, being
usually the smallest cells in the body; and, secondly, they have, as a
rule, a peculiarly lively motion, which is known as spermatozoic
motion. The shape of the cell has a good deal to do with this motion.
In most of the animals, and also in many of the lower plants (but not
the higher) each of these spermatozoa has a very small, naked
cell-body, enclosing an elongated nucleus, and a long thread hanging
from it (Fig. 20). It was long before we could recognise that these
structures are simple cells. They were formerly held to be special
organisms, and were called “seed animals” (spermato-zoa, or
spermato-zoidia); they are now scientifically known as _spermia_ or
_spermidia,_ or as _spermatosomata_ (seed-bodies) or _spermatofila_
(seed threads). It took a good deal of comparative research to convince
us that each of these spermatozoa is really a simple cell. They have
the same shape as in many other vertebrates and most of the
invertebrates. However, in many of the lower animals they have quite a
different shape. Thus, for instance, in the craw fish they are large
round cells, without any movement, equipped with stiff outgrowths like
bristles (Fig. 21 _ f_ ). They have also a peculiar form in some of the
worms, such as the thread-worms (_filaria_); in this case they are
sometimes
amœboid and like very small ova (Fig. 21 _ c_ to _e_). But in most of
the lower animals (such as the sponges and polyps) they have the same
pine-cone shape as in man and the other animals (Fig. 21 _a, h_).


Fig.21 Spermatozoa or spermidia of various animals. Fig. 21—Spermatozoa
or spermidia of various animals. (From _Lang_). _a_ of a fish, _b_ of a
turbellaria worm (with two side-lashes), _c_ to _e_ of a nematode worm
(amœboid spermatozoa), _f_ from a craw fish (star-shaped), _g_ from the
salamander (with undulating membrane), _h_ of an annelid (_a_ and _h_
are the usual shape).


When the Dutch naturalist Leeuwenhoek discovered these thread-like
lively particles in 1677 in the male sperm, it was generally believed
that they were special, independent, tiny animalcules, like the
infusoria, and that the whole mature organism existed already, with all
its parts, but very small and packed together, in each spermatozoon
(see p.12). We now know that the mobile spermatozoa are nothing but
simple and real cells, of the kind that we call “ciliated” (equipped
with lashes, or _cilia_). In the previous illustrations we have
distinguished in the spermatozoon a head, trunk, and tail. The “head”
(Fig. 20 _k_) is merely the oval nucleus of the cell; the body or
middle-part (_m_) is an accumulation of cell-matter; and the tail (_s_)
is a thread-like prolongation of the same.

Moreover, we now know that these spermatozoa are not at all a peculiar
form of cell; precisely similar cells are found in various other parts
of the body. If they have many short threads projecting, they are
called _ciliated_; if only one long, whip-shaped process (or, more
rarely, two or four), _caudate_ (tailed) cells.

Very careful recent examination of the spermia, under a very high
microscopic power (Fig. 22 a, b), has detected some further details in
the finer structure of the ciliated cell, and these are common to man
and the anthropoid ape. The head (_k_) encloses the elliptic nucleus in
a thin envelope of cytoplasm; it is a little flattened on one side, and
thus looks rather pear-shaped from the front (_b_). In the central
piece (_m_) we can distinguish a short neck and a longer connective
piece (with central body). The tail consists of a long main section
(_h_) and a short, very fine tail (_e_).


Fig.22 A single human spermatozoon. Fig. 22—A single human spermatozoon
magnified; a shows it from the broader and b from the narrower side.
_k_ head (with nucleus), _m_ middle-stem, _h_ long-stem, and _e_ tail.
(From _ Retzius._)

The process of fertilisation by sexual conception consists, therefore,
essentially in the coalescence and fusing together of two different
cells. The lively spermatozoon travels towards the ovum by its
serpentine movements, and bores its way into the female cell (Fig. 23).
The nuclei of both sexual cells, attracted by a certain “affinity,”
approach each other and melt into one.

The fertilised cell is quite another thing from the unfertilised cell.
For if we must regard the spermia as real cells no less than the ova,
and the process of conception as a coalescence of the two, we must
consider the resultant cell as a quite new and independent organism. It
bears in the cell and nuclear matter of the penetrating spermatozoon a
part of the father’s body, and in the protoplasm and caryoplasm of the
ovum a part of the mother’s body. This is clear from the fact that the
child inherits many features from both parents. It inherits from the
father by means of the spermatozoon, and from the mother by means of
the ovum. The
actual blending of the two cells produces a third cell, which is the
germ of the child, or the new organism conceived. One may also say of
this sexual coalescence that the _ stem-cell is a simple
hermaphrodite_; it unites both sexual substances in itself.


Fig.23 The fertilisation of the ovum by the spermatozoon. Fig. 23—The
fertilisation of the ovum by the spermatozoon (of a mammal). One of the
many thread-like, lively spermidia pierces through a fine pore-canal
into the nuclear yelk. The nucleus of the ovum is invisible.

I think it necessary to emphasise the fundamental importance of this
simple, but often unappreciated, feature in order to have a correct and
clear idea of conception. With that end, I have given a special name to
the new cell from which the child develops, and which is generally
loosely called “the fertilised ovum,” or “the first segmentation
sphere.” I call it “the stem-cell” (_cytula_). The name “stem-cell”
seems to me the simplest and most suitable, because all the other cells
of the body are derived from it, and because it is, in the strictest
sense, the stem-father and stem-mother of all the countless generations
of cells of which the multicellular organism is to be composed. That
complicated molecular movement of the protoplasm which we call “life”
is, naturally, something quite different in this stem-cell from what we
find in the two parent-cells, from the coalescence of which it has
issued. _The life of the stem-cell or cytula is the product or
resultant of the paternal life-movement that is conveyed in the
spermatozoon and the maternal life-movement that is contributed by the
ovum._

The admirable work done by recent observers has shown that the
individual development, in man and the other animals, commences with
the formation of a simple “stem-cell” of this character, and that this
then passes, by repeated segmentation (or cleavage), into a cluster of
cells, known as “the segmentation sphere” or “segmentation cells.” The
process is most clearly observed in the ova of the echinoderms
(star-fishes, sea-urchins, etc.). The investigations of Oscar and
Richard Hertwig were chiefly directed to these. The main results may be
summed up as follows:—

Conception is preceded by certain preliminary changes, which are very
necessary—in fact, usually indispensable—for its occurrence. They are
comprised under the general heading of “Changes prior to impregnation.”
In these the original nucleus of the ovum, the germinal vesicle, is
lost. Part of it is extruded, and part dissolved in the cell contents;
only a very small part of it is left to form the basis of a fresh
nucleus, the _pronucleus femininus._ It is the latter alone that
combines in conception with the invading nucleus of the fertilising
spermatozoon (the _pronucleus masculinus_).

The impregnation of the ovum commences with a decay of the germinal
vesicle, or the original nucleus of the ovum (Fig. 8). We have seen
that this is in most unripe ova a large, transparent, round vesicle.
This germinal vesicle contains a viscous fluid (the _caryolymph_). The
firm nuclear frame (_caryobasis_) is formed of the enveloping membrane
and a mesh-work of nuclear threads running across the interior, which
is filled with the nuclear sap. In a knot of the network is contained
the dark, stiff, opaque nuclear corpuscle or nucleolus. When the
impregnation of the ovum sets in, the greater part of the germinal
vesicle is dissolved in the cell; the nuclear membrane and mesh-work
disappear; the nuclear sap is distributed in the protoplasm; a small
portion of the nuclear base is extruded; another small portion is left,
and is converted into the secondary nucleus, or the female pro-nucleus
(Fig. 24 _e k_).

The small portion of the nuclear base which is extruded from the
impregnated ovum is known as the “directive bodies” or “polar cells”;
there are many disputes as to their origin and significance, but we are
as yet imperfectly acquainted with them. As a rule, they are two small
round granules, of the same size and appearance as the remaining
pro-nucleus. They are detached cell-buds; their separation from the
large mother-cell takes
place in the same way as in ordinary “indirect cell-division.” Hence,
the polar cells are probably to be conceived as “abortive ova,” or
“rudimentary ova,” which proceed from a simple original ovum by
cleavage in the same way that several sperm-cells arise from one
“sperm-mother-cell,” in reproduction from sperm. The male sperm-cells
in the testicles must undergo similar changes in view of the coming
impregnation as the ova in the female ovary. In this maturing of the
sperm each of the original seed-cells divides by double segmentation
into four daughter-cells, each furnished with a fourth of the original
nuclear matter (the hereditary chromatin); and each of these four
descendant cells becomes a _ spermatozoon,_ ready for impregnation.
Thus is prevented the doubling of the chromatin in the coalescence of
the two nuclei at conception. As the two polar cells are extruded and
lost, and have no further part in the fertilisation of the ovum, we
need not discuss them any further. But we must give more attention to
the female pro-nucleus which alone remains after the extrusion of the
polar cells and the dissolving of the germinal vesicle (Fig. 23 _e k_).
This tiny round corpuscle of chromatin now acts as a centre of
attraction for the invading spermatozoon in the large ripe ovum, and
coalesces with its “head,” the male pro-nucleus. The product of this
blending, which is the most important part of the act of impregnation,
is the stem-nucleus, or the first segmentation nucleus
(_archicaryon_)—that is to say, the nucleus of the new-born embryonic
stem-cell or “first segmentation cell.” This stem-cell is the starting
point of the subsequent embryonic processes.

Hertwig has shown that the tiny transparent ova of the echinoderms are
the most convenient for following the details of this important process
of impregnation. We can, in this case, easily and successfully
accomplish artificial impregnation, and follow the formation of the
stem-cell step by step within the space of ten minutes. If we put ripe
ova of the star-fish or sea-urchin in a watch glass with sea-water and
add a drop of ripe sperm-fluid, we find each ovum impregnated within
five minutes. Thousands of the fine, mobile ciliated cells, which we
have described as “sperm-threads” (Fig. 20), make their way to the ova,
owing to a sort of chemical sensitive action which may be called
“smell.” But only one of these innumerable spermatozoa is
chosen—namely, the one that first reaches the ovum by the serpentine
motions of its tail, and touches the ovum with its head. At the spot
where the point of its head touches the surface of the ovum the
protoplasm of the latter is raised in the form of a small wart, the
“impregnation rise” (Fig. 25 _A_). The spermatozoon then bores its way
into this with its head, the tail outside wriggling about all the time
(Fig. 25 _B, C_). Presently the tail also disappears within the ovum.
At the same time the ovum secretes a thin external yelk-membrane (Fig.
25 _ C_), starting from the point of impregnation; and this prevents
any more spermatozoa from entering.


Fig.24 An impregnated echinoderm ovum. Fig. 24—An impregnated
echinoderm ovum, with small homogeneous nucleus (_e k_). (From
_Hertwig._)


Inside the impregnated ovum we now see a rapid series of most important
changes. The pear-shaped head of the sperm-cell, or the “head of the
spermatozoon,” grows larger and rounder, and is converted into the male
pro-nucleus (Fig. 26 _s k_). This has an attractive influence on the
fine granules or particles which are distributed in the protoplasm of
the ovum; they arrange themselves in lines in the figure of a star. But
the attraction or the “affinity” between the two nuclei is even
stronger. They move towards each other inside the yelk with increasing
speed, the male (Fig. 27 _s k_) going more quickly than the female
nucleus (_e k_). The tiny male nucleus takes with it the radiating
mantle which spreads like a star about it. At last the two sexual
nuclei touch (usually in the centre of the globular ovum), lie close
together, are flattened at the points of contact, and coalesce into a
common mass. The small central particle of
nuclein which is formed from this combination of the nuclei is the
stem-nucleus, or the first segmentation nucleus; the new-formed cell,
the product of the impregnation, is our stem-cell, or “first
segmentation sphere” (Fig. 2).


Fig. 25 Impregnation of the ovum of a star-fish. Fig. 25—Impregnation
of the ovum of a star-fish. (From _Hertwig._) Only a small part of the
surface of the ovum is shown. One of the numerous spermatozoa
approaches the “impregnation rise” (_A_), touches it (_B_), and then
penetrates into the protoplasm of the ovum (_C_).


Hence the one essential point in the process of sexual reproduction or
impregnation is the formation of a new cell, the stem-cell, by the
combination of two originally different cells, the female ovum and the
male spermatozoon. This process is of the highest importance, and
merits our closest attention; all that happens in the later development
of this first cell and in the life of the organism that comes of it is
determined from the first by the chemical and morphological composition
of the stem-cell, its nucleus and its body. We must, therefore, make a
very careful study of the rise and structure of the stem-cell.

The first question that arises is as to the two different active
elements, the nucleus and the protoplasm, in the actual coalescence. It
is obvious that the nucleus plays the more important part in this.
Hence Hertwig puts his theory of conception in the principle:
“Conception consists in the copulation of two cell-nuclei, which come
from a male and a female cell.” And as the phenomenon of heredity is
inseparably connected with the reproductive process, we may further
conclude that these two copulating nuclei “convey the characteristics
which are transmitted from parents to offspring.” In this sense I had
in 1866 (in the ninth chapter of the _General Morphology_) ascribed to
the reproductive nucleus the function of generation and _heredity,_ and
to the nutritive protoplasm the duties of nutrition and _adaptation._
As, moreover, there is a complete coalescence of the mutually attracted
nuclear substances in conception, and the new nucleus formed (the
stem-nucleus) is the real starting-point for the development of the
fresh organism, the further conclusion may be drawn that the male
nucleus conveys to the child the qualities of the father, and the
female nucleus the features of the mother. We must not forget, however,
that the protoplasmic bodies of the copulating cells also fuse together
in the act of impregnation; the cell-body of the invading spermatozoon
(the trunk and tail of the male ciliated cell) is dissolved in the yelk
of the female ovum. This coalescence is not so important as that of the
nuclei, but it must not be overlooked; and, though this process is not
so well known to us, we see clearly at least the formation of the
star-like figure (the radial arrangement of the particles in the
plasma) in it (Figs. 26–27).

The older theories of impregnation generally went astray in regarding
the large ovum as the sole base of the new organism, and only ascribed
to the spermatozoon the work of stimulating and originating its
development. The stimulus which it gave to the ovum was sometimes
thought to be purely chemical, at other times rather physical (on the
principle of transferred movement), or again a mystic and
transcendental process. This error was partly due to the imperfect
knowledge at that time of the facts of impregnation, and partly to the
striking
difference in the sizes of the two sexual cells. Most of the earlier
observers thought that the spermatozoon did not penetrate into the
ovum. And even when this had been demonstrated, the spermatozoon was
believed to disappear in the ovum without leaving a trace. However, the
splendid research made in the last three decades with the finer
technical methods of our time has completely exposed the error of this.
It has been shown that the tiny sperm-cell is _not subordinated to, but
coordinated with,_ the large ovum. The nuclei of the two cells, as the
vehicles of the hereditary features of the parents, are of equal
physiological importance. In some cases we have succeeded in proving
that the mass of the active nuclear substance which combines in the
copulation of the two sexual nuclei is originally the same for both.

These morphological facts are in perfect harmony with the familiar
physiological truth that the child inherits from both parents, and that
on the average they are equally distributed. I say “on the average,”
because it is well known that a child may have a greater likeness to
the father or to the mother; that goes without saying, as far as the
primary sexual characters (the sexual glands) are concerned. But it is
also possible that the determination of the latter—the weighty
determination whether the child is to be a boy or a girl—depends on a
slight qualitative or quantitative difference in the nuclein or the
coloured nuclear matter which comes from both parents in the act of
conception.


Figs. 26 and 27 Impregnation of the ovum of the sea-urchin. Figs. 26
and 27.—Impregnation of the ovum of the sea-urchin. (From _Hertwig._)
In Fig. 26 the little sperm-nucleus (_sk_) moves towards the larger
nucleus of the ovum (_ek_). In Fig. 27 they nearly touch, and are
surrounded by the radiating mantle of protoplasm.


The striking differences of the respective sexual cells in size and
shape, which occasioned the erroneous views of earlier scientists, are
easily explained on the principle of division of labour. The inert,
motionless ovum grows in size according to the quantity of provision it
stores up in the form of nutritive yelk for the development of the
germ. The active swimming sperm-cell is reduced in size in proportion
to its need to seek the ovum and bore its way into its yelk. These
differences are very conspicuous in the higher animals, but they are
much less in the lower animals. In those protists (unicellular plants
and animals) which have the first rudiments of sexual reproduction the
two copulating cells are at first quite equal. In these cases the act
of impregnation is nothing more than a sudden _growth,_ in which the
originally simple cell doubles its volume, and is thus prepared for
reproduction (cell-division). Afterwards slight differences are seen in
the size of the copulating cells; though the smaller ones still have
the same shape as the larger ones. It is only when the difference in
size is very pronounced that a notable difference in shape is found:
the sprightly sperm-cell changes more in shape and the ovum in size.

Quite in harmony with this new conception of the _equivalence of the
two gonads,_ or the equal physiological importance of the male and
female sex-cells and their equal share in the process of heredity, is
the important fact established by Hertwig (1875), that in normal
impregnation only one single spermatozoon
copulates with one ovum; the membrane which is raised on the surface of
the yelk immediately after one sperm-cell has penetrated (Fig. 25 _C_)
prevents any others from entering. All the rivals of the fortunate
penetrator are excluded, and die without. But if the ovum passes into a
morbid state, if it is made stiff by a lowering of its temperature or
stupefied with narcotics (chloroform, morphia, nicotine, etc.), two or
more spermatozoa may penetrate into its yelk-body. We then witness
_polyspermism._ The more Hertwig chloroformed the ovum, the more
spermatozoa were able to bore their way into its unconscious body.


Fig.28 Stem-cell of a rabbit. Fig. 28—Stem-cell of a rabbit, magnified.
In the centre of the granular protoplasm of the fertilised ovum (_d_)
is seen the little, bright stem-nucleus, _z_ is the ovolemma, with a
mucous membrane (_h_). _s_ are dead spermatozoa.

These remarkable facts of impregnation are also of the greatest
interest in psychology, especially as regards the theory of the
cell-soul, which I consider to be its chief foundation. The phenomena
we have described can only be understood and explained by ascribing a
certain lower degree of psychic activity to the sexual principles. They
_feel_ each other’s proximity, and are drawn together by a _sensitive_
impulse (probably related to smell); they _move_ towards each other,
and do not rest until they fuse together. Physiologists may say that it
is only a question of a peculiar physico-chemical phenomenon, and not a
psychic action; but the two cannot be separated. Even the psychic
functions, in the strict sense of the word, are only complex physical
processes, or “psycho-physical” phenomena, which are determined in all
cases exclusively by the chemical composition of their material
substratum.

The monistic view of the matter becomes clear enough when we remember
the radical importance of impregnation as regards heredity. It is well
known that not only the most delicate bodily structures, but also the
subtlest traits of mind, are transmitted from the parents to the
children. In this the chromatic matter of the male nucleus is just as
important a vehicle as the large caryoplasmic substance of the female
nucleus; the one transmits the mental features of the father, and the
other those of the mother. The blending of the two parental nuclei
determines the individual psychic character of the child.

But there is another important psychological question—the most
important of all—that has been definitely answered by the recent
discoveries in connection with conception. This is the question of the
immortality of the soul. No fact throws more light on it and refutes it
more convincingly than the elementary process of conception that we
have described. For this copulation of the two sexual nuclei (Figs. 26
and 27) indicates the precise moment at which the individual begins to
exist. All the bodily and mental features of the new-born child are the
sum-total of the hereditary qualities which it has received in
reproduction from parents and ancestors. All that man acquires
afterwards in life by the exercise of his organs, the influence of his
environment, and education—in a word, by adaptation—cannot obliterate
that general outline of his being which he inherited from his parents.
But this hereditary disposition, the essence of every human soul, is
not “eternal,” but “temporal”; it comes into being only at the moment
when the sperm-nucleus of the father and the nucleus of the maternal
ovum meet and fuse together. It is clearly irrational to assume an
“eternal life without end” for an individual phenomenon, the
commencement of which we can indicate to a moment by direct visual
observation.

The great importance of the process of impregnation in answering such
questions is quite clear. It is true that conception has never been
studied microscopically in all its details in the human
case—notwithstanding its occurrence at every moment—for reasons that
are
obvious enough. However, the two cells which need consideration, the
female ovum and the male spermatozoon, proceed in the case of man in
just the same way as in all the other mammals; the human fœtus or
embryo which results from copulation has the same form as with the
other animals. Hence, no scientist who is acquainted with the facts
doubts that the processes of impregnation are just the same in man as
in the other animals.

The stem-cell which is produced, and with which every man begins his
career, cannot be distinguished in appearance from those of other
mammals, such as the rabbit (Fig. 28). In the case of man, also, this
stem-cell differs materially from the original ovum, both in regard to
form (morphologically), in regard to material composition (chemically),
and in regard to vital properties (physiologically). It comes partly
from the father and partly from the mother. Hence it is not surprising
that the child who is developed from it inherits from both parents. The
vital movements of each of these cells form a sum of mechanical
processes which in the last analysis are due to movements of the
smallest vital parts, or the molecules, of the living substance. If we
agree to call this active substance _plasson,_ and its molecules _
plastidules,_ we may say that the individual physiological character of
each of these cells is due to its molecular plastidule-movement.
_Hence, the plastidule-movement of the cytula is the resultant of the
combined plastidule-movements of the female ovum and the male
sperm-cell._[15]

 [15] The plasson of the stem-cell or cytula may, from the anatomical
 point of view, be regarded as homogeneous and structureless, like that
 of the monera. This is not inconsistent with our hypothetical
 ascription to the plastidules (or molecules of the plasson) of a
 complex molecular structure. The complexity of this is the greater in
 proportion to the complexity of the organism that is developed from it
 and the length of the chain of its ancestry, or to the multitude of
 antecedent processes of heredity and adaptation.



Chapter VIII.
THE GASTRÆA THEORY


There is a substantial agreement throughout the animal world in the
first changes which follow the impregnation of the ovum and the
formation of the stem-cell; they begin in all cases with the
segmentation of the ovum and the formation of the germinal layers. The
only exception is found in the protozoa, the very lowest and simplest
forms of animal life; these remain unicellular throughout life. To this
group belong the amœbae, gregarinæ, rhizopods, infusoria, etc. As their
whole organism consists of a single cell, they can never form germinal
layers, or definite strata of cells. But all the other animals—all the
tissue-forming animals, or _metazoa,_ as we call them, in
contradistinction to the protozoa—construct real germinal layers by the
repeated cleavage of the impregnated ovum. This we find in the lower
cnidaria and worms, as well as in the more highly-developed molluscs,
echinoderms, articulates, and vertebrates.

In all these metazoa, or multicellular animals, the chief embryonic
processes are substantially alike, although they often seem to a
superficial observer to differ considerably. The stem-cell that
proceeds from the impregnated ovum always passes by repeated cleavage
into a number of simple cells. These cells are all direct descendants
of the stem-cell, and are, for reasons we shall see presently, called
segmentation-cells. The repeated cleavage of the stem-cell, which gives
rise to these segmentation-spheres, has long been known as
“segmentation.” Sooner or later the segmentation-cells join together to
form a round (at first, globular) embryonic sphere (_blastula_); they
then form into two very different groups, and arrange themselves
in two separate strata—the two _primary germinal layers._ These enclose
a digestive cavity, the primitive gut, with an opening, the primitive
mouth. We give the name of the _gastrula_ to the important embryonic
form that has these primitive organs, and the name of _gastrulation_ to
the formation of it. This ontogenetic process has a very great
significance, and is the real starting-point of the construction of the
multicellular animal body.

The fundamental embryonic processes of the cleavage of the ovum and the
formation of the germinal layers have been very thoroughly studied in
the last thirty years, and their real significance has been
appreciated. They present a striking variety in the different groups,
and it was no light task to prove their essential identity in the whole
animal world. But since I formulated the gastræa theory in 1872, and
afterwards (1875) reduced all the various forms of segmentation and
gastrulation to one fundamental type, their identity may be said to
have been established. We have thus mastered the law of unity which
governs the first embryonic processes in all the animals.

Man is like all the other higher animals, especially the apes, in
regard to these earliest and most important processes. As the human
embryo does not essentially differ, even at a much later stage of
development—when we already perceive the cerebral vesicles, the eyes,
ears, gill-arches, etc.—from the similar forms of the other higher
mammals, we may confidently assume that they agree in the earliest
embryonic processes, segmentation and the formation of germinal layers.
This has not yet, it is true, been established by observation. We have
never yet had occasion to dissect a woman immediately after
impregnation and examine the stem-cell or the segmentation-cells in her
oviduct. However, as the earliest human embryos we have examined, and
the later and more developed forms, agree with those of the rabbit,
dog, and other higher mammals, no reasonable man will doubt but that
the segmentation and formation of layers are the same in both cases.

But the special form of segmentation and layer formation which we find
in the mammal is by no means the original, simple, palingenetic form.
It has been much modified and cenogenetically altered by a very complex
adaptation to embryonic conditions. We cannot, therefore, understand it
altogether in itself. In order to do this, we have to make a
_comparative_ study of segmentation and layer-formation in the animal
world; and we have especially to seek the original, _palingenetic_ form
from which the modified _cenogenetic_ (see p. 4) form has gradually
been developed.

This original unaltered form of segmentation and layer-formation is
found to-day in only one case in the vertebrate-stem to which man
belongs—the lowest and oldest member of the stem, the wonderful
lancelet or amphioxus (cf. Chapters XVI and XVII). But we find a
precisely similar palingenetic form of embryonic development in the
case of many of the invertebrate animals, as, for instance, the
remarkable ascidia, the pond-snail (_Limnæus_), and arrow-worm
(_Sagitta_), and many of the echinoderms and cnidaria, such as the
common star-fish and sea-urchin, many of the medusæ and corals, and the
simpler sponges (_Olynthus_). We may take as an illustration the
palingenetic segmentation and germinal layer-formation in an eight-fold
insular coral, which I discovered in the Red Sea, and described as
_Monoxenia Darwinii._

The impregnated ovum of this coral (Fig. 29 A, B) first splits into two
equal cells (C). First, the nucleus of the stem-cell and its central
body divide into two halves. These recede from and repel each other,
and act as centres of attraction on the surrounding protoplasm; in
consequence of this, the protoplasm is constricted by a circular
furrow, and, in turn, divides into two halves. Each of the two
segmentation-cells thus produced splits in the same way into two equal
cells. The four segmentation-cells (grand-daughters of the stem-cell)
lie in one plane. Now, however, each of them subdivides into two equal
halves, the cleavage of the nucleus again preceding that of the
surrounding protoplasm. The eight cells which thus arise break into
sixteen, these into thirty-two, and then (each being constantly halved)
into sixty-four, 128, and so on.[16] The final result of this
repeated cleavage is the formation of a globular cluster of similar
segmentation-cells, which we call the mulberry-formation or morula. The
cells are thickly pressed together like the parts of a mulberry or
blackberry, and this gives a lumpy appearance to the surface of the
sphere (Fig. E).[17]

 [16] The number of segmentation-cells thus produced increases
 geometrically in the original gastrulation, or the purest palingenetic
 form of cleavage. However, in different animals the number reaches a
 different height, so that the morula, and also the blastula, may
 consist sometimes of thirty-two, sometimes of sixty-four, and
 sometimes of 128, or more, cells.


 [17] The segmentation-cells which make up the morula after the close
 of the palingenetic cleavage seem usually to be quite similar, and to
 present no differences as to size, form, and composition. That,
 however, does not prevent them from differentiating into animal and
 vegetative cells, even during the cleavage.



Gastrulation of a coral. Fig. 29—Gastrulation of a coral (_Monoxenia
Darwinii_). A, B, stem-cell (cytula) or impregnated ovum. In Figure A
(immediately after impregnation) the nucleus is invisible. In Figure B
(a little later) it is quite clear. C two segmentation-cells. D four
segmentation-cells. E mulberry-formation (morula). F blastosphere
(blastula). G blastula (transverse section). H depula, or hollowed
blastula (transverse section). I gastrula (longitudinal section). K
gastrula, or cup-sphere, external appearance.)


When the cleavage is thus ended, the mulberry-like mass changes into a
hollow globular sphere. Watery fluid or jelly gathers inside the
globule; the segmentation-cells are loosened, and all rise to the
surface. There they are flattened by mutual pressure, and assume the
shape of truncated pyramids, and arrange themselves side by side in one
regular layer (Figs. F, G). This layer of cells is called the germinal
membrane (or blastoderm); the homogeneous cells which compose its
simple structure are called blastodermic cells; and the whole hollow
sphere, the walls of which are made of the preceding, is called the
_blastula_ or _ blastosphere._[18]

 [18] The blastula of the lower animals must not be confused with the
 very different blastula of the mammal, which is properly called the
 _gastrocystis_ or _blastocystis._ This _cenogenetic_ gastrocystis and
 the _palingenetic_ blastula are sometimes very wrongly comprised under
 the common name of blastula or vesicula blastodermica.


In the case of our coral, and of many other lower forms of animal life,
the young embryo begins at once to move independently and swim about in
the water. A fine, long, thread-like process, a sort of whip or lash,
grows out of each blastodermic cell, and this independently executes
vibratory movements, slow at first, but quicker after a time (Fig. F).
In this way each blastodermic cell becomes a ciliated cell. The
combined force of all these vibrating lashes causes the whole blastula
to move about in a rotatory fashion. In many other animals, especially
those in which the embryo develops within enclosed membranes, the
ciliated cells are only formed at a later stage, or even not formed at
all. The blastosphere may grow and expand by the blastodermic cells (at
the surface of the sphere) dividing and increasing, and more fluid is
secreted in the internal cavity. There are still to-day some organisms
that remain throughout life at the structural stage of the
blastula—hollow vesicles that swim about by a ciliary movement in the
water, the wall of which is composed of a single layer of cells, such
as the volvox, the magosphæra, synura, etc. We shall speak further of
the great phylogenetic significance of this fact in Chapter XIX.

A very important and remarkable process now follows—namely, the curving
or invagination of the blastula (Fig. H). The vesicle with a single
layer of cells for wall is converted into a cup with a wall of two
layers of cells (cf. Figs. G, H, I). A certain spot at the surface of
the sphere is flattened, and then bent inward. This depression sinks
deeper and deeper, growing at the cost of the internal cavity. The
latter decreases as the hollow deepens. At last the internal cavity
disappears altogether, the inner side of the blastoderm (that which
lines the depression) coming to lie close on the outer side. At the
same time, the cells of the two sections assume different sizes and
shapes; the inner cells are more round and the outer more oval (Fig.
I). In this way the embryo takes the form of a cup or jar-shaped body,
with a wall made up of two layers of cells, the inner cavity of which
opens to the outside at one end (the spot where the depression was
originally formed). We call this very important and interesting
embryonic form the “cup-embryo” or “cup-larva” (_gastrula,_ Fig. 29, I
longitudinal section, K external view). I have in my _Natural History
of Creation_ given the name of _depula_ to the remarkable intermediate
form which appears at the passage of the blastula into the gastrula. In
this intermediate stage there are two cavities in the embryo—the
original cavity (_blastocœl_) which is disappearing, and the primitive
gut-cavity (_progaster_) which is forming.

I regard the gastrula as the most important and significant embryonic
form in the animal world. In all real animals (that is, excluding the
unicellular protists) the segmentation of the ovum produces either a
pure, primitive, palingenetic gastrula (Fig. 29 I, K) or an equally
instructive cenogenetic form, which has been developed in time from the
first, and can be directly reduced to it. It is certainly a fact of the
greatest interest and instructiveness that animals of the most
different stems—vertebrates and tunicates, molluscs and articulates,
echinoderms and annelids, cnidaria and sponges—proceed from one and the
same embryonic form. In illustration I give a few
pure gastrula forms from various groups of animals (Figs. 30–35,
explanation given below each).



Fig.30 Gastrula of a very simple primitive-gut animal or gastræad. Fig.
31 Gastrula of a worm. Fig. 32 Gastrula of an echinoderm. Fig. 33
Gastrula of an arthropod. Fig. 34 Gastrula of a mollusc. Fig. 35
Gastrula of a vertebrate. Fig. 30 (_A_)—Gastrula of a very simple
primitive-gut animal or gastræad (gastrophysema). (_Haeckel._) Fig. 31
(_B_)—Gastrula of a worm (_Sagitta_). (From _Kowalevsky._) Fig. 32
(_C_)—Gastrula of an echinoderm (star-fish, _Uraster_), not completely
folded in (depula). (From _Alexander Agassiz._) Fig. 33 (_D_)—Gastrula
of an arthropod (primitive crab, _Nauplius_) (as 32). Fig. 34
(_E_)—Gastrula of a mollusc (pond-snail, _Linnæus_). (From _Karl
Rabl._) Fig. 35 (_F_)—Gastrula of a vertebrate (lancelet, _Amphioxus_).
(From _Kowalevsky._) (Front view.) In each figure _d_ is the
primitive-gut cavity, _o_ primitive mouth, _s_ segmentation-cavity, _i_
entoderm (gut-layer), _e_ ectoderm (skin layer).


In view of this extraordinary significance of the gastrula, we must
make a very careful study of its original structure. As a rule, the
typical gastrula is very small, being invisible to the naked eye, or at
the most only visible as a fine point under very favourable conditions,
and measuring generally 1/500 to 1/250 of an inch (less frequently 1/50
inch, or even more) in diameter. In shape it is usually like a roundish
drinking-cup. Sometimes it is rather oval, at other times more
ellipsoid or spindle-shaped; in some cases it is half round, or even
almost round, and in others lengthened out, or almost cylindrical.

I give the name of primitive gut (_progaster_) and primitive mouth
(_prostoma_) to the internal cavity of the gastrula-body and its
opening; because this cavity is the first rudiment of the digestive
cavity of the organism, and the opening originally served to take food
into it. Naturally, the primitive gut and mouth change very
considerably afterwards in the various classes of animals. In most of
the cnidaria and many of the annelids (worm-like animals) they remain
unchanged throughout life. But in most of the
higher animals, and so in the vertebrates, only the larger central part
of the later alimentary canal develops from the primitive gut; the
later mouth is a fresh development, the primitive mouth disappearing or
changing into the anus. We must therefore distinguish carefully between
the primitive gut and mouth of the gastrula and the later alimentary
canal and mouth of the fully developed vertebrate.[19]

 [19] My distinction (1872) between the primitive gut and mouth and the
 later permanent stomach (_metagaster_) and mouth (_metastoma_) has
 been much criticised; but it is as much justified as the distinction
 between the primitive kidneys and the permanent kidneys. Professor E.
 Ray-Lankester suggested three years afterwards (1875) the name
 _archenteron_ for the primitive gut, and _blastoporus_ for the
 primitive mouth.


Fig.36 Gastrula of a lower sponge (olynthus). Fig. 36—Gastrula of a
lower sponge (lynthus). _A_ external view, _B_ longitudinal section
through the axis, _g_ primitive-gut cavity, a primitive mouth-aperture,
_i_ inner cell-layer (entoderm, endoblast, gut-layer), _e_ external
cell-layer (outer germinal layer, ectoderm, ectoblast, or skin-layer).


The two layers of cells which line the gut-cavity and compose its wall
are of extreme importance. These two layers, which are the sole
builders of the whole organism, are no other than the two primary
germinal layers, or the primitive germ-layers. I have spoken in the
introductory section (Chapter III) of their radical importance. The
outer stratum is the skin-layer, or _ectoderm_ (Figs. 30–35_e_); the
inner stratum is the gut-layer, or _entoderm_ (_i_). The former is
often also called the ectoblast, or epiblast, and the latter the
endoblast, or hypoblast. _From these two primary germinal layers alone
is developed the entire organism of all the metazoa or multicellular
animals._ The skin-layer forms the external skin, the gut-layer forms
the internal skin or lining of the body. Between these two germinal
layers are afterwards developed the middle germinal layer (_mesoderma_)
and the body-cavity (_cœloma_) filled with blood or lymph.

The two primary germinal layers were first distinguished by Pander in
1817 in the incubated chick. Twenty years later (1849) Huxley pointed
out that in many of the lower zoophytes, especially the medusæ, the
whole body consists throughout life of these two primary germinal
layers. Soon afterwards (1853) Allman introduced the names which have
come into general use; he called the outer layer the _ectoderm_
(“outer-skin”), and the inner the _entoderm_ (“inner-skin”). But in
1867 it was shown, particularly by Kowalevsky, from comparative
observation, that even in invertebrates, also, of the most different
classes—annelids, molluscs, echinoderms, and articulates—the body is
developed out of the same two primary layers. Finally, I discovered
them (1872) in the lowest tissue-forming animals, the sponges, and
proved in my gastræa theory that these two layers must be regarded as
identical throughout the animal world, from the sponges and corals to
the insects and vertebrates, including man. This fundamental “homology
[identity] of the primary germinal layers and the primitive gut” has
been confirmed during the last thirty years by the careful research of
many able observers, and is now pretty generally admitted for the whole
of the metazoa.

As a rule, the cells which compose the two primary germinal layers show
appreciable differences even in the gastrula stage. Generally (if not
always) the cells of the skin-layer or ectoderm (Figs. 36 _c_ and 37
_e_) are the smaller, more numerous, and clearer; while the cells of
the gut-layer, or entoderm (_i_), are larger, less numerous, and
darker. The protoplasm of the ectodermic (outer) cells is clearer and
firmer than the thicker and softer cell-matter of the entodermic
(inner) cells; the latter are, as a rule, much richer in yelk-granules
(albumen and fatty particles) than the former. Also the cells of the
gut-layer have, as a rule, a stronger affinity for colouring matter,
and take on a tinge in a solution of carmine, aniline, etc., more
quickly and appreciably than the cells of the skin-layer. The nuclei of
the entoderm-cells are usually roundish, while those of the
ectoderm-cells are oval.

When the doubling-process is complete, very striking histological
differences between the cells of the two layers are found (Fig. 37).
The tiny, light ectoderm-cells (_e_) are sharply distinguished from the
larger and darker entoderm-cells (_i_). Frequently this differentiation
of the cell-forms sets in at a very early stage, during the
segmentation-process, and is already very appreciable in the blastula.

We have, up to the present, only considered that form of segmentation
and gastrulation which, for many and weighty reasons, we may regard as
the original, primordial, or palingenetic form. We might call it
“equal” or homogeneous segmentation, because the divided cells retain a
resemblance to each other at first (and often until the formation of
the blastoderm). We give the name of the “bell-gastrula,” or _
archigastrula,_ to the gastrula that succeeds it. In just the same form
as in the coral we considered (_Monoxenia,_ Fig. 29), we find it in the
lowest zoophyta (the gastrophysema, Fig. 30), and the simplest sponges
(olynthus, Fig. 36); also in many of the medusæ and hydrapolyps, lower
types of worms of various classes (brachiopod, arrow-worm, Fig. 31),
tunicates (ascidia), many of the echinoderms (Fig. 32), lower
articulates (Fig. 33), and molluscs (Fig. 34), and, finally, in a
slightly modified form, in the lowest vertebrate (the amphioxus, Fig.
35).


Fig.37 Cells from the two primary germinal layers. Fig. 37—Cells from
the two primary germinal layers of the mammal (from both layers of the
blastoderm). _i_ larger and darker cells of the inner stratum, the
vegetal layer or entoderm. _e_ smaller and clearer cells from the outer
stratum, the animal layer or ectoderm.


The gastrulation of the amphioxus is especially interesting because
this lowest and oldest of all the vertebrates is of the highest
significance in connection with the evolution of the vertebrate stem,
and therefore with that of man (compare Chapters XVI and XVII). Just as
the comparative anatomist traces the most elaborate features in the
structures of the various classes of vertebrates to divergent
development from this simple primitive vertebrate, so comparative
embryology traces the various secondary forms of vertebrate
gastrulation to the simple, primary formation of the germinal layers in
the amphioxus. Although this formation, as distinguished from the
cenogenetic modifications of the vertebrate, may on the whole be
regarded as palingenetic, it is nevertheless different in some features
from the quite primitive gastrulation such as we have, for instance, in
the _Monoxenia_ (Fig. 29) and the _Sagitta._ Hatschek rightly observes
that the segmentation of the ovum in the amphioxus is not strictly
equal, but almost equal, and approaches the unequal. The difference in
size between the two groups of cells continues to be very noticeable in
the further course of the segmentation; the smaller animal cells of the
upper hemisphere divide more quickly than the larger vegetal cells of
the lower (Fig. 38 _A, B_). Hence the blastoderm, which forms the
single-layer wall of the globular blastula at the end of the
cleavage-process, does not consist of
homogeneous cells of equal size, as in the Sagitta and the Monoxenia;
the cells of the upper half of the blastoderm (the mother-cells of the
ectoderm) are more numerous and smaller, and the cells of the lower
half (the mother-cells of the entoderm) less numerous and larger.
Moreover, the segmentation-cavity of the blastula (Fig. 38 _C, h_) is
not quite globular, but forms a flattened spheroid with unequal poles
of its vertical axis. While the blastula is being folded into a cup at
the vegetal pole of its axis, the difference in the size of the
blastodermic cells increases (Fig. 38 _D, E_); it is most conspicuous
when the invagination is complete and the segmentation-cavity has
disappeared (Fig. 38 _F_). The larger vegetal cells of the entoderm are
richer in granules, and so darker than the smaller and lighter animal
cells of the ectoderm.


Fig.38 Gastrulation of the amphioxus. Fig. 38—Gastrulation of the
amphioxus, from _ Hatschek_ (vertical section through the axis of the
ovum). _A, B, C_ three stages in the formation of the blastula; _D, E_
curving of the blastula; _F_ complete gastrula. _h_
segmentation-cavity. _g_ primitive gut-cavity.


But the unequal gastrulation of the amphioxus diverges from the typical
equal cleavage of the _Sagitta,_ the _Monoxenia_ (Fig. 29), and the
_Olynthus_ (Fig. 36), in another important particular. The pure
archigastrula of the latter forms is uni-axial, and it is round in its
whole length in transverse section. The vegetal pole of the vertical
axis is just in the centre of the primitive mouth. This is not the case
in the gastrula of the amphioxus. During the folding of the blastula
the ideal axis is already bent on one side, the growth of the
blastoderm (or the increase of its cells) being brisker on one side
than on the other; the side that grows more quickly, and so is more
curved (Fig. 39 _ v_), will be the anterior or belly-side, the
opposite, flatter side will form the back (_d_). The primitive mouth,
which at first, in the typical archigastrula, lay at the vegetal pole
of the main axis, is forced away to the dorsal side; and whereas its
two lips lay at first in a plane at right angles to the chief axis,
they are now so far thrust aside that their plane cuts the axis at a
sharp angle. The dorsal lip is therefore the upper and more forward,
the ventral lip the lower and hinder. In the latter, at the ventral
passage of the entoderm into the ectoderm, there lie side by side a
pair of very large cells, one to the right and one to the left (Fig. 39
_p_): these are the important polar cells of the primitive mouth, or
“the primitive cells of the mesoderm.” In consequence of these
considerable variations arising in the course of the gastrulation, the
primitive uni-axial form of the archigastrula in the amphioxus has
already become tri-axial, and thus the two-sidedness, or bilateral
symmetry, of the vertebrate body has already been determined. This has
been transmitted from the amphioxus to all the other modified
gastrula-forms of the vertebrate stem.

Apart from this bilateral structure, the gastrula of the amphioxus
resembles the typical archigastrula of the lower animals (Figs. 30–36)
in developing the two primary germinal layers from a single layer of
cells. This is clearly the oldest and original form of the metazoic
embryo. Although the animals I have mentioned belong to the most
diverse classes, they nevertheless agree with each other, and many more
animal forms, in having retained to the present day, by a conservative
heredity, this palingenetic form of gastrulation which they have from
their
earliest common ancestors. But this is not the case with the great
majority of the animals. With these the original embryonic process has
been gradually more or less altered in the course of millions of years
by adaptation to new conditions of development. Both the segmentation
of the ovum and the subsequent gastrulation have in this way been
considerably changed. In fact, these variations have become so great in
the course of time that the segmentation was not rightly understood in
most animals, and the gastrula was unrecognised. It was not until I had
made an extensive comparative study, lasting a considerable time (in
the years 1866–75), in animals of the most diverse classes, that I
succeeded in showing the same common typical process in these
apparently very different forms of gastrulation, and tracing them all
to one original form. I regard all those that diverge from the primary
palingenetic gastrulation as secondary, modified, and cenogenetic. The
more or less divergent form of gastrula that is produced may be called
a secondary, modified gastrula, or a _ metagastrula._ The reader will
find a scheme of these different kinds of segmentation and gastrulation
at the close of this chapter.

By far the most important process that determines the various
cenogenetic forms of gastrulation is the change in the nutrition of the
ovum and the accumulation in it of nutritive yelk. By this we
understand various chemical substances (chiefly granules of albumin and
fat-particles) which serve exclusively as reserve-matter or food for
the embryo. As the metazoic embryo in its earlier stages of development
is not yet able to obtain its food and so build up the frame, the
necessary material has to be stored up in the ovum. Hence we
distinguish in the ova two chief elements—the active formative yelk
(protoplasm) and the passive food-yelk (deutoplasm, wrongly spoken of
as “the yelk”). In the little palingenetic ova, the segmentation of
which we have already considered, the yelk-granules are so small and so
regularly distributed in the protoplasm of the ovum that the even and
repeated cleavage is not affected by them. But in the great majority of
the animal ova the food-yelk is more or less considerable, and is
stored in a certain part of the ovum, so that even in the unfertilised
ovum the “granary” can clearly be distinguished from the formative
plasm. As a rule, the formative-yelk (with the germinal vesicle) then
usually gathers at one pole and the food-yelk at the other. The first
is the _ animal,_ and the second the _vegetal,_ pole of the vertical
axis of the ovum.


Fig.39 Gastrula of the amphioxus, seen from left side. Fig. 39—Gastrula
of the amphioxus, seen from left side (diagrammatic median section).
(From _Hatschek._) _g_ primitive gut, _u_ primitive mouth, _p_
peristomal pole-cells, _i_ entoderm, _e_ ectoderm, _d_ dorsal side, _v_
ventral side.

In these “telolecithal” ova, or ova with the yelk at one end (for
instance, in the cyclostoma and amphibia), the gastrulation then
usually takes place in such a way that in the cleavage of the
impregnated ovum the animal (usually the upper) half splits up more
quickly than the vegetal (lower). The contractions of the active
protoplasm, which effect this continual cleavage of the cells, meet a
greater resistance in the lower vegetal half from the passive
deutoplasm than in the upper animal half. Hence we find in the latter
more but smaller, and in the former fewer but larger, cells. The animal
cells produce the external, and the vegetal cells the internal,
germinal layer.

Although this unequal segmentation of the cyclostoma, ganoids, and
amphibia seems at first sight to differ from the original equal
segmentation (for instance, in the monoxenia, Fig. 29), they both have
this in common, that the cleavage process throughout affects the
_whole_ cell; hence Remak called it _total_ segmentation, and the ova
in question _holoblastic,_ or “whole-cleaving.” It is otherwise with
the second chief group of ova, which he distinguished from these as _
meroblastic,_ or “partially-cleaving ”: to this class belong the
familiar large eggs of birds and reptiles, and of most fishes. The
inert mass of the passive food-yelk is so
large in these cases that the protoplasmic contractions of the active
yelk cannot effect any further cleavage. In consequence, there is only
a partial segmentation. While the protoplasm in the animal section of
the ovum continues briskly to divide, multiplying the nuclei, the
deutoplasm in the vegetal section remains more or less undivided; it is
merely consumed as food by the forming cells. The larger the
accumulation of food, the more restricted is the process of
segmentation. It may, however, continue for some time (even after the
gastrulation is more or less complete) in the sense that the vegetal
cell-nuclei distributed in the deutoplasm slowly increase by cleavage;
as each of them is surrounded by a small quantity of protoplasm, it may
afterwards appropriate a portion of the food-yelk, and thus form a real
“yelk-cell” (_merocyte_). When this vegetal cell-formation continues
for a long time, after the two primary germinal layers have been
formed, it takes the name of the “after-segmentation.”

The meroblastic ova are only found in the larger and more highly
developed animals, and only in those whose embryo needs a longer time
and richer nourishment within the fœtal membranes. According as the
yelk-food accumulates at the centre or at the side of the ovum, we
distinguish two groups of dividing ova, periblastic and discoblastic.
In the periblastic the food-yelk is in the centre, enclosed inside the
ovum (hence they are also called “centrolecithal” ova): the formative
yelk surrounds the food-yelk, and so suffers itself a superficial
cleavage. This is found among the articulates (crabs, spiders, insects,
etc.). In the discoblastic ova the food-yelk gathers at one side, at
the vegetal or lower pole of the vertical axis, while the nucleus of
the ovum and the great bulk of the formative yelk lie at the upper or
animal pole (hence these ova are also called “telolecithal”). In these
cases the cleavage of the ovum begins at the upper pole, and leads to
the formation of a dorsal discoid embryo. This is the case with all
meroblastic vertebrates, most fishes, the reptiles and birds, and the
oviparous mammals (the monotremes).

The gastrulation of the discoblastic ova, which chiefly concerns us,
offers serious difficulties to microscopic investigation and
philosophic consideration. These, however, have been mastered by the
comparative embryological research which has been conducted by a number
of distinguished observers during the last few decades—especially the
brothers Hertwig, Rabl, Kupffer, Selenka, Rückert, Goette, Rauber, etc.
These thorough and careful studies, aided by the most perfect modern
improvements in technical method (in tinting and dissection), have
given a very welcome support to the views which I put forward in my
work, _On the Gastrula and the Segmentation of the Animal Ovum_ [not
translated], in 1875. As it is very important to understand these views
and their phylogenetic foundation clearly, not only as regards
evolution in general, but particularly in connection with the genesis
of man, I will give here a brief statement of them as far as they
concern the vertebrate-stem:—

1. All the vertebrates, including man, are phylogenetically (or
genealogically) related—that is, are members of one single natural
stem.

2. Consequently, the embryonic features in their individual development
must also have a genetic connection.

3. As the gastrulation of the amphioxus shows the original palingenetic
form in its simplest features, that of the other vertebrates must have
been derived from it.

4. The cenogenetic modifications of the latter are more appreciable the
more food-yelk is stored up in the ovum.

5. Although the mass of the food-yelk may be very large in the ova of
the discoblastic vertebrates, nevertheless in every case a blastula is
developed from the morula, as in the holoblastic ova.

6. Also, in every case, the gastrula develops from the blastula by
curving or invagination.

7. The cavity which is produced in the fœtus by this curving is, in
each case, the primitive gut (progaster), and its opening the primitive
mouth (prostoma).

8. The food-yelk, whether large or small, is always stored in the
ventral wall of the primitive gut; the cells (called “merocytes”) which
may be formed in it subsequently (by “after-segmentation”) also belong
to the inner germinal layer, like the cells which immediately enclose
the primitive gut-cavity.

9. The primitive mouth, which at first lies below at the lower pole of
the vertical axis, is forced, by the growth of the yelk, backwards and
then upwards,
towards the dorsal side of the embryo; the vertical axis of the
primitive gut is thus gradually converted into horizontal.

10. The primitive mouth is closed sooner or later in all the
vertebrates, and does not evolve into the permanent mouth-aperture; it
rather corresponds to the “properistoma,” or region of the anus. From
this important point the formation of the middle germinal layer
proceeds, between the two primary layers.

The wide comparative studies of the scientists I have named have
further shown that in the case of the discoblastic higher vertebrates
(the three classes of amniotes) the primitive mouth of the embryonic
disc, which was long looked for in vain, is found always, and is
nothing else than the familiar “primitive groove.” Of this we shall see
more as we proceed. Meantime we realise that gastrulation may be
reduced to one and the same process in all the vertebrates. Moreover,
the various forms it takes in the invertebrates can always be reduced
to one of the four types of segmentation described above. In relation
to the distinction between total and partial segmentation, the grouping
of the various forms is as follows:—

I. Palingenetic
    (primitive) segmentation.     1. Equal segmentation
    (bell-gastrula).     A. Total segmentation (without independent
    food-yelk). II. Cenogenetic segmentation
     (modified by adaptation).     2. Unequal segmentation (hooded
     gastrula). 3. Discoid segmentation (discoid gastrula).     B.
     Partial segmentation (with independent food-yelk). 4. Superficial
     segmentation
     (spherical gastrula).

The lowest metazoa we know—namely, the lower zoophyta (sponges, simple
polyps, etc.)—remain throughout life at a stage of development which
differs little from the gastrula; their whole body consists of two
layers of cells. This is a fact of extreme importance. We see that man,
and also other vertebrates, pass quickly through a stage of development
in which they consist of two layers, just as these lower zoophyta do
throughout life. If we apply our biogenetic law to the matter, we at
once reach this important conclusion. “Man and all the other animals
which pass through the two-layer stage, or gastrula-form, in the course
of their embryonic development, must descend from a primitive simple
stem-form, the whole body of which consisted throughout life (as is the
case with the lower zoophyta to-day) merely of two cell-strata or
germinal layers.” We will call this primitive stem-form, with which we
shall deal more fully later on, the _ gastræa_—that is to say,
“primitive-gut animal.”

According to this gastræa-theory there was originally in all the
multicellular animals _one organ_ with the same structure and function.
This was the primitive gut; and the two primary germinal layers which
form its wall must also be regarded as identical in all. This important
homology or identity of the primary germinal layers is proved, on the
one hand, from the fact that the gastrula was originally formed in the
same way in all cases—namely, by the curving of the blastula; and, on
the other hand, by the fact that in every case the same fundamental
organs arise from the germinal layers. The outer or animal layer, or
ectoderm, always forms the chief organs of animal life—the skin,
nervous system, sense-organs, etc.; the inner or vegetal layer, or
entoderm, gives rise to the chief organs of vegetative life—the organs
of nourishment, digestion, blood-formation, etc.

In the lower zoophyta, whose body remains at the two-layer stage
throughout life, the gastræads, the simplest sponges (_Olynthus_), and
polyps (_Hydra_), these two groups of functions, animal and vegetative,
are strictly divided between the two simple primary layers. Throughout
life the outer or animal layer acts simply as a covering for the body,
and accomplishes its movement and sensation. The inner or vegetative
layer of cells acts throughout life as a gut-lining, or nutritive layer
of enteric cells, and often also yields the reproductive cells.

The best known of these “gastræads,” or “gastrula-like animals,” is the
common fresh-water polyp (_Hydra_). This simplest of all the cnidaria
has, it is true, a crown of tentacles round its mouth. Also its outer
germinal layer has certain special modifications. But these are
secondary additions, and the inner germinal layer is a simple stratum
of cells. On the whole, the hydra has preserved to our day by heredity
the simple structure of our primitive ancestor, the _ gastræa_ (cf.
Chapter XIX).

In all other animals, particularly the vertebrates, the gastrula is
merely a brief transitional stage. Here the two-layer stage of the
embryonic development is quickly succeeded by a three-layer, and then a
four-layer, stage. With the appearance of the four superimposed
germinal layers we reach again a firm and steady standing-ground, from
which we may follow the further, and much more difficult and
complicated, course of embryonic development.

SUMMARY OF THE CHIEF DIFFERENCES IN THE OVUM-SEGMENTATION AND
GASTRULATION OF ANIMALS.
The animal stems are indicated by the letters _ a–g_: _a_ Zoophyta. _b_
Annelida. _c_ Mollusca. _d_ Echinoderma. _e_ Articulata. _ f_ Tunicata.
_g_ Vertebrata.

I. Total
Segmentation. Holoblastic ova.



 Gastrula without separate food-yelk.
Hologastrula.     I. Primitive
Segmentation. Archiblastic ova.
 Bell-gastrula
(archigastrula.)     _a._ Many lower zoophyta (sponges, hydrapolyps,
medusæ, simpler corals). _b._ Many lower annelids (sagitta, phoronis,
    many nematoda, etc., terebratula, argiope,
    pisidium). _c._ Some lower molluscs.
_d._ Many echinoderms. _e._ A few lower articulata (some brachiopods,
copepods: Tardigrades, pteromalina). _f._ Many tunicata. _g._ The
acrania (amphioxus). II. Unequal Segmentation.
Amphiblastic ova.
 Hooded-gastrula (amphigastrula).     _a._ Many zoophyta (sponges,
 medusæ,
    corals, siphonophoræ, ctenophora). _b._ Most worms. _c._ Most
    molluscs. _d._ Many echinoderms (viviparous species and some
    others).
_e._ Some of the lower articulata (both crustacea
    and tracheata). _f._ Many tunicata.
_g._ Cyclostoma, the oldest fishes, amphibia,
    mammals (not including man). II. Partial Segmentation. Meroblastic
    ova.
 Gastrula with
separate food-yelk. Merogastrula.     III. Discoid Segmentation.
Discoblastic ova.
 Discoid gastrula.     _c._ Cephalopods or cuttlefish. _e._ Many
 articulata, wood-lice, scorpions, etc. _g._ Primitive fishes, bony
 fishes, reptiles, birds, monotremes. IV. Superficial
Segmentation. Periblastic ova. Spherical-gastrula.     _e._ The great
majority of the articulata
    (crustaceans, myriapods, arachnids, insects).



Chapter IX.
THE GASTRULATION OF THE VERTEBRATE[20]


 [20] Cf. Balfour’s _Manual of Comparative Embryology,_ vol. ii;
 Theodore Morgan’s _The Development of the Frog’s Egg._


The remarkable processes of gastrulation, ovum-segmentation, and
formation of germinal layers present a most conspicuous variety. There
is to-day only the lowest of the vertebrates, the amphioxus, that
exhibits the original form of those processes, or the palingenetic
gastrulation which we have considered in the preceding chapter, and
which culminates in the formation of the archigastrula (Fig. 38). In
all other extant vertebrates these fundamental processes have been more
or less modified by adaptation to the conditions of embryonic
development (especially by changes in the food-yelk); they exhibit
various cenogenetic types of the formation of germinal layers. However,
the different classes vary considerably from each other. In order to
grasp the unity that underlies the manifold differences in these
phenomena and their historical connection, it is necessary to bear in
mind always the unity of the vertebrate-stem. This “phylogenetic
unity,” which I developed in my _General Morphology_ in 1866, is now
generally admitted. All impartial zoologists agree to-day that all the
vertebrates, from the amphioxus and the fishes to the ape and man,
descend from a common ancestor, “the primitive vertebrate.” Hence the
embryonic processes, by which each individual vertebrate is developed,
must also be capable of being reduced to one common type of embryonic
development; and this primitive type is most certainly exhibited to-day
by the amphioxus.

It must, therefore, be our next task to make a comparative study of the
various forms of vertebrate gastrulation, and trace them backwards to
that of the lancelet. Broadly speaking, they fall first into two
groups: the older cyclostoma, the earliest fishes, most of the
amphibia, and the viviparous mammals, have _ holoblastic_ ova—that is
to say, ova with total, unequal segmentation; while the younger
cyclostoma, most of the fishes, the cephalopods, reptiles, birds, and
monotremes, have _ meroblastic_ ova, or ova with partial discoid
segmentation. A closer study of them shows, however, that these two
groups do not present a natural unity, and that the historical
relations between their several divisions are very complicated. In
order to understand them properly, we must first consider the various
modifications of gastrulation in these classes. We may begin with that
of the amphibia.

The most suitable and most available objects of study in this class are
the eggs of our indigenous amphibia, the tailless frogs and toads, and
the tailed salamander. In spring they are to be found in clusters in
every pond, and careful examination of the ova with a lens is
sufficient to show at least the external features of the segmentation.
In order to understand the whole process rightly and follow the
formation of the germinal layers and the gastrula, the ova of the frog
and salamander must be carefully hardened; then the thinnest possible
sections must be made of the hardened ova with the microtome, and the
tinted sections must be very closely compared under a powerful
microscope.

The ova of the frog or toad are globular in shape, about the twelfth of
an inch in diameter, and are clustered in jelly-like masses, which are
lumped together in the case of the frog, but form long strings in the
case of the toad. When we examine the opaque, grey, brown, or blackish
ova closely, we find that the upper half is darker than the lower. The
middle of the upper half is in many species black, while the middle of
the lower half is white.[21] In this way we get a definite axis of the
ovum with two poles. To give a clear
idea of the segmentation of this ovum, it is best to compare it with a
globe, on the surface of which are marked the various parallels of
longitude and latitude. The superficial dividing lines between the
different cells, which come from the repeated segmentation of the ovum,
look like deep furrows on the surface, and hence the whole process has
been given the name of furcation. In reality, however, this
“furcation,” which was formerly regarded as a very mysterious process,
is nothing but the familiar, repeated cell-segmentation. Hence also the
segmentation-cells which result from it are real cells.

 [21] The colouring of the eggs of the amphibia is caused by the
 accumulation of dark-colouring matter at the animal pole of the ovum.
 In consequence of this, the animal cells of the ectoderm are darker
 than the vegetal cells of the entoderm. We find the reverse of this in
 the case of most animals, the protoplasm of the entoderm cells being
 usually darker and coarser-grained.


Fig.40. The cleavage of the frog’s ovum. Fig. 40—The cleavage of the
frog’s ovum (magnified). A stem-cell. _ B_ the first two
segmentation-cells. _C_ four cells. _ D_ eight cells (4 animal and 4
vegetative). _E_ twelve cells (8 animal and 4 vegetative). _F_ sixteen
cells (8 animal and 8 vegetative). _G_ twenty-four cells (16 animal and
8 vegetative). _H_ thirty-two cells. _I_ forty-eight cells. _K_
sixty-four cells. _L_ ninety-six cells. _M_ 160 cells (128 animal and
32 vegetative).


The unequal segmentation which we observe in the ovum of the amphibia
has the special feature of beginning at the upper and darker pole (the
north pole of the terrestrial globe in our illustration), and slowly
advancing towards the lower and brighter pole (the south pole). Also
the upper and darker hemisphere remains in this position throughout the
course of the segmentation, and its cells multiply much more briskly.
Hence the cells of the lower hemisphere are found to be larger and less
numerous. The cleavage of the stem-cell (Fig. 40 _A_) begins with the
formation of a complete furrow, which starts from the north pole and
reaches to the south (_B_). An hour later a second furrow arises in the
same way, and this cuts the first at a right angle (Fig. 40 _ C_). The
ovum is thus divided into four equal parts. Each of these four
“segmentation cells” has an upper and darker and a lower, brighter
half. A few hours later a third furrow appears, vertically to the first
two (Fig. 40 _D_). The globular germ now consists of eight cells, four
smaller ones above (northern) and four larger ones below (southern).
Next, each of the four upper ones divides into two halves by a cleavage
beginning from the north pole, so that we now have eight above and four
below (Fig. 40 _E_). Later, the
four new longitudinal divisions extend gradually to the lower cells,
and the number rises from twelve to sixteen (_F_). Then a second
circular furrow appears, parallel to the first, and nearer to the north
pole, so that we may compare it to the north polar circle. In this way
we get twenty-four segmentation-cells—sixteen upper, smaller, and
darker ones, and eight smaller and brighter ones below (_G_). Soon,
however, the latter also sub-divide into sixteen, a third or “meridian
of latitude” appearing, this time in the southern hemisphere: this
makes thirty-two cells altogether (_H_). Then eight new longitudinal
lines are formed at the north pole, and these proceed to divide, first
the darker cells above and afterwards the lighter southern cells, and
finally reach the south pole. In this way we get in succession forty,
forty-eight, fifty-six, and at last sixty-four cells (_I, K_). In the
meantime, the two hemispheres differ more and more from each other.
Whereas the sluggish lower hemisphere long remains at thirty-two cells,
the lively northern hemisphere briskly sub-divides twice, producing
first sixty-four and then 128 cells (_L, M_). Thus we reach a stage in
which we count on the surface of the ovum 128 small cells in the upper
half and thirty-two large ones in the lower half, or 160 altogether.
The dissimilarity of the two halves increases: while the northern
breaks up into a great number of small cells, the southern consists of
a much smaller number of larger cells. Finally, the dark cells of the
upper half grow almost over the surface of the ovum, leaving only a
small circular spot


at the south pole, where the large and clear cells of the lower half
are visible. This white region at the south pole corresponds, as we
shall see afterwards, to the primitive mouth of the gastrula. The whole
mass of the inner and larger and clearer cells (including the white
polar region) belongs to the entoderm or ventral layer. The outer
envelope of dark smaller cells forms the ectoderm or skin-layer.


Figs. 41-44. Four vertical sections of the fertilised ovum of the toad,
in four successive stages of development. Figs. 41–44—Four vertical
sections of the fertilised ovum of the toad, in four successive stages
of development. The letters have the same meaning throughout: _F_
segmentation-cavity. _D_ covering of same (_D_ dorsal half of the
embryo, _P_ ventral half). _P_ yelk-stopper (white round field at the
lower pole). _Z_ yelk-cells of the entoderm (Remak’s “glandular
embryo”). _N_ primitive gut cavity (progaster or Rusconian alimentary
cavity). The primitive mouth (prostoma) is closed by the yelk-stopper,
_P. s_ partition between the primitive gut cavity (_N_) and the
segmentation cavity (_F_). _k k′,_ section of the large circular
lip-border of the primitive mouth (the Rusconian anus). The line of
dots between _k_ and _k′_ indicates the earlier connection of the
yelk-stopper (_P_) with the central mass of the yelk-cells (_Z_). In
Fig. 44 the ovum has turned 90°, so that the back of the embryo is
uppermost and the ventral side down. (From _Stricker._).


Blastula of the water-salamander. Fig. 45—Blastula of the
water-salamander (_Triton_). _fh_ segmentation-cavity, _dz_ yelk-cells,
_rz_ border-zone. (From _Hertwig._)


In the meantime, a large cavity, full of fluid, has been formed within
the globular body—the segmentation-cavity or embryonic cavity
(_blastocœl,_ Figs. 41–44 _F_). It extends considerably as the cleavage
proceeds, and afterwards assumes an almost semi-circular form (Fig. 41
_F_). The frog-embryo now represents a modified embryonic vesicle or
_blastula,_ with hollow animal half and solid vegetal half.

Now a second, narrower but longer, cavity arises by a process of
folding at the lower pole, and by the falling away from each other of
the white entoderm-cells (Figs. 41–44 _N_). This is the primitive
gut-cavity or the gastric cavity of the gastrula, progaster or
archenteron. It was first observed in the ovum of the amphibia by
Rusconi, and so called the Rusconian cavity. The reason of its peculiar
narrowness here is that it is, for the most part, full of yelk-cells of
the entoderm. These also stop up the whole of the wide opening of the
primitive mouth, and form what is known as the “yelk-stopper,” which is
seen freely at the white round spot at the south pole (_P_). Around it
the ectoderm is much thicker, and forms the border of the primitive
mouth, the most important part of the embryo (Fig. 44 _k, k′_). Soon
the primitive gut-cavity stretches further and further at the expense
of the segmentation-cavity (_F_), until at last the latter disappears
altogether. The two cavities are only separated by a thin partition
(Fig. 43 _s_). With the formation of the primitive gut our frog-embryo
has reached the gastrula stage, though it is clear that this
cenogenetic amphibian gastrula is very different from the real
palingenetic gastrula we have considered (Figs. 30–36).

In the growth of this hooded gastrula we cannot sharply mark off the
various stages which we distinguish successively in the bell-gastrula
as morula and gastrula. Nevertheless, it is not difficult to reduce the
whole cenogenetic or disturbed development of this amphigastrula to the
true palingenetic formation of the archigastrula of the amphioxus.


Fig.46. Embryonic vesicle of triton. Fig. 46—Embryonic vesicle of
triton (_blastula_), outer view, with the transverse fold of the
primitive mouth (_u_). (From _Hertwig._)

This reduction becomes easier if, after considering the gastrulation of
the tailless amphibia (frogs and toads), we glance for a moment at that
of the tailed amphibia, the salamanders. In some of the latter, that
have only recently been carefully studied, and that are
phylogenetically older, the process is much simpler and clearer than is
the case with the former and longer known. Our common water-salamander
(_Triton taeniatus_) is a particularly good subject for observation.
Its nutritive yelk is much smaller and its formative yelk less obscured
with black pigment-cells than in the case of the frog; and its
gastrulation has better retained the original palingenetic character.
It was first described by Scott and Osborn (1879), and Oscar Hertwig
especially made a careful study of it (1881), and rightly pointed out
its great importance in helping us to understand the vertebrate
development. Its globular blastula (Fig. 45) consists of
loosely-aggregated,
yelk-filled entodermic cells or yelk-cells (_dz_) in the lower vegetal
half; the upper, animal half encloses the hemispherical
segmentation-cavity (_fh_), the curved roof of which is formed of two
or three strata of small ectodermic cells. At the point where the
latter pass into the former (at the equator of the globular vesicle) we
have the border zone (_rz_). The folding which leads to the formation
of the gastrula takes place at a spot in this border zone, the
primitive mouth (Fig. 46 _u_).


Fig. 47 Sagittal section of a hooded-embryo (depula) of triton. Fig.
47—Sagittal section of a hooded-embryo (_depula_) of triton (blastula
at the commencement of gastrulation). _ak_ outer germinal layer, _ik_
inner germinal layer, _fh_ segmentation-cavity, ud primitive gut, _u_
primitive mouth, _dl_ and _vl_ dorsal and ventral lips of the mouth,
_dz_ yelk-cells. (From _ Hertwig._)


Unequal segmentation takes place in some of the cyclostoma and in the
oldest fishes in just the same way as in most of the amphibia. Among
the cyclostoma (“round-mouthed”) the familiar lampreys are particularly
interesting. In respect of organisation and development they are
half-way between the acrania (lancelet) and the lowest real fishes
(_Selachii_); hence I divided the group of the cyclostoma in 1886 from
the real fishes with which they were formerly associated, and formed of
them a special class of vertebrates. The ovum-segmentation in our
common river-lamprey (_Petromyzon fluviatilis_) was described by Max
Schultze in 1856, and afterwards by Scott (1882) and Goette (1890).

Unequal total segmentation follows the same lines in the oldest fishes,
the selachii and ganoids, which are directly descended from the
cyclostoma. The primitive fishes (_Selachii_), which we must regard as
the ancestral group of the true fishes, were generally considered,
until a short time ago, to be discoblastic. It was not until the
beginning of the twentieth century that Bashford Dean made the
important discovery in Japan that one of the oldest living fishes of
the shark type (_Cestracion japonicus_) has the same total unequal
segmentation as the amphiblastic plated fishes (_ganoides_).[22] This
is particularly interesting in connection with our subject, because the
few remaining survivors of this division, which was so numerous in
paleozoic times, exhibit three different types of gastrulation.

 [22] Bashford Dean, _Holoblastic Cleavage in the Egg of a Shark,
 Cestracion japonicus Macleay. Annotationes zoologicae japonenses,_
 vol. iv, Tokio, 1901.


Fig. 48 Sagittal section of the gastrula of the water-salamander. Fig.
48—Sagittal section of the gastrula of the water-salamander (_Triton_).
(From _Hertwig._) Letters as in Fig. 47; except—_p_ yelk-stopper, _ mk_
beginning of the middle germinal layer.)


The oldest and most conservative forms of the modern ganoids are the
scaly sturgeons (Sturiones), plated fishes of great evolutionary
importance, the eggs of which are eaten as caviar; their cleavage is
not essentially different from that of the lampreys and the amphibia.
On the other hand, the most modern of the plated fishes, the
beautifully scaled bony pike of the North American rivers
(Lepidosteus), approaches the osseous fishes, and is discoblastic like
them. A third genus (Amia) is midway between the sturgeons and the
latter.

The group of the lung-fishes (_Dipneusta_ or _Dipnoi_) is closely
connected with the older ganoids. In respect of their whole
organisation they are midway between the gill-breathing fishes and the
lung-breathing amphibia; they share with the former the shape of the
body and limbs, and with the latter the form of the heart
and lungs. Of the older dipnoi (_Paladipneusta_) we have now only one
specimen, the remarkable Ceratodus of East Australia; its amphiblastic
gastrulation has been recently explained by Richard Semon (cf. Chapter
XXI). That of the two modern dipneusta, of which _ Protopterus_ is
found in Africa and _Lepidosiren_ in America, is not materially
different. (Cf. Fig. 51.)


Fig. 49. Ovum-segmentation in the lamprey. Fig. 49—Ovum-segmentation of
the lamprey (_Petromyzon fluviatalis_), in four successive stages. The
small cells of the upper (animal) hemisphere divide much more quickly
than the cells of the lower (vegetal) hemisphere.


Fig.50. Gastrulation of the lamprey. Fig. 50—Gastrulation of the
lamprey (_Petromyzon fluviatilis_). A blastula, with wide embryonic
cavity (blastocoel, _bl_), _g_ incipient invagination. _B_ depula, with
advanced invagination, from the primitive mouth (_g_). _C_ gastrula,
with complete primitive gut: the embryonic cavity has almost
disappeared in consequence of invagination.


All these amphiblastic vertebrates, _Petromyzon_ and _ Cestracion,
Accipenser_ and _Ceratodus,_ and also the salamanders and batrachia,
belong to the old, conservative groups of our stem. Their unequal
ovum-segmentation and gastrulation have many peculiarities in detail,
but can always be reduced with comparative ease to the original
cleavage and gastrulation of the lowest vertebrate, the amphioxus; and
this is little removed, as we have seen, from the very simple
archigastrula of the _Sagitta_ and _Monoxenia_ (see Fig. 29–36). All
these and many other classes of animals generally agree in the
circumstance that in segmentation their
ovum divides into a large number of cells by repeated cleavage. All
such ova have been called, after Remak, “whole-cleaving”
(_holoblasta_), because their division into cells is complete or total.


Fig.51. Gastrulation of ceratodus. Fig. 51—Gastrulation of ceratodus
(from _Semon_). _A_ and _C_ stage with four cells, _B_ and _D_ with
sixteen cells. _A_ and _B_ are seen from above, _ C_ and _D_ sideways.
_E_ stage with thirty-two cells; _F_ blastula; _G_ gastrula in
longitudinal section. _ fh_ segmentation-cavity. _gh_ primitive gut or
gastric cavity.


In a great many other classes of animals this is not the case, as we
find (in the vertebrate stem) among the birds, reptiles, and most of
the fishes; among the insects and most of the spiders and crabs (of the
articulates); and the cephalopods (of the molluscs). In all these
animals the mature ovum, and the stem-cell that arises from it in
fertilisation, consist of two different and separate parts, which we
have called formative yelk and nutritive yelk. The formative yelk alone
consists of living protoplasm, and is the active, evolutionary, and
nucleated part of the ovum; this alone divides in segmentation, and
produces the numerous cells which make up the embryo. On the other
hand, the nutritive yelk is merely a passive part of the contents of
the ovum, a subordinate element which contains nutritive material
(albumin, fat, etc.), and so represents in a sense the provision-store
of the developing embryo. The latter takes a quantity of food out of
this store, and finally consumes it all. Hence the nutritive yelk is of
great indirect importance in embryonic development, though it has no
direct share in it. It either does not divide at all, or only later on,
and does not generally consist of cells. It is sometimes large and
sometimes small, but generally many times larger than the formative
yelk; and hence it is
that it was formerly thought the more important of the two. As the
respective significance of these two parts of the ovum is often wrongly
described, it must be borne in mind that the nutritive yelk is only a
secondary addition to the primary cell, it is an inner enclosure, not
an external appendage. All ova that have this independent nutritive
yelk are called, after Remak, “partially-cleaving” (_meroblasta_).
Their segmentation is incomplete or partial.


Fig.52. Ovum of a deep-sea bony fish. Fig. 52—Ovum of a deep-sea bony
fish. _b_ protoplasm of the stem-cell, _k_ nucleus of same, _d_ clear
globule of albumin, the nutritive yelk, _f_ fat-globule of same, _c_
outer membrane of the ovum, or ovolemma.)


There are many difficulties in the way of understanding this partial
segmentation and the gastrula that arises from it. We have only
recently succeeded, by means of comparative research, in overcoming
these difficulties, and reducing this cenogenetic form of gastrulation
to the original palingenetic type. This is comparatively easy in the
small meroblastic ova which contain little nutritive yelk—for instance,
in the marine ova of a bony fish, the development of which I observed
in 1875 at Ajaccio in Corsica. I found them joined together in lumps of
jelly, floating on the surface of the sea; and, as the little ovula
were completely transparent, I could easily follow the development of
the germ step by step. These ovula are glossy and colourless globules
of little more than the 50th of an inch. Inside a structureless, thin,
but firm membrane (_ovolemma,_ Fig. 52 _c_) we find a large, quite
clear, and transparent globule of albumin (_d_). At both poles of its
axis this globule has a pit-like depression. In the pit at the upper,
animal pole (which is turned downwards in the floating ovum) there is a
bi-convex lens composed of protoplasm, and this encloses the nucleus
(_k_); this is the formative yelk of the stem-cell, or the germinal
disk (_b_). The small fat-globule (_f_) and the large albumin-globule
(_d_) together form the nutritive yelk. Only the formative yelk
undergoes cleavage, the nutritive yelk not dividing at all at first.

The segmentation of the lens-shaped formative yelk (_b_) proceeds quite
independently of the nutritive yelk, and in perfect geometrical order.

When the mulberry-like cluster of cells has been formed, the
border-cells of the lens separate from the rest and travel into the
yelk and the border-layer. From this the blastula is developed; the
regular bi-convex lens being converted into a disk, like a watch-glass,
with thick borders. This lies on the upper and less curved polar
surface of the nutritive yelk like the watch glass on the yelk. Fluid
gathers between the outer layer and the border, and the
segmentation-cavity is formed. The gastrula is then formed by
invagination, or a kind of turning-up of the edge of the blastoderm. In
this process the segmentation-cavity disappears.

The space underneath the entoderm corresponds to the primitive
gut-cavity, and is filled with the decreasing food-yelk (_n_). Thus the
formation of the gastrula of our fish is complete. In contrast to the
two chief forms of gastrula we considered previously, we give the name
of discoid gastrula (_discogastrula,_ Fig. 54) to this third principal
type.

Very similar to the discoid gastrulation of the bony fishes is that of
the hags or myxinoida, the remarkable cyclostomes that live
parasitically in the body-cavity of fishes, and are distinguished by
several notable peculiarities from their nearest relatives, the
lampreys. While the amphiblastic ova of the latter are small and
develop like those of the amphibia, the cucumber-shaped ova of the hag
are about an inch long, and form a discoid gastrula. Up to the present
it has only been observed in one species (_Bdellostoma Stouti_), by
Dean and Doflein (1898).

It is clear that the important features which distinguish the discoid
gastrula from the other chief forms we have considered are determined
by the large food-yelk. This takes no direct part in the building of
the germinal layers, and completely fills the primitive gut-cavity of
the gastrula, even protruding at the mouth-opening. If we imagine the
original bell-gastrula (Figs. 30–36) trying to swallow a
ball of food which is much bigger than itself, it would spread out
round it in discoid shape in the attempt, just as we find to be the
case here (Fig. 54). Hence we may derive the discoid gastrula from the
original bell-gastrula, through the intermediate stage of the hooded
gastrula. It has arisen through the accumulation of a store of
food-stuff at the vegetal pole, a “nutritive yelk” being thus formed in
contrast to the “formative yelk.” Nevertheless, the gastrula is formed
here, as in the previous cases, by the folding or invagination of the
blastula. We can, therefore, reduce this cenogenetic form of the
discoid segmentation to the palingenetic form of the primitive
cleavage.


Fig.53. Ovum-segmentation of a bony fish. Fig. 53—Ovum-segmentation of
a bony fish. _A_ first cleavage of the stem-cell (_cytula_), _B_
division of same into four segmentation-cells (only two visible), _C_
the germinal disk divides into the blastoderm (_b_) and the periblast
(_p_). _d_ nutritive yelk, _f_ fat-globule, _c_ ovolemma, _z_ space
between the ovolemma and the ovum, filled with a clear fluid.)


This reduction is tolerably easy and confident in the case of the small
ovum of our deep-sea bony fish, but it becomes difficult and uncertain
in the case of the large ova that we find in the majority of the other
fishes and in all the reptiles and birds. In these cases the food-yelk
is, in the first place, comparatively colossal, the formative yelk
being almost invisible beside it; and, in the second place, the
food-yelk contains a quantity of different elements, which are known as
“yelk-granules, yelk-globules, yelk-plates, yelk-flakes,
yelk-vesicles,” and so on. Frequently these definite elements in the
yelk have been described as real cells, and it has been wrongly stated
that a portion of the embryonic body is built up from these cells. This
is by no means the case. In every case, however large it is—and even
when cell-nuclei travel into it during the cleavage of the border—the
nutritive yelk remains a dead accumulation of food, which is taken into
the gut during embryonic development and consumed by the embryo. The
latter develops solely from the living formative yelk of the stem-cell.
This is equally true of the ova of our small bony fishes and of the
colossal ova of the primitive fishes, reptiles, and birds.

The gastrulation of the primitive fishes or selachii (sharks and rays)
has been carefully studied of late years by Ruckert, Rabl, and H.E.
Ziegler in particular, and is very important in the sense that this
group is the oldest among living fishes, and their gastrulation can be
derived directly from that of the cyclostoma by the accumulation of a
large quantity of food-yelk. The oldest sharks (_Cestracion_) still
have the unequal segmentation inherited from the cyclostoma. But while
in this case, as in the case of the amphibia, the small ovum completely
divides into cells in segmentation, this is no longer so in the great
majority of the selachii (or _Elasmobranchii_). In these the
contractility of the active protoplasm no longer suffices to break up
the huge mass of the passive deutoplasm completely into cells; this is
only possible in the upper or dorsal part, but not in the lower or
ventral section. Hence we find in the primitive fishes a blastula with
a small eccentric segmentation-cavity (Fig. 55 _b_), the wall of which
varies greatly in composition. The circular border of the germinal disk
which connects the roof and floor of the segmentation-cavity
corresponds to the border-zone at the equator of the amphibian ovum. In
the middle of its hinder border we have the beginning of the
invagination of the primitive gut
(Fig. 56 _ud_); it extends gradually from this spot (which corresponds
to the Rusconian anus of the amphibia) forward and around, so that the
primitive mouth becomes first crescent-shaped and then circular, and,
as it opens wider, surrounds the ball of the larger food-yelk.


Fig.54. Discoid gastrula (discogastrula) of a bony fish. Fig.
54—Discoid gastrula (_discogastrula_) of a bony fish. _e_ ectoderm, _i_
entoderm, _w_ border-swelling or primitive mouth, _n_ albuminous
globule of the nutritive yelk, _f_ fat-globule of same, _c_ external
membrane (ovolemma), _d_ partition between entoderm and ectoderm
(earlier the segmentation-cavity.)


Essentially different from the wide-mouthed discoid gastrula of most of
the selachii is the narrow-mouthed discoid gastrula (or _ epigastrula_)
of the amniotes, the reptiles, birds, and monotremes; between the
two—as an intermediate stage—we have the _amphigastrula_ of the
amphibia. The latter has developed from the amphigastrula of the
ganoids and dipneusts, whereas the discoid amniote gastrula has been
evolved from the amphibian gastrula by the addition of food-yelk. This
change of gastrulation is still found in the remarkable ophidia
(_Gymnophiona, Cœcilia,_ or _Peromela_), serpent-like amphibia that
live in moist soil in the tropics, and in many respects represent the
transition from the gill-breathing amphibia to the lung-breathing
reptiles. Their embryonic development has been explained by the fine
studies of the brothers Sarasin of _Ichthyophis glutinosa_ at Ceylon
(1887), and those of August Brauer of the _Hypogeophis rostrata_ in the
Seychelles (1897). It is only by the historical and comparative study
of these that we can understand the difficult and obscure gastrulation
of the amniotes.


Fig. 55 Longitudinal section through the blastula of a shark. Fig.
55—Longitudinal section through the blastula of a shark (_Pristiuris_).
(From _Ruckert._) (Looked at from the left; to the right is the hinder
end, _H,_ to the left the fore end, _V._) _B_ segmentation-cavity, _
kz_ cells of the germinal membrane, _dk_ yelk-nuclei.


The bird’s egg is particularly important for our purpose, because most
of the chief studies of the development of the vertebrates are based on
observations of the hen’s egg during hatching. The mammal ovum is much
more difficult to obtain and study, and for this practical and obvious
reason very rarely thoroughly investigated. But we can get hens’ eggs
in any quantity at any time, and, by means of artificial incubation,
follow the development of the embryo step by step. The bird’s egg
differs considerably from the tiny mammal ovum in size, a large
quantity of food-yelk accumulating within the original yelk or the
protoplasm of the ovum. This is the yellow ball which we commonly call
the yolk of the egg. In order to understand the bird’s egg aright—for
it is very often quite wrongly explained—we must examine it in its
original condition, and follow it from the very beginning of its
development in the bird’s ovary. We then see that the original ovum is
a quite small, naked, and simple cell with a nucleus, not differing in
either size or shape from the original ovum of the mammals and other
animals (cf. Fig. 13 _ E_). As in the case of all the craniota (animals
with a skull), the original or primitive ovum (_protovum_) is covered
with a continuous layer of small cells. This membrane is the follicle,
from which the ovum afterwards issues. Immediately underneath it the
structureless yelk-membrane is secreted from the yelk.

The small primitive ovum of the bird begins very early to take up into
itself a quantity of food-stuff through the yelk-membrane, and work it
up into the “yellow yelk.” In this way the ovum
enters on its second stage (the metovum), which is many times larger
than the first, but still only a single enlarged cell. Through the
accumulation of the store of yellow yelk within the ball of protoplasm
the nucleus it contains (the germinal vesicle) is forced to the surface
of the ball. Here it is surrounded by a small quantity of protoplasm,
and with this forms the lens-shaped formative yelk (Fig. 15 _b_). This
is seen on the yellow yelk-ball, at a certain point of the surface, as
a small round white spot—the “tread” (_cicatricula_). From this point a
thread-like column of white nutritive yelk (_d_), which contains no
yellow yelk-granules, and is softer than the yellow food-yelk, proceeds
to the middle of the yellow yelk-ball, and forms there a small central
globule of white yelk (Fig. 15 _ d_). The whole of this white yelk is
not sharply separated from the yellow yelk, which shows a slight trace
of concentric layers in the hard-boiled egg (Fig. 15 _c_). We also find
in the hen’s egg, when we break the shell and take out the yelk, a
round small white disk at its surface which corresponds to the tread.
But this small white “germinal disk” is now further developed, and is
really the gastrula of the chick. The body of the chick is formed from
it alone. The whole white and yellow yelk-mass is without any
significance for the formation of the embryo, it being merely used as
food by the developing chick. The clear, glarous mass of albumin that
surrounds the yellow yelk of the bird’s egg, and also the hard chalky
shell, are only formed within the oviduct round the impregnated ovum.


Fig.56. Longitudinal section of the blastula of a shark (Pristiurus) at
the beginning of gastrulation. Fig. 56—Longitudinal section of the
blastula of a shark (_Pristiurus_) at the beginning of gastrulation.
(From _ Ruckert._) (Seen from the left.) _V_ fore end, _H_ hind end,
_B_ segmentation-cavity, _ud_ first trace of the primitive gut, _dk_
yelk-nuclei, _fd_ fine-grained yelk, _gd_ coarse-grained yelk.


When the fertilisation of the bird’s ovum has taken place within the
mother’s body, we find in the lens-shaped stem-cell the progress of
flat, discoid segmentation (Fig. 57). First two equal
segmentation-cells (_A_) are formed from the ovum. These divide into
four (_B_), then into eight, sixteen (_C_), thirty-two, sixty-four, and
so on. The cleavage of the cells is always preceded by a division of
their nuclei. The cleavage surfaces between the segmentation-cells
appear at the free surface of the tread as clefts. The first two
divisions are vertical to each other, in the form of a cross (_B_).
Then there are two more divisions, which cut the former at an angle of
forty-five degrees. The tread, which thus becomes the germinal disk,
now has the appearance of an eight-rayed star. A circular cleavage next
taking place round the middle, the eight triangular cells divide into
sixteen, of which eight are in the middle and eight distributed around
(_C_). Afterwards circular clefts and radial clefts, directed towards
the centre, alternate more or less irregularly (_D, E_). In most of the
amniotes the formation of concentric and radial clefts is irregular
from the very first; and so also in the hen’s egg. But the final
outcome of the cleavage-process is once more the formation of a large
number of small cells of a similar nature. As in the case of the
fish-ovum, these segmentation-cells form a round, lens-shaped disk,
which corresponds to the morula, and is embedded in a small depression
of the white yelk. Between the lens-shaped disk of the morula-cells and
the underlying white yelk a small cavity is now formed by the
accumulation of fluid, as in the fishes. Thus we get the peculiar and
not easily recognisable blastula of the bird (Fig. 58). The small
segmentation-cavity (_fh_) is very flat and much compressed. The upper
or dorsal wall (_dw_) is formed of a single layer of clear, distinctly
separated cells; this
corresponds to the upper or animal hemisphere of the triton-blastula
(Fig. 45). The lower or ventral wall of the flat dividing space (_vw_)
is made up of larger and darker segmentation-cells; it corresponds to
the lower or vegetal hemisphere of the blastula of the water-salamander
(Fig. 45 _dz_). The nuclei of the yelk-cells, which are in this case
especially numerous at the edge of the lens-shaped blastula, travel
into the white yelk, increase by cleavage, and contribute even to the
further growth of the germinal disk by furnishing it with food-stuff.


Fig. 57 Diagram of discoid segmentation in the bird’s ovum. Fig.
57—Diagram of discoid segmentation in the bird’s ovum (magnified). Only
the formative yelk (the tread) is shown in these six figures (_A_ to
_F_), because cleavage only takes place in this. The much larger
food-yelk, which does not share in the cleavage, is left out and merely
indicated by the dark ring without.


The invagination or the folding inwards of the bird-blastula takes
place in this case also at the hinder pole of the subsequent chief
axis, in the middle of the hind border of the round germinal disk (Fig.
59 _s_). At this spot we have the most brisk cleavage of the cells;
hence the cells are more numerous and smaller here than in the
fore-half of the germinal disk. The border-swelling or thick edge of
the disk is less clear but whiter behind, and is more sharply separated
from contiguous parts. In the middle of its hind border there is a
white, crescent-shaped groove—Koller’s sickle-groove (Fig. 59 _s_); a
small projecting process in the centre of it is called the sickle-knob
(_sk_). This important cleft is the primitive mouth, which was
described for a long time as the “primitive groove.” If we make a
vertical section through this part, we see that a flat and broad cleft
stretches under the germinal disk forwards from the primitive mouth;
this is the primitive gut (Fig. 60 _ud_). Its roof or dorsal wall is
formed by the folded upper part of the blastula, and its floor or
ventral wall by the white yelk (_wd_), in which a number of yelk-nuclei
(_dk_) are distributed. There is a brisk multiplication of these at the
edge of the germinal disk, especially in the neighbourhood of the
sickle-shaped primitive mouth.

We learn from sections through later stages of this discoid
bird-gastrula that the primitive gut-cavity, extending forward from the
primitive mouth as a flat pouch, undermines the whole region of the
round flat lens-shaped blastula (Fig. 61 _ ud_). At the same time, the
segmentation-cavity gradually disappears altogether, the folded inner
germinal layer (_ik_) placing itself from underneath on the overlying
outer germinal layer (_ak_). The typical process of invagination,
though greatly disguised, can thus be clearly seen in this case, as
Goette and Rauber, and more recently Duval (Fig. 61), have shown.

The older embryologists (Pander, Baer, Remak), and, in recent times
especially,
His, Kölliker, and others, said that the two primary germinal layers of
the hen’s ovum—the oldest and most frequent subject of
observation!—arose by horizontal cleavage of a simple germinal disk. In
opposition to this accepted view, I affirmed in my _Gastræa Theory_
(1873) that the discoid bird-gastrula, like that of all other
vertebrates, is formed by folding (or invagination), and that this
typical process is merely altered in a peculiar way and disguised by
the immense accumulation of food-yelk and the flat spreading of the
discoid blastula at one part of its surface. I endeavoured to establish
this view by the derivation of the vertebrates from one source, and
especially by proving that the birds descend from the reptiles, and
these from the amphibia. If this is correct, the discoid gastrula of
the amniotes must have been formed by the folding-in of a hollow
blastula, as has been shown by Remak and Rusconi of the discoid
gastrula of the amphibia, their direct ancestors. The accurate and
extremely careful observations of the authors I have mentioned (Goette,
Rauber, and Duval) have decisively proved this
recently for the birds; and the same has been done for the reptiles by
the fine studies of Kupffer, Beneke, Wenkebach, and others. In the
shield-shaped germinal disk of the lizard (Fig. 62), the crocodile, the
tortoise, and other reptiles, we find in the middle of the hind border
(at the same spot as the sickle groove in the bird) a transverse furrow
(_u_), which leads into a flat, pouch-like, blind sac, the primitive
gut. The fore (dorsal) and hind (ventral) lips of the transverse furrow
correspond exactly to the lips of the primitive mouth (or
sickle-groove) in the birds.


Fig.58. Vertical section of the bastula of a hen. Fig. 59. The germinal
disk of the hen’s ovum at the beginning of gastrulation. Fig. 60.
Longitudinal section of the germinal disk of a siskin. Fig. 58—Vertical
section of the blastula of a hen (_discoblastula_). _ fh_
segmentation-cavity, _dw_ dorsal wall of same, _ vw_ ventral wall,
passing directly into the white yelk (_wd_). (From _Duval._) Fig.
59—The germinal disk of the hen’s ovum at the beginning of
gastrulation; _A_ before incubation, _B_ in the first hour of
incubation. (From _Koller._) _ks_ germinal-disk, _V_ its fore and _H_
its hind border; _ es_ embryonic shield, _s_ sickle-groove, _sk_ sickle
knob, _d_ yelk. Fig. 60—Longitudinal section of the germinal disk of a
siskin (_discogastrula_). (From _Duval._) _ud_ primitive gut, _vl, hl_
fore and hind lips of the primitive mouth (or sickle-edge); _ak_ outer
germinal layer, _ik_ inner germinal layer, _dk_ yelk-nuclei, _wd_ white
yelk.


Fig.61. Longitudinal section of the discoid gastrula of the
nightingale. Fig. 61—Longitudinal section of the discoid gastrula of
the nightingale. (From _Duval._) _ud_ primitive gut, _ vl, hl_ fore and
hind lips of the primitive mouth; _ak, ik_ outer and inner germinal
layers; _vr_ fore-border of the discogastrula.


Fig.62. Germinal disk of the lizard. Fig. 62—Germinal disk of the
lizard (_Lacerta agilis_). (From _Kupffer._) _u_ primitive mouth, _ s_
sickle, _es_ embryonic shield, _hf_ and _df_ light and dark germinative
area.


The gastrulation of the mammals must be derived from this special
embryonic development of the reptiles and birds. This latest and most
advanced class of the vertebrates has, as we shall see afterwards,
evolved at a comparatively recent date from an older group of reptiles;
and all these amniotes must have come originally from a common
stem-form. Hence the distinctive embryonic process of the mammal must
have arisen by cenogenetic modifications from the older form of
gastrulation of the reptiles and birds. Until we admit this thesis we
cannot understand the formation of the germinal layers in the mammal,
and therefore in man.

I first advanced this fundamental principle in my essay _On the
Gastrulation of Mammals_ (1877), and sought to show in this way that I
assumed a gradual degeneration of the food-yelk and the yelk-sac on the
way from the proreptiles to the mammals. “The cenogenetic process of
adaptation,” I said, “which has occasioned the atrophy of the
rudimentary yelk-sac of the mammal, is perfectly clear. It is due to
the fact that the young of the mammal, whose ancestors were certainly
oviparous, now remain a long time in the womb. As the great store of
food-yelk, which the oviparous ancestors gave to the egg, became
superfluous in their descendants owing to the long carrying in the
womb, and the maternal blood in the wall of the uterus made itself the
chief source of nourishment, the now useless yelk-sac was bound to
atrophy by embryonic adaptation.”

My opinion met with little approval at the time; it was vehemently
attacked by Kölliker, Hensen, and His in particular. However, it has
been gradually accepted, and has recently been firmly established by a
large number of excellent studies of mammal gastrulation, especially by
Edward Van Beneden’s studies of the rabbit and bat, Selenka’s on the
marsupials and rodents, Heape’s and Lieberkühn’s on the mole, Kupffer
and Keibel’s on the rodents, Bonnet’s on the ruminants, etc. From the
general comparative point of view, Carl Rabl in his theory of the
mesoderm, Oscar Hertwig in the latest edition of his Manual (1902), and
Hubrecht in his _Studies in Mammalian Embryology_ (1891), have
supported the opinion, and sought to derive the peculiarly modified
gastrulation of the mammal from that of the reptile.

In the meantime (1884) the studies of Wilhelm Haacke and Caldwell
provided a proof of the long-suspected and very interesting fact, that
the lowest mammals, the monotremes, _lay eggs,_ like the birds and
reptiles, and are not viviparous like the other mammals. Although the
gastrulation of the monotremes was not really known until studied by
Richard
Semon in 1894, there could be little doubt, in view of the great size
of their food-yelk, that their ovum-segmentation was discoid, and led
to the formation of a sickle-mouthed discogastrula, as in the case of
the reptiles and birds. Hence I had, in 1875 (in my essay on _The
Gastrula and Ovum-segmentation of Animals_), counted the monotremes
among the discoblastic vertebrates. This hypothesis was established as
a fact nineteen years afterwards by the careful observations of Semon;
he gave in the second volume of his great work, _Zoological Journeys in
Australia_ (1894), the first description and correct explanation of the
discoid gastrulation of the monotremes. The fertilised ova of the two
living monotremes (_Echidna_ and _ Ornithorhynchus_) are balls of
one-fifth of an inch in diameter, enclosed in a stiff shell; but they
grow considerably during development, so that when laid the egg is
three times as large. The structure of the plentiful yelk, and
especially the relation of the yellow and the white yelk, are just the
same as in the reptiles and birds. As with these, partial cleavage
takes place at a spot on the surface at which the small formative yelk
and the nucleus it encloses are found. First is formed a lens-shaped
circular germinal disk. This is made up of several strata of cells, but
it spreads over the yelk-ball, and thus becomes a one-layered blastula.


Fig.63. Ovum of the opossum (Didelphys) divided into four. Fig. 63—Ovum
of the opossum (_Didelphys_)  divided into four. (From _Selenka._) _b_
the four segmentation-cells, _r_ directive body, _c_ unnucleated
coagulated matter, _p,_ albumin-membrane.


If we then imagine the yelk it contains to be dissolved and replaced by
a clear liquid, we have the characteristic blastula of the higher
mammals. In these the gastrulation proceeds in two phases, as Semon
rightly observes: firstly, formation of the entoderm by cleavage at the
centre and further growth at the edge; secondly, invagination. In the
monotremes more primitive conditions have been retained better than in
the reptiles and birds. In the latter, before the commencement of the
gastrula-folding, we have, at least at the periphery, a two-layered
embryo forming from the cleavage. But in the monotremes the formation
of the cenogenetic entoderm does not precede the invagination; hence in
this case the construction of the germinal layers is less modified than
in the other amniota.


Fig.64. Blastula of the opossum (Didelphys). Fig. 64—Blastula of the
opossum (_Didelphys_). (From _Selenka._) _a_ animal pole of the
blastula, _ v_ vegetal pole, _en_ mother-cell of the entoderm, _ ex_
ectodermic cells, _s_ spermia, _ib_ unnucleated yelk-balls (remainder
of the food-yelk), _p_ albumin membrane.


The marsupials, a second sub-class, come next to the oviparous
monotremes, the oldest of the mammals. But as in their case the
food-yelk is already atrophied, and the little ovum develops within the
mother’s body, the partial cleavage has been reconverted into total.
One section of the marsupials still show points of agreement with the
monotremes, while another section of them, according to the splendid
investigations of Selenka, form a connecting-link between these and the
placentals.

The fertilised ovum of the opossum (_Didelphys_) divides, according to
Selenka, first into two, then four, then eight equal cells; hence the
segmentation is at first equal or homogeneous. But in the course of the
cleavage a larger cell, distinguished by its less clear plasm and its
containing more yelk-granules (the mother cell of the entoderm, Fig. 64
_en_),
separates from the others; the latter multiply more rapidly than the
former. As, further, a quantity of fluid gathers in the morula, we get
a round blastula, the wall of which is of varying thickness, like that
of the amphioxus (Fig. 38 _E_) and the amphibia (Fig. 45). The upper or
animal hemisphere is formed of a large number of small cells; the lower
or vegetal hemisphere of a small number of large cells. One of the
latter, distinguished by its size (Fig. 64 _en_), lies at the vegetal
pole of the blastula-axis, at the point where the primitive mouth
afterwards appears. This is the mother-cell of the entoderm; it now
begins to multiply by cleavage, and the daughter-cells (Fig. 65 _ i_)
spread out from this spot over the inner surface of the blastula,
though at first only over the vegetal hemisphere. The less clear
entodermic cells (_i_) are distinguished at first by their rounder
shape and darker nuclei from the higher, clearer, and longer entodermic
cells (_e_), afterwards both are greatly flattened, the inner
blastodermic cells more than the outer.


Fig.65. Blastula of the opossum (Didelphys) at the beginning of
gastrulation. Fig. 66. Oval gastrula of the opossum (Didelphys), about
eight hours old. Fig. 65—Blastula of the opossum (_Didelphys_) at the
beginning of gastrulation. (From _Selenka._) _e_ ectoderm, _i_
entoderm; _a_ animal pole, _u_ primitive mouth at the vegetal pole, _f_
segmentation-cavity, _d_ unnucleated yelk-balls (relics of the reduced
food-yelk), c nucleated curd (without yelk-granules) Fig. 66—Oval
gastrula of the opossum (_Didelphys_), about eight hours old. (From
_Selenka_) (external view).)


The unnucleated yelk-balls and curd (Fig. 65 _d_) that we find in the
fluid of the blastula in these marsupials are very remarkable; they are
the relics of the atrophied food-yelk, which was developed in their
ancestors, the monotremes, and in the reptiles.

In the further course of the gastrulation of the opossum the oval shape
of the gastrula (Fig. 66) gradually changes into globular, a larger
quantity of fluid accumulating in the vesicle. At the same time, the
entoderm spreads further and further over the inner surface of the
ectoderm (_e_). A globular vesicle is formed, the wall of which
consists of two thin simple strata of cells; the cells of the outer
germinal layer are rounder, and those of the inner layer flatter. In
the region of the primitive mouth (_p_) the cells are less flattened,
and multiply briskly. From this point—from the hind (ventral) lip of
the primitive mouth, which extends in a central cleft, the primitive
groove—the construction of the mesoderm proceeds.

Gastrulation is still more modified and curtailed cenogenetically in
the placentals than in the marsupials. It was first accurately known to
us by the distinguished investigations of Edward Van Beneden in 1875,
the first object of study being the ovum of the rabbit. But as man also
belongs to this sub-class, and as his as yet unstudied gastrulation
cannot be materially different from that of the other placentals, it
merits the closest attention. We have, in the first place, the peculiar
feature that the two first segmentation-cells that proceed from the
cleavage of the fertilised ovum (Fig. 68) are of different sizes and
natures; the difference is sometimes greater, sometimes less (Fig. 69).
One of these first daughter-cells of the ovum is a little
larger, clearer, and more transparent than the other. Further, the
smaller cell takes a colour in carmine, osmium, etc., more strongly
than the larger. By repeated cleavage of it a morula is formed, and
from this a blastula, which changes in a very characteristic way into
the greatly modified gastrula. When the number of the
segmentation-cells in the mammal embryo has reached ninety-six (in the
rabbit, about seventy hours after impregnation) the fœtus assumes a
form very like the archigastrula (Fig. 72). The spherical embryo
consists of a central mass of thirty-two soft, round cells with dark
nuclei, which are flattened into polygonal shape by mutual pressure,
and colour dark-brown with osmic acid (Fig. 72 _i_). This dark central
group of cells is surrounded by a lighter spherical membrane,
consisting of sixty-four cube-shaped, small, and fine-grained cells
which lie close together in a single stratum, and only colour slightly
in osmic acid (Fig. 72 _e_). The authors who regard this embryonic form
as the primary gastrula of the placental conceive the outer layer as
the ectoderm and the inner as the entoderm. The entodermic membrane is
only interrupted at one spot, one, two, or three of the ectodermic
cells being loose there. These form the yelk-stopper, and fill up the
mouth of the gastrula (_a_). The central primitive gut-cavity (_d_) is
full of entodermic cells. The uni-axial type of the mammal gastrula is
accentuated in this way. However, opinions still differ considerably as
to the real nature of this “provisional gastrula” of the placental and
its relation to the blastula into which it is converted.

As the gastrulation proceeds a large spherical blastula is formed from
this peculiar solid amphigastrula of the placental, as we saw in the
case of the marsupial. The accumulation of fluid in the solid gastrula
(Fig. 73 A) leads to the formation of an eccentric cavity, the group of
the darker entodermic cells (_hy_) remaining directly attached at one
spot with the round enveloping stratum of the lighter ectodermic cells
(_ep_). This spot corresponds to the original primitive mouth (prostoma
or blastoporus). From this important spot the inner germinal layer
spreads all round on the inner surface of the outer layer, the
cell-stratum of which forms the wall of the hollow sphere; the
extension proceeds from the vegetal towards the animal pole.


Fig.67. Longitudinal section through the oval gastrula of the opossum.
Fig. 67—Longitudinal section through the oval gastrula of the opossum
(Fig. 69). (From _Selenka._) _p_ primitive mouth, _e_ ectoderm, _i_
entoderm, _d_ yelk remains in the primitive gut-cavity (_u_).


The cenogenetic gastrulation of the placental has been greatly modified
by secondary adaptation in the various groups of this most advanced and
youngest sub-class of the mammals. Thus, for instance, we find in many
of the rodents (guinea-pigs, mice, etc.) _ apparently_ a temporary
inversion of the two germinal layers. This is due to a folding of the
blastodermic wall by what is called the “girder,” a plug-shaped growth
of Rauber’s “roof-layer.” It is a thin layer of flat epithelial cells,
that is freed from the surface of the blastoderm in some of the
rodents; it has no more significance in connection with the general
course of placental gastrulation than the conspicuous departure from
the usual globular shape in the blastula of some of the ungulates. In
some pigs and ruminants it grows into a thread-like, long and thin
tube.

Thus the gastrulation of the placentals, which diverges most from that
of the amphioxus, the primitive form, is reduced to the original type,
the invagination of a modified blastula. Its chief peculiarity is that
the folded part of the blastoderm does not form a completely closed
(only open at the primitive mouth) blind sac, as is usual; but this
blind sac has a wide opening at the ventral curve (opposite to the
dorsal mouth); and through this opening the primitive gut communicates
from the first with the embryonic cavity of the blastula. The folded
crest-shaped
entoderm grows with a free circular border on the inner surface of the
entoderm towards the vegetal pole; when it has reached this, and the
inner surface of the blastula is completely grown over, the primitive
gut is closed. This remarkable direct transition of the primitive
gut-cavity into the segmentation-cavity is explained simply by the
assumption that in most of the mammals the yelk-mass, which is still
possessed by the oldest forms of the class (the monotremes) and their
ancestors (the reptiles), is atrophied. This proves the essential unity
of gastrulation in all the vertebrates, in spite of the striking
differences in the various classes.


Fig.68. Stem-cell of the mammal ovum (from the rabbit). Fig. 69.
Incipient cleavage of the mammal ovum (from the rabbit). Fig. 70. The
first four segmentation-cells of the mammal ovum (from the rabbit).
Fig. 71. Mammal ovum with eight segmentation-cells (from the rabbit).
Fig. 68—Stem-cell of the mammal ovum (from the rabbit). _k_
stem-nucleus, _n_ nuclear corpuscle, _p_ protoplasm of the stem-cell,
_z_ modified zona pellucida, _h_ outer albuminous membrane, _s_ dead
sperm-cells.


Fig. 69 Incipient cleavage of the mammal ovum (from the rabbit). Fig.
69—Incipient cleavage of the mammal ovum (from the rabbit). The
stem-cell has divided into two unequal cells, one lighter (_e_) and one
darker (_i_). _z_ zona pellucida, _h_ outer albuminous membrane, _s_
dead sperm-cell.


Fig. 70 The first four segmentation-cells of the mammal ovum (from the
rabbit). Fig. 70—The first four segmentation-cells of the mammal ovum
(from the rabbit). _e_ the two larger (and lighter) cells, _i_ the two
smaller (and darker) cells, _z_ zona pellucida, _h_ outer albuminous
membrane.


Fig. 71 Mammal ovum with eight segmentation-cells (from the rabbit).
Fig. 71—Mammal ovum with eight segmentation-cells (from the rabbit).
_e_ four larger and lighter cells, _i_ four smaller and darker cells,
_z_ zona pellucida, _h_ outer albuminous membrane.


In order to complete our consideration of the important processes of
segmentation and gastrulation, we will, in conclusion, cast a brief
glance at the fourth chief type—superficial segmentation. In the
vertebrates this form is not found at all. But it plays the chief part
in the large stem of the articulates—the insects,
spiders, myriapods, and crabs. The distinctive form of gastrula that
comes of it is the “vesicular gastrula” (_Perigastrula_).

In the ova which undergo this superficial cleavage the formative yelk
is sharply divided from the nutritive yelk, as in the preceding cases
of the ova of birds, reptiles, fishes, etc.; the formative yelk alone
undergoes cleavage. But while in the ova with discoid gastrulation the
formative yelk is not in the centre, but at one pole of the uni-axial
ovum, and the food-yelk gathered at the other pole, in the ova with
superficial cleavage we find the formative yelk spread over the whole
surface of the ovum; it encloses spherically the food-yelk, which is
accumulated in the middle of the ova. As the segmentation only affects
the former and not the latter, it is bound to be entirely
“superficial”; the store of food in the middle is quite untouched by
it. As a rule, it proceeds in regular geometrical progression. In the
end the whole of the formative yelk divides into a number of small and
homogeneous cells, which lie close together in a single stratum on the
entire surface of the ovum, and form a superficial blastoderm. This
blastoderm is a simple, completely closed vesicle, the internal cavity
of which is entirely full of food-yelk. This real blastula only differs
from that of the primitive ova in its chemical composition. In the
latter the content is water or a watery jelly; in the former it is a
thick mixture, rich in food-yelk, of albuminous and fatty substances.
As this quantity of food-yelk fills the centre of the ovum before
cleavage begins, there is no difference in this respect between the
morula and the blastula. The two stages rather agree in this.

When the blastula is fully formed, we have again in this case the
important folding or invagination that determines gastrulation. The
space between the skin-layer and the gut-layer (the remainder of the
segmentation-cavity) remains full of food-yelk, which is gradually used
up. This is the only material difference between our vesicular gastrula
(_perigastrula_) and the original form of the bell-gastrula
(_archigastrula_). Clearly the one has been developed from the other in
the course of time, owing to the accumulation of food-yelk in the
centre of the ovum.[23]

 [23] On the reduction of all forms of gastrulation to the original
 palingenetic form see especially the lucid treatment of the subject in
 Arnold Lang’s _Manual of Comparative Anatomy_ (1888), Part I.


We must count it an important advance that we are thus in a position to
reduce all the various embryonic phenomena in the different groups of
animals to these four principal forms of segmentation and gastrulation.
Of these four forms we must regard one only as the original
palingenetic, and the other three as cenogenetic and derivative. The
unequal, the discoid, and the superficial segmentation have all clearly
arisen by secondary adaptation from the primary segmentation; and the
chief cause of their development has been the gradual formation of the
food-yelk, and the increasing antithesis between animal and vegetal
halves of the ovum, or between ectoderm (skin-layer) and entoderm
(gut-layer).


Fig.72. Gastrula of the placental mammal (epigastrula from the rabbit),
longitudinal section through the axis. Fig. 72—Gastrula of the
placental mammal (epigastrula from the rabbit), longitudinal section
through the axis.  _e_ ectodermic cells (sixty-four, lighter and
smaller), _i_ entodermic cells (thirty-two, darker and larger), _ d_
central entodermic cell, filling the primitive gut-cavity, _o_
peripheral entodermic cell, stopping up the opening of the primitive
mouth (yelk-stopper in the Rusconian anus).


The numbers of careful studies of animal gastrulation that have been
made in the last few decades have completely established the views I
have expounded, and which I first advanced in the years 1872–76. For a
time they were greatly disputed by many embryologists. Some said that
the original embryonic form of the metazoa was not the gastrula, but
the “planula”—a double-walled vesicle with closed cavity and without
mouth-aperture; the latter was supposed to pierce through gradually. It
was afterwards shown that this planula (found in several sponges, etc.)
was a later evolution from the gastrula.


Fig.73. Gastrula of the rabbit. Fig. 73—Gastrula of the rabbit. A as a
solid, spherical cluster of cells, B changing into the embryonic
vesicle, _bp_ primitive mouth, _ ep_ ectoderm, _hy_ entoderm.


It was also shown that what is called delamination—the rise of the two
primary germinal layers by the folding of the surface of the blastoderm
(for instance, in the _Geryonidæ_ and other medusæ)—was a secondary
formation, due to cenogenetic variations from the original invagination
of the blastula. The same may be said of what is called “immigration,”
in which certain cells or groups of cells are detached from the simple
layer of the blastoderm, and travel into the interior of the blastula;
they attach themselves to the inner wall of the blastula, and form a
second internal epithelial layer—that is to say, the entoderm. In these
and many other controversies of modern embryology the first requisite
for clear and natural explanation is a careful and discriminative
distinction between palingenetic (hereditary) and cenogenetic
(adaptive) processes. If this is properly attended to, we find evidence
everywhere of the biogenetic law.



Chapter X.
THE CŒLOM THEORY


The two “primary germinal layers” which the gastræa theory has shown to
be the first foundation in the construction of the body are found in
this simplest form throughout life only in animals of the lowest
grade—in the gastræads, olynthus (the stem-form of the sponges), hydra,
and similar very simple animals. In all the other animals new strata of
cells are formed subsequently between these two primary body-layers,
and these are generally comprehended under the title of the middle
layer, or _mesoderm._ As a rule, the various products of this middle
layer afterwards constitute the great bulk of the animal frame, while
the original entoderm, or internal germinal layer, is restricted to the
clothing of the alimentary canal and its glandular appendages; and, on
the other hand, the ectoderm, or external germinal layer, furnishes the
outer clothing of the body, the skin and nervous system.

In some large groups of the lower animals, such as the sponges, corals,
and flat-worms, the middle germinal layer
remains a single connected mass, and most of the body is developed from
it; these have been called the three-layered metazoa, in opposition to
the two-layered animals described. Like the two-layered animals, they
have no body-cavity—that is to say, no cavity distinct from the
alimentary system. On the other hand, all the higher animals have this
real body-cavity (_cœloma_), and so are called _cœlomaria._ In all
these we can distinguish _four_ secondary germinal layers, which
develop from the two primary layers. To the same class belong all true
vermalia (excepting the platodes), and also the higher typical animal
stems that have been evolved from them—molluscs, echinoderms,
articulates, tunicates, and vertebrates.


Figs. 74 and 75. Diagram of the four secondary terminal layers. Figs.
74 and 75—Diagram of the four secondary germinal layers, transverse
section through the metazoic embryo: Fig. 74 of an annelid, Fig. 75 of
a vermalian. _a_ primitive gut, _ dd_ ventral glandular layer, _df_
ventral fibre-layer, _ hm_ skin-fibre-layer, _hs_ skin-sense-layer, _u_
beginning of the rudimentary kidneys, _n_ beginning of the
nerve-plates.


The body-cavity (_cœloma_) is therefore a new acquisition of the animal
body, much younger than the alimentary system, and of great importance.
I first pointed out this fundamental significance of the cœlom in my
_Monograph on the Sponges_ (1872), in the section which draws a
distinction between the body-cavity and the gut-cavity, and which
follows immediately on the germ-layer theory and the ancestral tree of
the animal kingdom (the first sketch of the gastræa theory). Up to that
time these two principal cavities of the animal body had been confused,
or very imperfectly distinguished; chiefly because Leuckart, the
founder of the cœlenterata group (1848), has attributed a body-cavity,
but not a gut-cavity, to these lowest metazoa. In reality, the truth is
just the other way about.

The ventral cavity, the original organ of nutrition in the
multicellular animal-body, is the oldest and most important organ of
all the metazoa, and, together with the primitive mouth, is formed in
every case in the gastrula as the primitive gut; it is only at a much
later stage that the body-cavity, which is entirely wanting in the
cœlenterata, is developed in some of the metazoa between the ventral
and the body wall. The two cavities are entirely different in content
and purport. The alimentary cavity (_enteron_) serves the purpose of
digestion; it contains water and food taken from without, as well as
the pulp (chymus) formed from this by digestion. On the other hand, the
body-cavity, quite distinct from the gut and closed externally, has
nothing to do with digestion; it encloses the gut itself and its
glandular appendages, and also contains the sexual products and a
certain amount of blood or lymph, a fluid that is transuded through the
ventral wall.

As soon as the body-cavity appears, the ventral wall is found to be
separated from the enclosing body-wall, but the two continue to be
directly connected at various points. We can also then always
distinguish a number of different layers of tissue in both walls—at
least two in each. These tissue-layers are formed originally from four
different simple cell-layers, which are the much-discussed four
secondary germinal layers. The outermost of these, the skin-sense-layer
(Figs. 74, 75 _hs_), and the innermost, the gut-gland-layer (_dd_),
remain at first simple epithelia or covering-layers. The one covers the
outer surface of the body, the other the inner
surface of the ventral wall; hence they are called confining or
limiting layers. Between them are the two middle-layers, or mesoblasts,
which enclose the body-cavity.


Fig.76. Coelomula of sagitta. Fig. 76—Cœlomula of sagitta (gastrula
with a couple of cœlom-pouches. (From _Kowalevsky._) _ bl.p_ primitive
mouth, _al_ primitive gut, _pv_ cœlom-folds, _m_ permanent mouth.


The four secondary germinal layers are so distributed in the structure
of the body in all the cœlomaria (or all metazoa that have a
body-cavity) that the outer two, joined fast together, constitute the
body-wall, and the inner two the ventral wall; the two walls are
separated by the cavity of the cœlom. Each of the walls is made up of a
limiting layer and a middle layer. The two limiting layers chiefly give
rise to epithelia, or covering-tissues, and glands and nerves, while
the middle layers form the great bulk of the fibrous tissue, muscles,
and connective matter. Hence the latter have also been called fibrous
or muscular layers. The outer middle layer, which lies on the inner
side of the skin-sense-layer, is the skin fibre-layer; the inner middle
layer, which attaches from without to the ventral glandular layer, is
the ventral fibre layer. The former is usually called briefly the
parietal, and the latter the visceral layer or mesoderm. Of the many
different names that have been given to the four secondary germinal
layers, the following are those most in use to-day:—

1. Skin-sense-layer (outer limiting layer).     I. Neural layer
    (_neuroblast_).     The two secondary germinal layers of the
    body-wall:
I. Epithelial. II. Fibrous. 2. Skin-fibre-layer (outer middle
layer).     II. Parietal layer
      (_myoblast_). 3. Gut-fibre-layer (inner middle layer).     III.
      Visceral layer
        (_genoblast_).     The two secondary germinal layers of the
        gut-wall: III. Fibrous. IV. Epithelial. 4. Gut-gland-layer
        (inner limiting layer).     IV. Enteral layer
      (_enteroblast_)

The first scientist to recognise and clearly distinguish the four
secondary germinal layers was Baer. It is true that he was not quite
clear as to their origin and further significance, and made several
mistakes in detail in explaining them. But, on the whole, their great
importance did not escape him. However, in later years his view had to
be given up in consequence of more accurate observations. Remak then
propounded a three-layer theory, which was generally accepted. These
theories of cleavage, however, began to give way thirty years ago, when
Kowalevsky (1871) showed that in the case of _Sagitta_ (a very clear
and typical subject of gastrulation) the two middle germinal layers and
the two limiting layers arise not by cleavage, but by folding—by a
secondary invagination of the primary inner germ-layer. This
invagination or folding proceeds from the primitive mouth, at the two
sides of which (right and left) a couple of pouches are formed. As
these cœlom-pouches or cœlom-sacs detach themselves from the primitive
gut, a double body-cavity is formed (Figs. 74–76).


Fig.77. Coelomula of sagitta, in section. Fig. 77—Cœlomula of sagitta,
in section. (From _Hertwig._) _D_ dorsal side, _V_ ventral side, _ ik_
inner germinal layer, _mv_ visceral mesoblast, _ lh_ body-cavity, _mp_
parietal mesoblast, _ak_ outer germinal layer.


The same kind of cœlom-formation as in sagitta was afterwards found by
Kowalevsky in brachiopods and other invertebrates, and in the lowest
vertebrate—the amphioxus. Further instances were discovered by two
English embryologists, to whom we owe very considerable advance in
ontogeny—E. Ray-Lankester and F. Balfour. On the strength of these and
other studies, as well as most extensive research of their own, the
brothers Oscar and Richard Hertwig constructed in 1881
the Cœlom Theory. In order to appreciate fully the great merit of this
illuminating and helpful theory, one must remember what a chaos of
contradictory views was then represented by the “problem of the
mesoderm,” or the much-disputed “question of the origin of the middle
germinal layer.” The cœlom theory brought some light and order into
this infinite confusion by establishing the following points: 1. The
body-cavity originates in the great majority of animals (especially in
all the vertebrates) in the same way as in sagitta: a couple of pouches
or sacs are formed by folding inwards at the primitive mouth, between
the two primary germinal layers; as these pouches detach from the
primitive gut, a pair of cœlom-sacs (right and left) are formed; the
coalescence of these produces a simple body-cavity. 2. When these
cœlom-embryos develop, not as a pair of hollow pouches, but as solid
layers of cells (in the shape of a pair of mesodermal streaks)—as
happens in the higher vertebrates—we have a secondary (cenogenetic)
modification of the primary (palingenetic) structure; the two walls of
the pouches, inner and outer, have been pressed together by the
expansion of the large food-yelk. 3. Hence the mesoderm consists from
the first of _two_ genetically distinct layers, which do not originate
by the cleavage of a primary simple middle layer (as Remak supposed).
4. These two middle layers have, in all vertebrates, and the great
majority of the invertebrates, the same radical significance for the
construction of the animal body; the inner middle layer, or the
visceral mesoderm, (gut-fibre layer), attaches itself to the original
entoderm, and forms the fibrous, muscular, and connective part of the
visceral wall; the outer middle layer, or the parietal mesoderm
(skin-fibre-layer), attaches itself to the original ectoderm and forms
the fibrous, muscular, and connective part of the body-wall. 5. It is
only at the point of origination, the primitive mouth and its vicinity,
that the four secondary germinal layers are directly connected; from
this point the two middle layers advance forward separately between the
two primary germinal layers, to which they severally attach themselves.
6. The further separation or differentiation of the four secondary
germinal layers and their division into the various tissues and organs
take place especially in the later fore-part or head of the embryo, and
extend backwards from there towards the primitive mouth.


Fig.78. Section of a young sagitta. Fig. 78—Section of a young sagitta.
(From _ Hertwig._) _dh_ visceral cavity, _ik_ and _ak_ inner and outer
limiting layers, _mv_ and _mp_ inner and outer middle layers, _lk_
body-cavity, _dm_ and _vm_ dorsal and visceral mesentery.


All animals in which the body-cavity demonstrably arises in this way
from the primitive gut (vertebrates, tunicates, echinoderms,
articulates, and a part of the vermalia) were comprised by the Hertwigs
under the title of enterocœla, and were contrasted with the other
groups of the pseudocœla (with false body-cavity) and the cœlenterata
(with no body-cavity). However, this radical distinction and the views
as to classification which it occasioned have been shown to be
untenable. Further, the absolute differences in tissue-formation which
the Hertwigs set up between the enterocœla and pseudocœla cannot be
sustained in this connection. For these and other reasons their
cœlom-theory has been much criticised and partly abandoned.
Nevertheless, it has rendered a great and lasting service in the
solution of the difficult problem of the mesoderm, and a material part
of it will certainly be retained. I consider it an especial merit of
the theory that it has established the identity of the development of
the two middle layers in all the vertebrates, and has traced them as
cenogenetic modifications back to the original palingenetic form of
development that we still find in the amphioxus. Carl Rabl comes to the
same conclusion in his able Theory of the Mesoderm, and so do
Ray-Lankester, Rauber, Kupffer, Ruckert, Selenka, Hatschek, and others.
There is a general agreement in these and many other recent writers
that all the different forms of cœlom-construction, like those of
gastrulation, follow one and the same strict hereditary law in the vast
vertebrate stem; in spite of their apparent differences, they
are all only cenogenetic modifications of one palingenetic type, and
this original type has been preserved for us down to the present day by
the invaluable amphioxus.

But before we go into the regular cœlomation of the amphioxus, we will
glance at that of the arrow-worm (_Sagitta_), a remarkable deep-sea
worm that is interesting in many ways for comparative anatomy and
ontogeny. On the one hand, the transparency of the body and the embryo,
and, on the other hand, the typical simplicity of its embryonic
development, make the sagitta a most instructive object in connection
with various problems. The class of the _chætogatha,_ which is only
represented by the cognate genera of _Sagitta_ and _ Spadella,_ is in
another respect also a most remarkable branch of the extensive vermalia
stem. It was therefore very gratifying that Oscar Hertwig (1880) fully
explained the anatomy, classification, and evolution of the chætognatha
in his careful monograph.


Figs. 79 and 80. Transverse section of amphioxus-larvae. Figs. 79 and
80.—Transverse section of amphioxus-larvæ. (From _Hatschek._) Fig. 79
at the commencement of cœlom formation (still without segments), Fig.
80 at the stage with four primitive segments. _ak, ik, mk_ outer,
inner, and middle germinal layer, _hp_ horn plate, _mp_ medullary
plate, _ch_ chorda, * and * disposition of the cœlom-pouches, _lh_
body-cavity.)


The spherical blastula that arises from the impregnated ovum of the
sagitta is converted by a folding at one pole into a typical
archigastrula, entirely similar to that of the _Monoxenia_ which I
described (Chapter VIII, Fig. 29). This oval, uni-axial cup-larva
(circular in section) becomes bilateral (or tri-axial) by the growth of
a couple of cœlom-pouches from the primitive gut (Figs. 76, 77). To the
right and left a sac-shaped fold appears towards the top pole (where
the permanent mouth, _m,_ afterwards arises). The two sacs are at first
separated by a couple of folds of the entoderm (Fig. 76 _ pv_), and are
still connected with the primitive gut by wide apertures; they also
communicate for a short time with the dorsal side (Fig. 77 _d_). Soon,
however, the cœlom-pouches completely separate from each other and from
the primitive gut; at the same time they enlarge so much that they
close round the primitive gut (Fig. 78). But in the middle line of the
dorsal and ventral sides the pouches remain separated, their
approaching walls joining here to form a thin vertical partition, the
mesentery (_dm_ and _vm_). Thus _ Sagitta_ has throughout life a double
body-cavity (Fig. 78 _ lk_), and the gut is fastened to the body-wall
both above and below by a mesentery—below by the ventral mesentery
(_vm_), and above by the dorsal mesentery (_dm_). The inner layer of
the two cœlom-pouches (_mv_) attaches itself to the entoderm (_ik_),
and forms with it the visceral wall. The outer layer (_mp_) attaches
itself to the ectoderm (_ak_), and forms with it the outer body-wall.
Thus we have in _Sagitta_ a perfectly clear and simple illustration of
the original cœlomation of the enterocœla. This palingenetic fact is
the more important, as the greater part of the two body-cavities in
_Sagitta_ changes afterwards into sexual glands—the fore or female part
into a pair of ovaries, and the hind or male part into a pair of
testicles.

Cœlomation takes place with equal clearness and transparency in the
case of
the amphioxus, the lowest vertebrate, and its nearest relatives, the
invertebrate tunicates, the sea-squirts. However, in these two stems,
which we class together as _ Chordonia,_ this important process is more
complex, as two other processes are associated with it—the development
of the chorda from the entoderm and the separation of the medullary
plate or nervous centre from the ectoderm. Here again the skulless
amphioxus has preserved to our own time by tenacious heredity the chief
phenomena in their original form, while it has been more or less
modified by embryonic adaptation in all the other vertebrates (with
skulls). Hence we must once more thoroughly understand the palingenetic
embryonic features of the lancelet before we go on to consider the
cenogenetic forms of the craniota.


Figs. 81 and 82. Transverse section of amphioxus embryo. Figs. 81 and
82.—Transverse section of amphioxus embryo. Fig. 81 at the stage with
five somites, Fig. 82 at the stage with eleven somites. (From
_Hatschek._) _ak_ outer germinal layer, _mp_ medullary plate, _n_
nerve-tube, _ik_ inner germinal layer, _dh_ visceral cavity, _lh_
body-cavity, _mk_ middle germinal layer (_mk_1 parietal, _mk_2
visceral), _us_ primitive segment, _ ch_ chorda.


The cœlomation of the amphioxus, which was first observed by Kowalevsky
in 1867, has been very carefully studied since by Hatschek (1881).
According to him, there are first formed on the bilateral gastrula we
have already considered (Figs. 36, 37) three parallel longitudinal
folds—one single ectodermal fold in the central line of the dorsal
surface, and a pair of entodermic folds at the two sides of the former.
The broad ectodermal fold that first appears in the middle line of the
flattened dorsal surface, and forms a shallow longitudinal groove, is
the beginning of the central nervous system, the medullary tube. Thus
the primary outer germinal layer divides into two parts, the middle
medullary plate (Fig. 81 _mp_) and the horny-plate (_ak_), the
beginning of the outer skin or epidermis. As the parallel borders of
the concave medullary plate fold towards each other and grow underneath
the horny-plate, a cylindrical tube is formed, the medullary tube (Fig.
82 _n_); this quickly detaches itself altogether from the horny-plate.
At each side of the medullary tube, between it and the alimentary tube
(Figs. 79–82 _dh_), the two parallel longitudinal folds grow out of the
dorsal wall of the alimentary tube, and these form the two
cœlom-pouches (Figs. 80, 81 _ lh_). This part of the entoderm, which
thus represents the first structure of the middle germinal layer, is
shown darker than the rest of the inner germinal layer in Figs. 79–82.
The edges of the folds meet, and thus form closed tubes (Fig. 81 in
section).

During this interesting process the outline of a third very important
organ, the chorda or axial rod, is being formed between the two
cœlom-pouches. This first foundation of the skeleton, a solid
cylindrical cartilaginous rod, is formed in the middle line of the
dorsal primitive gut-wall, from the entodermal cell-streak that remains
here between the two cœlom-pouches (Figs. 79–82 _ch_). The chorda
appears at first in the shape of a flat longitudinal fold or a shallow
groove (Figs. 80, 81); it does not become a solid cylindrical cord
until after separation from the primitive gut (Fig. 82). Hence we might
say that the dorsal wall of the primitive gut forms three parallel
longitudinal folds at this important period—one single fold and a pair
of folds. The single middle fold becomes the chorda, and lies
immediately below the groove of the ectoderm, which becomes the
medullary
tube; the pair of folds to the right and left lie at the sides between
the former and the latter, and form the cœlom-pouches. The part of the
primitive gut that remains after the cutting off of these three dorsal
primitive organs is the permanent gut; its entoderm is the
gut-gland-layer or enteric layer.


Figs. 83 and 84. Chordula of the amphioxus. Figs. 83 and 84—Chordula of
the amphioxus. Fig. 83 median longitudinal section (seen from the
left). Fig. 84 transverse section. (From _Hatschek._) In Fig. 83 the
cœlom-pouches are omitted, in order to show the chordula more clearly.
Fig. 84 is rather diagrammatic. _h_ horny-plate, _m_ medullary tube,
_n_ wall of same (_n′_ dorsal, _n″_ ventral), _ ch_ chorda, _np_
neuroporus, _ne_ canalis neurentericus, _d_ gut-cavity, _r_ gut dorsal
wall, _ b_ gut ventral wall, _z_ yelk-cells in the latter, _u_
primitive mouth, _o_ mouth-pit, _p_ promesoblasts (primitive or polar
cells of the mesoderm), _w_ parietal layer, _v_ visceral layer of the
mesoderm, _c_ cœlom, _f_ rest of the segmentation-cavity.


Figs. 85 and 86. Chordula of the amphibia (the ringed adder). Figs. 85
and 86—Chordula of the amphibia (the ringed adder). (From _Goette._)
Fig. 85 median longitudinal section (seen from the left), Fig. 86
transverse section (slightly diagrammatic). Lettering as in Figs. 83
and 84.


I give the name of _chordula_ or _chorda-larva_ to the embryonic stage
of the vertebrate organism which is represented by the amphioxus larva
at this period (Figs. 83, 84, in the third period of development
according to Hatschek). (Strabo and Plinius give the name of _cordula_
or _cordyla_ to young fish larvæ.) I ascribe the utmost phylogenetic
significance to it, as it is found in all the chorda-animals (tunicates
as well as vertebrates) in essentially the same form. Although the
accumulation of food-yelk greatly modifies the form of the chordula in
the higher vertebrates, it remains the same in its main features
throughout. In all
cases the nerve-tube (_m_) lies on the dorsal side of the bilateral,
worm-like body, the gut-tube (_d_) on the ventral side, the chorda
(_ch_) between the two, on the long axis, and the cœlom pouches (_c_)
at each side. In every case these primitive organs develop in the same
way from the germinal layers, and the same organs always arise from
them in the mature chorda-animal. Hence we may conclude, according to
the laws of the theory of descent, that all these chordonia or chordata
(tunicates and vertebrates) descend from an ancient common ancestral
form, which we may call _Chordæa._ We should regard this long-extinct
_Chordæa,_ if it were still in existence, as a special class of
unarticulated worm (_chordaria_). It is especially noteworthy that
neither the dorsal nerve-tube nor the ventral gut-tube, nor even the
chorda that lies between them, shows any trace of articulation or
segmentation; even the two cœlom-sacs are not segmented at first
(though in the amphioxus they quickly divide into a series of parts by
transverse
folding). These ontogenetic facts are of the greatest importance for
the purpose of learning those ancestral forms of the vertebrates which
we have to seek in the group of the unarticulated vermalia. The
cœlom-pouches were originally sexual glands in these ancient chordonia.


Figs. 87 and 88. Diagrammatic vertical section of coelomula-embryos of
vertebrates. Figs. 87 and 88—Diagrammatic vertical section of
cœlomula-embryos of vertebrates. (From _Hertwig._) Fig. 87, vertical
section _through_ the primitive mouth, Fig. 88, vertical section
_before_ the primitive mouth. _u_ primitive mouth, _ud_ primitive gut.
_d_ yelk, _dk_ yelk-nuclei, _dh_ gut-cavity, _lh_ body-cavity, _mp_
medullary plate, _ch_ chorda plate, _ak_ and _ik_ outer and inner
germinal layers, _pb_ parietal and _vb_ visceral mesoblast.


Figs. 89 and 90. Transverse section of coelomula embryos of triton.
Figs. 89 and 90—Transverse section of cœlomula embryos of triton. (From
_Hertwig._) Fig. 89, section _through_ the primitive mouth. Fig. 90,
section in front of the primitive mouth, _u_ primitive mouth. _dh_
gut-cavity, _dz_ yelk-cells, _dp_ yelk-stopper, _ak_ outer and _ik_
inner germinal layer, _pb_ parietal and _vb_ visceral middle layer, _m_
medullary plate, _ch_ chorda.


Fig.91 A, B, C. Vertical section of the dorsal part of three
triton-embryos. Fig. 91. _A, B, C._—Vertical section of the dorsal part
of three triton-embryos. (From _Hertwig._) In Fig. _A_ the medullary
swellings (the parallel borders of the medullary plate) begin to rise;
in Fig. _B_ they grow towards each other; in Fig. _C_ they join and
form the medullary tube. _mp_ medullary plate, _mf_ medullary folds,
_n_ nerve-tube, _ch_ chorda, _lh_ body-cavity, _mk_1 and _mk_2 parietal
and visceral mesoblasts, _uv_ primitive-segment cavities, _ak_
ectoderm, _ik_ entoderm, _dz_ yelk-cells, _dh_ gut-cavity.


From the evolutionary point of view the cœlom-pouches are, in any case,
older than the chorda; since they also develop in the same way as in
the chordonia in a number of invertebrates which have no chorda (for
instance, _Sagitta,_ Figs. 76–78). Moreover, in the amphioxus the first
outline of the chorda appears later than that of the cœlom-sacs. Hence
we must, according to the biogenetic law, postulate a special
intermediate form between the gastrula and the chordula, which we will
call _ cœlomula,_ an unarticulated, worm-like body with primitive gut,
primitive mouth, and a double body-cavity, but no chorda. This
embryonic form, the bilateral _cœlomula_ (Fig. 81), may in turn be
regarded as the ontogenetic reproduction (maintained by heredity) of an
ancient ancestral form of the cœlomaria, the _ Cœlomæa_ (cf. Chapter
XX).

In _Sagitta_ and other worm-like animals the two cœlom-pouches
(presumably gonads or sex-glands) are separated by a complete median
partition, the dorsal and ventral mesentery (Fig. 78 _dm, vm_); but in
the vertebrates only the upper part of this vertical partition is
maintained, and forms the dorsal mesentery. This mesentery afterwards
takes the form of a thin membrane, which fastens the visceral tube to
the chorda (or the vertebral column). At the under side of the visceral
tube the cœlom-sacs blend together, their inner or median walls
breaking down and disappearing. The body-cavity then forms a single
simple hollow, in which the gut is quite free, or only attached to the
dorsal wall by means of the mesentery.

The development of the body-cavity and the formation of the _ chordula_
in the higher vertebrates is, like that of the _ gastrula,_ chiefly
modified by the pressure of the food-yelk on the embryonic structures,
which forces its hinder part into
a discoid expansion. These cenogenetic modifications seem to be so
great that until twenty years ago these important processes were
totally misunderstood. It was generally believed that the body-cavity
in man and the higher vertebrates was due to the division of a simple
middle layer, and that the latter arose by cleavage from one or both of
the primary germinal layers. The truth was brought to light at last by
the comparative embryological research of the Hertwigs. They showed in
their _Cœlom Theory_ (1881) that all vertebrates are true enterocœla,
and that in every case a pair of cœlom-pouches are developed from the
primitive gut by folding. The cenogenetic chordula-forms of the
craniotes must therefore be derived from the palingenetic embryology of
the amphioxus in the same way as I had previously proved for their
gastrula-forms.

The chief difference between the cœlomation of the acrania
(_amphioxus_) and the other vertebrates (with skulls—craniotes) is that
the two cœlom-folds of the primitive gut in the former are from the
first hollow vesicles, filled with fluid, but in the latter are empty
pouches, the layers of which (inner and outer) close with each other.
In common parlance we still call a pouch or pocket by that name,
whether it is full or empty. It is different in ontogeny; in some of
our embryological literature ordinary logic does not count for very
much. In many of the manuals and large treatises on this science it is
proved that vesicles, pouches, or sacs deserve that name only when they
are inflated and filled with a clear fluid. When they are not so filled
(for instance, when the primitive gut of the gastrula is filled with
yelk, or when the walls of the empty cœlom-pouches are pressed
together), these vesicles must not be cavities any longer, but “solid
structures.”

The accumulation of food-yelk in the ventral wall of the primitive gut
(Figs. 85, 86) is the simple cause that converts the sac-shaped
cœlom-pouches of the acrania into the leaf-shaped cœlom-streaks of the
craniotes. To convince ourselves of this we need only compare, with
Hertwig, the palingenetic cœlomula of the amphioxus (Figs. 80, 81) with
the corresponding cenogenetic form of the amphibia (Figs. 89–90), and
construct the simple diagram that connects the two (Figs. 87, 88). If
we imagine the ventral half of the primitive gut-wall in the amphioxus
embryo (Figs. 79–84) distended with food-yelk, the vesicular
cœlom-pouches (_lh_) must be pressed together by this, and forced to
extend in the shape of a thin double plate between the gut-wall and
body-wall (Figs. 86, 87). This expansion follows a downward and forward
direction. They are not directly connected with these two walls. The
real unbroken connection between the two middle layers and the primary
germ-layers is found right at the back, in the region of the primitive
mouth (Fig. 87 _u_). At this important spot we have the source of
embryonic development (_blastocrene_), or “zone of growth,” from which
the cœlomation (and also the gastrulation) originally proceeds.


Fig.92. Transverse section of the chordula-embryo of a bird (from a
hen’s egg at the close of the first day of incubation). Fig.
92—Transverse section of the chordula-embryo of a bird (from a hen’s
egg at the close of the first day of incubation). (From _Kölliker._)
_h_ horn-plate (ectoderm), _m_ medullary plate, _Rf_ dorsal folds of
same, _Pv_ medullary furrow, _ch_ chorda, _uwp_ median (inner) part of
the middle layer (median wall of the cœlom-pouches), _sp_ lateral
(outer) part of same, or lateral plates, _uwh_ structure of the
body-cavity, _dd_ gut-gland-layer.


Hertwig even succeeded in showing, in the cœlomula-embryo of the water
salamander (_Triton_), between the first structures of the two middle
layers, the relic of the body-cavity, which is represented in the
diagrammatic transitional form (Figs. 87, 88). In sections both through
the primitive mouth itself (Fig. 89) and in front of it (Fig. 90) the
two middle layers (_pb_ and _vb_) diverge from each other, and disclose
the two body-cavities as narrow clefts. At the primitive-mouth itself
(Fig. 90 _u_) we can penetrate into them from without. It is only here
at the border of the primitive mouth that we can show the direct
transition of the two middle layers into the two limiting layers or
primary germinal layers.

The structure of the chorda also shows the same features in these
cœlomula-embryos of the amphibia (Fig. 91) as in the amphioxus (Figs.
79–82). It arises from the entodermic cell-streak, which forms the
middle dorsal-line of the primitive gut, and occupies the space between
the flat cœlom-pouches (Fig. 91 _A_).
While the nervous centre is formed here in the middle line of the back
and separated from the ectoderm as “medullary tube,” there takes place
at the same time, directly underneath, the severance of the chorda from
the entoderm (Fig. 91 _A, B, C_). Under the chorda is formed (out of
the ventral entodermic half of the gastrula) the permanent gut or
visceral cavity (_enteron_) (Fig. 91 _B, dh_). This is done by the
coalescence, under the chorda in the median line, of the two dorsal
side-borders of the gut-gland-layer (_ik_), which were previously
separated by the chorda-plate (Fig. 91 _A, ch_); these now alone form
the clothing of the visceral cavity (_dh_) (enteroderm, Fig. 91 _C_).
All these important modifications take place at first in the fore or
head-part of the embryo, and spread backwards from there; here at the
hinder end, the region of the primitive mouth, the important border of
the mouth (or _properistoma_) remains for a long time the source of
development or the zone of fresh construction, in the further
building-up of the organism. One has only to compare carefully the
illustrations given (Figs. 85–91) to see that, as a fact, the
cenogenetic cœlomation of the amphibia can be deduced directly from the
palingenetic form of the acrania (Figs. 79–84).


Fig.93. Transverse section of the vertebrate-embryo of a bird (from a
hen’s egg on the second day of incubation). Fig. 93—Transverse section
of the vertebrate-embryo of a bird (from a hen’s egg on the second day
of incubation). (From _Kölliker._) _h_ horn-plate, _mr_ medullary tube,
_ch_ chorda, _uw_ primitive segments, _ uwh_ primitive-segment cavity
(median relic of the cœlom), _sp_ lateral cœlom-cleft, _hpl_
skin-fibre-layer, _df_ gut-fibre-layer, _ung_ primitive-kidney passage,
_ ao_ primitive aorta, _dd_ gut-gland-layer.


The same principle holds good for the amniotes, the reptiles, birds,
and mammals, although in this case the processes of cœlomation are more
modified and more difficult to identify on account of the colossal
accumulation of food-yelk and the corresponding notable flattening of
the germinal disk. However, as the whole group of the amniotes has been
developed at a comparatively late date from the class of the amphibia,
their cœlomation must also be directly traceable to that of the latter.
This is really possible as a matter of fact; even the older
illustrations showed an essential identity of features. Thus forty
years ago Kölliker gave, in the first edition of his _Human Embryology_
(1861), some sections of the chicken-embryo, the features of which
could at once be reduced to those already described and explained in
the sense of Hertwig’s cœlom-theory. A section through the embryo in
the hatched hen’s egg towards the close of the first day of incubation
shows in the middle of the dorsal surface a broad ectodermic medullary
groove (Fig. 92 _Rf_), and underneath the middle of the chorda (_ch_)
and at each side of it a couple of broad mesodermic layers (_sp_).
These enclose a narrow space or cleft (_uwh_), which is nothing else
than the structure of the body-cavity. The two layers that enclose
it—the upper parietal layer (_hpl_) and the lower visceral layer
(_df_)—are pressed together from without, but clearly distinguishable.
This is even clearer a little later, when the medullary furrow is
closed into the nerve-tube (Fig. 93 _mr_).

Special importance attaches to the fact that here again the four
secondary germinal layers are already sharply distinct, and easily
separated from each other. There is only one very restricted area in
which they are connected, and actually pass into each other; this is
the region of the primitive mouth, which is contracted in the amniotes
into a dorsal longitudinal cleft, the primitive groove. Its two lateral
lip-borders form the _primitive streak,_ which has long been recognised
as the most important embryonic source and starting-point of further
processes. Sections through this primitive streak (Figs. 94 and 95)
show that the two primary germinal layers grow at an early stage (in
the discoid gastrula of the chick, a few hours after incubation) into
the primitive
streak (_x_), and that the two middle layers extend outward from this
thickened axial plate (_y_) to the right and left between the former.
The plates of the cœlom-layers, the parietal skin-fibre-layer (_m_) and
the visceral gut-fibre-layer (_f_), are seen to be still pressed close
together, and only diverge later to form the body-cavity. Between the
inner borders of the two flat cœlom-pouches lies the chorda (Fig. 95
_x_), which here again develops from the middle line of the dorsal wall
of the primitive gut.


Transverse section of the primitive streak (primitive mouth) of the
chick. Figs. 94 and 95—Transverse section of the primitive-streak
(primitive mouth) of the chick. Fig. 94 a few hours after the
commencement of incubation, Fig. 95 a little later. (From _ Waldeyer._)
_h_ horn-plate, _n_ nerve-plate, _m_ skin-fibre-layer, _f_
gut-fibre-layer, _d_ gut-gland-layer, _y_ primitive streak or axial
plate, in which all four germinal layers meet, _x_ structure of the
chorda, _u_ region of the later primitive kidneys.


Cœlomation takes place in the vertebrates in just the same way as in
the birds and reptiles. This was to be expected, as the characteristic
gastrulation of the mammal has descended from that of the reptiles. In
both cases a discoid gastrula with primitive streak arises from the
segmented ovum, a two-layered germinal disk with long and small hinder
primitive mouth. Here again the two primary germinal layers are only
directly connected (Fig. 96 _ pr_) along the primitive streak (at the
folding-point of the blastula), and from this spot (the border of the
primitive mouth) the middle germinal layers (_mk_) grow out to right
and left between the preceding. In the fine illustration of the
cœlomula of the rabbit which Van Beneden has given us (Fig. 96) one can
clearly see that each of the four secondary germinal layers consists of
a single stratum of cells.

Finally, we must point out, as a fact of the utmost importance for our
anthropogeny and of great general interest, that the four-layered
cœlomula of man has just the same construction as that of the rabbit
(Fig. 96). A vertical section that Count Spee made through the
primitive mouth or streak of a very young human germinal disk (Fig. 97)
clearly shows that here again the four secondary germ-layers are
inseparably connected only at the primitive streak, and that here also
the two flattened cœlom-pouches (_mk_) extend outwards to right and
left from the primitive mouth between the outer and inner germinal
layers. In this case, too, the middle germinal layer consists from the
first of two separate strata of cells, the parietal (_mp_) and visceral
(_mv_) mesoblasts.

These concordant results of the best recent investigations (which have
been confirmed by the observations of a number of scientists I have not
enumerated) prove the unity of the vertebrate-stem in point of
cœlomation, no less than of gastrulation. In both respects the
invaluable amphioxus—the sole survivor of the acrania—is found to be
the original model that has preserved for us in palingenetic form by a
tenacious heredity these
most important embryonic processes. From this primary model of
construction we can cenogenetically deduce all the embryonic forms of
the other vertebrates, the craniota, by secondary modifications. My
thesis of the universal formation of the gastrula by folding of the
blastula has now been clearly proved for all the vertebrates; so also
has been Hertwig’s thesis of the origin of the middle germinal layers
by the folding of a couple of cœlom-pouches which appear at the border
of the primitive mouth. Just as the gastræa-theory explains the origin
and identity of the two primary layers, so the cœlom-theory explains
those of the four secondary layers. The point of origin is always the
properistoma, the border of the original primitive mouth of the
gastrula, at which the two primary layers pass directly into each
other.


Fig.96. Transverse section of the primitive groove (or primitive mouth)
of a rabbit. Fig. 96—Transverse section of the primitive groove (or
primitive mouth) of a rabbit. (From _Van Beneden._) _ pr_ primitive
mouth, _ul_ lips of same (primitive lips), _ak_ and _ik_ outer and
inner germinal layers, _mk_ middle germinal layer, _mp_ parietal layer,
_mv_ visceral layer of the mesoderm.


Fig.97. Transverse section of the primitive mouth (or groove) of a
human embryo (at the coelomula stage). Fig. 97—Transverse section of
the primitive mouth (or groove) of a human embryo (at the cœlomula
stage). (From _Count Spee._) _pr_ primitive mouth, _ul_ lips of same
(primitive folds), _ak_ and _ik_ outer and inner germinal layers, _mk_
middle layer, _mp_ parietal layer, _mv_ visceral layer of the
mesoblasts.


Moreover, the cœlomula is important as the immediate source of the
chordula, the embryonic reproduction of the ancient, typical,
unarticulated, worm-like form, which has an axial chorda between the
dorsal nerve-tube and the ventral gut-tube. This instructive chordula
(Figs. 83–86) provides a valuable support of our phylogeny; it
indicates the important moment in our stem-history at which the stem of
the chordonia (tunicates and vertebrates) parted for ever from the
divergent stems of the other metazoa (articulates, echinoderms, and
molluscs).

I may express here my opinion, in the form of a chordæa-theory, that
the characteristic chordula-larva of the chordonia has in reality this
great significance—it is the typical reproduction (preserved by
heredity) of the ancient common stem-form of all the vertebrates and
tunicates, the long-extinct _Chordæa._ We will return in Chapter XX to
these worm-like ancestors, which stand out as luminous points in the
obscure stem-history of the invertebrate ancestors of our race.



Chapter XI.
THE VERTEBRATE CHARACTER OF MAN


We have now secured a number of firm standing-places in the
labyrinthian course of our individual development by our study of the
important embryonic forms which we have called the cytula, morula,
blastula, gastrula, cœlomula, and chordula. But we have still in front
of us the difficult task of deriving the complicated frame of the human
body, with all its different parts, organs, members, etc., from the
simple form of the chordula. We have previously considered the origin
of this four-layered embryonic form from the two-layered gastrula. The
two primary germinal layers, which form the entire body of the
gastrula, and the two middle layers of the cœlomula that develop
between them, are the four simple cell-strata, or epithelia, which
alone go to the formation of the complex body of man and the higher
animals. It is so difficult to understand this construction that we
will first seek a companion who may help us out of many difficulties.

This helpful associate is the science of comparative anatomy. Its task
is, by comparing the fully-developed bodily forms in the various groups
of animals, to learn the general laws of organisation according to
which the body is constructed; at the same time, it has to determine
the affinities of the various groups by critical appreciation of the
degrees of difference between them. Formerly, this work was conceived
in a teleological sense, and it was sought to find traces of the plan
of the Creator in the actual purposive organisation of animals. But
comparative anatomy has gone much deeper since the establishment of the
theory of descent; its philosophic aim now is to explain the variety of
organic forms by adaptation, and their similarity by heredity. At the
same time, it has to recognise in the shades of difference in form the
degree of blood-relationship, and make an effort to construct the
ancestral tree of the animal world. In this way, comparative anatomy
enters into the closest relations with comparative embryology on the
one hand, and with the science of classification on the other.

Now, when we ask what position man occupies among the other organisms
according to the latest teaching of comparative anatomy and
classification, and how man’s place in the zoological system is
determined by comparison of the mature bodily forms, we get a very
definite and significant reply; and this reply gives us extremely
important conclusions that enable us to understand the embryonic
development and its evolutionary purport. Since Cuvier and Baer, since
the immense progress that was effected in the early decades of the
nineteenth century by these two great zoologists, the opinion has
generally prevailed that the whole animal kingdom may be distributed in
a small number of great divisions or types. They are called types
because a certain typical or characteristic structure is constantly
preserved within each of these large sections. Since we applied the
theory of descent to this doctrine of types, we have learned that this
common type is an outcome of heredity; all the animals of one type are
blood-relatives, or members of one stem, and can be traced to a common
ancestral form. Cuvier and Baer set up four of these types: the
vertebrates, articulates, molluscs, and radiates. The first three of
these are still retained, and may be conceived as natural phylogenetic
unities, as stems or _phyla_ in the sense of the theory of descent. It
is quite otherwise with the fourth type—the radiata. These animals,
little known as yet at the beginning of the nineteenth century, were
made to form a sort of lumber-room, into which were cast all the lower
animals that did not belong to the other three types. As we obtained a
closer acquaintance with them in the course of the last sixty years, it
was found that we must distinguish among them from four to eight
different types. In this way the total number of animal stems or phyla
has been raised to eight or twelve (cf. Chapter XX).


These twelve stems of the animal kingdom are, however, by no means
co-ordinate and independent types, but have definite relations, partly
of subordination, to each other, and a very different phylogenetic
meaning. Hence they must not be arranged simply in a row one after the
other, as was generally done until thirty years ago, and is still done
in some manuals. We must distribute them in three subordinate principal
groups of very different value, and arrange the various stems
phylogenetically on the principles which I laid down in my _Monograph
on the Sponges,_ and developed in the _Study of the Gastræa Theory._ We
have first to distinguish the unicellular animals (_protozoa_) from the
multicellular tissue-forming (_metazoa_). Only the latter exhibit the
important processes of segmentation and gastrulation; and they alone
have a primitive gut, and form germinal layers and tissues.

The metazoa, the tissue-animals or gut-animals, then sub-divide into
two main sections, according as a body-cavity is or is not developed
between the primary germinal layers. We may call these the _cœlenteria_
and _cœlomaria,_ the former are often also called _zoophytes_ or
_cœlenterata,_ and the latter _bilaterals._ This division is the more
important as the cœlenteria (without cœlom) have no blood and
blood-vessels, nor an anus. The cœlomaria (with body-cavity) have
generally an anus, and blood and blood-vessels. There are four stems
belonging to the cœlenteria: the gastræads (“primitive-gut animals”),
sponges, cnidaria, and platodes. Of the cœlomaria we can distinguish
six stems: the vermalia at the bottom represent the common stem-group
(derived from the platodes) of these, the other five typical stems of
the cœlomaria—the molluscs, echinoderms, articulates, tunicates, and
vertebrates—being evolved from them.

Man is, in his whole structure, a true vertebrate, and develops from an
impregnated ovum in just the same characteristic way as the other
vertebrates. There can no longer be the slightest doubt about this
fundamental fact, nor of the fact that all the vertebrates form a
natural phylogenetic unity, a single stem. The whole of the members of
this stem, from the amphioxus and the cyclostoma to the apes and man,
have the same characteristic disposition, connection, and development
of the central organs, and arise in the same way from the common
embryonic form of the chordula. Without going into the difficult
question of the origin of this stem, we must emphasise the fact that
the vertebrate stem has no direct affinity whatever to five of the
other ten stems; these five isolated phyla are the sponges, cnidaria,
molluscs, articulates, and echinoderms. On the other hand, there are
important and, to an extent, close phylogenetic relations to the other
five stems—the protozoa (through the amœbæ), the gastræads (through the
blastula and gastrula), the platodes and vermalia (through the
cœlomula), and the tunicates (through the chordula).

How we are to explain these phylogenetic relations in the present state
of our knowledge, and what place is assigned to the vertebrates in the
animal ancestral tree, will be considered later (Chapter XX). For the
present our task is to make plainer the vertebrate character of man,
and especially to point out the chief peculiarities of organisation by
which the vertebrate stem is profoundly separated from the other eleven
stems of the animal kingdom. Only after these comparative-anatomical
considerations shall we be in a position to attack the difficult
question of our embryology. The development of even the simplest and
lowest vertebrate from the simple chordula (Figs. 83–86) is so
complicated and difficult to follow that it is necessary to understand
the organic features of the fully-formed vertebrate in order to grasp
the course of its embryonic evolution. But it is equally necessary to
confine our attention, in this general anatomic description of the
vertebrate-body, to the essential facts, and pass by all the
unessential. Hence, in giving now an ideal anatomic description of the
chief features of the vertebrate and its internal organisation, I omit
all the subordinate points, and restrict myself to the most important
characteristics.

Much, of course, will seem to the reader to be essential that is only
of subordinate and secondary interest, or even not essential at all, in
the light of comparative anatomy and embryology. For instance, the
skull and vertebral column and the extremities are non-essential in
this sense. It is true that these parts are very important
_physiologically_; but for the _morphological_ conception of the
vertebrate they are not essential, because they are only found in the
higher, not the lower, vertebrates. The lowest vertebrates have
neither skull nor vertebræ, and no extremities or limbs. Even the human
embryo passes through a stage in which it has no skull or vertebræ; the
trunk is quite simple, and there is yet no trace of arms and legs. At
this stage of development man, like every other higher vertebrate, is
essentially similar to the simplest vertebrate form, which we now find
in only one living specimen. This one lowest vertebrate that merits the
closest study—undoubtedly the most interesting of all the vertebrates
after man—is the famous lancelet or amphioxus, to which we have already
often referred. As we are going to study it more closely later on
(Chapters XVI and XVII), I will only make one or two passing
observations on it here.

The amphioxus lives buried in the sand of the sea, is about one or two
inches in length, and has, when fully developed, the shape of a very
simple, longish, lancet-like leaf; hence its name of the lancelet. The
narrow body is compressed on both sides, almost equally pointed at the
fore and hind ends, without any trace of external appendages or
articulation of the body into head, neck, breast, abdomen, etc. Its
whole shape is so simple that its first discoverer thought it was a
naked snail. It was not until much later—half a century ago—that the
tiny creature was studied more carefully, and was found to be a true
vertebrate. More recent investigations have shown that it is of the
greatest importance in connection with the comparative anatomy and
ontogeny of the vertebrates, and therefore with human phylogeny. The
amphioxus reveals the great secret of the origin of the vertebrates
from the invertebrate vermalia, and in its development and structure
connects directly with certain lower tunicates, the ascidia.

When we make a number of sections of the body of the amphioxus, firstly
vertical longitudinal sections through the whole body from end to end,
and secondly transverse sections from right to left, we get anatomic
pictures of the utmost instructiveness (cf. Figs. 98–102). In the main
they correspond to the ideal which we form, with the aid of comparative
anatomy and ontogeny, of the primitive type or build of the
vertebrate—the long-extinct form to which the whole stem owes its
origin. As we take the phylogenetic unity of the vertebrate stem to be
beyond dispute, and assume a common origin from a primitive stem-form
for all the vertebrates, from amphioxus to man, we are justified in
forming a definite morphological idea of this primitive vertebrate
(_Prospondylus_ or _Vertebræa_). We need only imagine a few slight and
unessential changes in the real sections of the amphioxus in order to
have this ideal anatomic figure or diagram of the primitive vertebrate
form, as we see in Figs. 98–102. The amphioxus departs so little from
this primitive form that we may, in a certain sense, describe it as a
modified “primitive vertebrate.”[24]

 [24] The ideal figure of the vertebrate as given in Figs. 98–102 is a
 hypothetical scheme or diagram, that has been chiefly constructed on
 the lines of the amphioxus, but with a certain attention to the
 comparative anatomy and ontogeny of the ascidia and appendicularia on
 the one hand, and of the cyclostoma and selachii on the other. This
 diagram has no pretension whatever to be an “exact picture,” but
 merely an attempt to reconstruct hypothetically the unknown and long
 extinct vertebrate stem-form, an ideal “archetype.”


The outer form of our hypothetical primitive vertebrate was at all
events very simple, and probably more or less similar to that of the
lancelet. The bilateral or bilateral-symmetrical body is stretched out
lengthways and compressed at the sides (Figs. 98–100), oval in section
(Figs. 101, 102). There are no external articulation and no external
appendages, in the shape of limbs, legs, or fins. On the other hand,
the division of the body into two sections, head and trunk, was
probably clearer in _Prospondylus_ than it is in its little-changed
ancestor, the amphioxus. In both animals the fore or head-half of the
body contains different organs from the trunk, and different on the
dorsal from on the ventral side. As this important division is found
even in the sea-squirt, the remarkable invertebrate stem-relative of
the vertebrates, we may assume that it was also found in the
prochordonia, the common ancestors of both stems. It is also very
pronounced in the young larvæ of the cyclostoma; this fact is
particularly interesting, as this palingenetic larva-form is in other
respects also an important connecting-link between the higher
vertebrates and the acrania.

The head of the acrania, or the anterior half of the body (both of the
real amphioxus and the ideal prospondylus), contains the branchial
(gill) gut and heart in the ventral section and the brain and
sense-organs in the dorsal section. The trunk, or posterior half of the
body, contains the hepatic (liver) gut and sexual-glands

in the ventral part, and the spinal marrow and most of the muscles in
the dorsal part.


Figs. 98-102. The ideal primitive vertebrate (prospondylus). Diagram.
Figs. 98–102.—The ideal primitive vertebrate (prospondylus). Diagram.
Fig. 98 side-view (from the left). Fig. 99 back-view. Fig. 100 front
view. Fig. 101 transverse section through the head (to the left through
the gill-pouches, to the right through the gill-clefts). Fig. 102
transverse section of the trunk (to the right a pro-renal canal is
affected). _a_ aorta, _af_ anus, _au_ eye, _b_ lateral furrow
(primitive renal process), _c_ cœloma (body-cavity), _d_ small
intestine, _e_ parietal eye (epiphysis), _f_ fin border of the skin,
_g_ auditory vesicle, _gh_ brain, _h_ heart, _i_ muscular cavity
(dorsal cœlom-pouch), _k_ gill-gut, _ka_ gill-artery, _kg_ gill-arch,
_ks_ gill-folds, _l_ liver, _ma_ stomach, _md_ mouth, _ms_ muscles,
_na_ nose (smell pit), _n_ renal canals, _u_ apertures of same, _o_
outer skin, _p_ gullet, _r_ spinal marrow, a sexual glands (gonads),
_t_ corium, _u_ kidney-openings (pores of the lateral furrow), _v_
visceral vein (chief vein). _x_ chorda, _y_ hypophysis (urinary
appendage), _z_ gullet-groove or gill-groove (hypobranchial groove).


In the longitudinal section of the ideal vertebrate (Fig. 98) we have
in the middle of the body a thin and flexible, but stiff, cylindrical
rod, pointed at both ends (_ch_). It goes the whole length through the
middle of the body, and forms, as the central skeletal axis, the
original structure of the later vertebral column. This is the axial
rod, or _chorda dorsalis,_ also called _chorda vertebralis,_ vertebral
cord, axial cord, dorsal cord, _notochorda,_ or, briefly, _chorda._
This solid, but flexible and elastic, axial rod consists of a
cartilaginous mass of cells, and forms the inner axial skeleton or
central frame of the body; it is only found in vertebrates and
tunicates, not in any other animals. As the first structure of the
spinal column it has the same radical significance in all vertebrates,
from the amphioxus to man. But it is only in the amphioxus and the
cyclostoma that the axial rod retains its simplest form throughout
life. In man and all the higher vertebrates it is found only in the
earlier embryonic period, and is afterwards replaced by the articulated
vertebral column.

The axial rod or chorda is the real solid chief axis of the vertebrate
body, and at the same time corresponds to the ideal long-axis, and
serves to direct us with some confidence in the orientation of the
principal organs. We therefore take the vertebrate-body in its
original, natural disposition, in which the long-axis lies
horizontally, the dorsal side upward and the ventral side downward
(Fig. 98). When we make a vertical section through the whole length of
this long axis, the body divides into two equal and symmetrical halves,
right and left. In each half we have _originally_ the same organs in
the same disposition and connection; only their disposal in relation to
the vertical plane of section, or median plane, is exactly reversed:
the left half is the reflection of the right. We call the two halves
_antimera_ (opposed-parts). In the vertical plane of section that
divides the two halves the sagittal (“arrow”) axis, or “dorsoventral
axis,” goes from the back to the belly, corresponding to the sagittal
seam of the skull. But when we make a horizontal longitudinal section
through the chorda, the whole body divides into a dorsal and a ventral
half. The line of section that passes through the body from right to
left is the transverse, frontal, or lateral axis.

The two halves of the vertebrate body that are separated by this
horizontal transverse axis and by the chorda have quite different
characters. The dorsal half is mainly the animal part of the body, and
contains the greater part of what are called the animal organs, the
nervous system, muscular system, osseous system, etc.—the instruments
of movement and sensation. The ventral half is essentially the
vegetative half of the body, and contains the greater part of the
vertebrate’s vegetal organs, the visceral and vascular systems, sexual
system, etc.—the instruments of nutrition and reproduction. Hence in
the construction of the dorsal half it is chiefly the outer, and in the
construction of the ventral half chiefly the inner, germinal layer that
is engaged. Each of the two halves develops in the shape of a tube, and
encloses a cavity in which another tube is found. The dorsal half
contains the narrow spinal-column cavity or vertebral canal _above_ the
chorda, in which lies the tube-shaped central nervous system, the
medullary tube. The ventral half contains the much more spacious
visceral cavity or body-cavity _underneath_ the chorda, in which we
find the alimentary canal and all its appendages.

The medullary tube, as the central nervous system or psychic organ of
the vertebrate is called in its first stage, consists, in man and all
the higher vertebrates, of two different parts: the large brain,
contained in the skull, and the long spinal cord which stretches from
there over the whole dorsal part of the trunk. Even in the primitive
vertebrate this composition is plainly indicated. The fore half of the
body, which corresponds to the head, encloses a knob-shaped vesicle,
the brain (_gh_); this is prolonged backwards into the thin cylindrical
tube of the spinal marrow (_r_). Hence we find here this very important
psychic organ, which accomplishes sensation, will, and thought, in the
vertebrates, in its simplest form. The thick wall of the nerve-tube,
which runs through the long axis of the body immediately over the axial
rod, encloses a narrow central canal filled with fluid (Figs. 98–102
_r_). We still find the medullary tube in this very simple form for a
time in the embryo of all the vertebrates, and it retains this form in
the amphioxus throughout life;
only in the latter case the cylindrical medullary tube barely indicates
the separation of brain and spinal cord. The lancelet’s medullary tube
runs nearly the whole length of the body, above the chorda, in the
shape of a long thin tube of almost equal diameter throughout, and
there is only a slight swelling of it right at the front to represent
the rudiment of a cerebral lobe. It is probable that this peculiarity
of the amphioxus is connected with the partial atrophy of its head, as
the ascidian larvæ on the one hand and the young cyclostoma on the
other clearly show a division of the vesicular brain, or head marrow,
from the thinner, tubular spinal marrow.

Probably we must trace to the same phylogenetic cause the defective
nature of the sense organs of the amphioxus, which we will describe
later (Chapter XVI). Prospondylus, on the other hand, probably had
three pairs of sense-organs, though of a simple character, a pair of,
or a single olfactory depression, right in front (Figs. 98, 99, _na_),
a pair of eyes (_au_) in the lateral walls of the brain, and a pair of
simple auscultory vesicles (_g_) behind. There was also, perhaps, a
single parietal or “pineal” eye at the top of the skull (_epiphysis,
e_).

In the vertical median plane (or middle plane, dividing the bilateral
body into right and left halves) we have in the acrania, underneath the
chorda, the mesentery and visceral tube, and above it the medullary
tube; and above the latter a membranous partition of the two halves of
the body. With this partition is connected the mass of connective
tissue which acts as a sheath both for the medullary tube and the
underlying chorda, and is, therefore, called the chord-sheath
(_perichorda_); it originates from the dorsal and median part of the
cœlom-pouches, which we shall call the skeleton plate or “sclerotom” in
the craniote embryo. In the latter the chief part of the skeleton—the
vertebral column and skull—develops from this chord-sheath; in the
acrania it retains its simple form as a soft connective matter, from
which are formed the membranous partitions between the various muscular
plates or myotomes (Figs. 98, 99 _ms_).

To the right and left of the cord-sheath, at each side of the medullary
tube and the underlying axial rod, we find in all the vertebrates the
large masses of muscle that constitute the musculature of the trunk and
effect its movements. Although these are very elaborately
differentiated and connected in the developed vertebrate (corresponding
to the various parts of the bony skeleton), in our ideal primitive
vertebrate we can distinguish only two pairs of these principal
muscles, which run the whole length of the body parallel to the chorda.
These are the upper (dorsal) and lower (ventral) lateral muscles of the
trunk. The upper (dorsal) muscles, or the original dorsal muscles (Fig.
102 _ms_), form the thick mass of flesh on the back. The lower
(ventral) muscles, or the original muscles of the belly, form the
fleshy wall of the abdomen. Both sets are segmented, and consist of a
double row of muscular plates (Figs. 98, 99 _ms_); the number of these
myotomes determines the number of joints in the trunk, or metamera. The
myotomes are also developed from the thick wall of the cœlom-pouches
(Fig. 102 _i_).

Outside this muscular tube we have the external envelope of the
vertebrate body, which is known as the corium or cutis. This strong and
thick envelope consists, in its deeper strata, chiefly of fat and loose
connective tissue, and in its upper layers of cutaneous muscles and
firmer connective tissue. It covers the whole surface of the fleshy
body, and is of considerable thickness in all the craniota. But in the
acrania the corium is merely a thin plate of connective tissue, an
insignificant “corium-plate” (_lamella corii,_ Figs. 98–102 _t_).

Immediately above the corium is the outer skin (_epidermis, o_), the
general covering of the whole outer surface. In the higher vertebrates
the hairs, nails, feathers, claws, scales, etc., grow out of this
epidermis. It consists, with all its appendages and products, of simple
cells, and has no blood-vessels. Its cells are connected with the
terminations of the sensory nerves. Originally, the outer skin is a
perfectly simple covering of the outer surface of the body, composed
only of homogeneous cells—a permanent horn-plate. In this simplest
form, as a one-layered epithelium, we find it, at first, in all the
vertebrates, and throughout life in the acrania. It afterwards grows
thicker in the higher vertebrates, and divides into two strata—an
outer, firmer corneous (horn) layer and an inner, softer mucus-layer;
also a number of external and internal appendages grow out of it:
outwardly, the hairs, nails, claws, etc., and
inwardly, the sweat-glands, fat-glands, etc.

It is probable that in our primitive vertebrate the skin was raised in
the middle line of the body in the shape of a vertical fin border
(_f_). A similar fringe, going round the greater part of the body, is
found to-day in the amphioxus and the cyclostoma; we also find one in
the tail of fish-larvæ and tadpoles.

Now that we have considered the external parts of the vertebrate and
the animal organs, which mainly lie in the dorsal half, above the
chorda, we turn to the vegetal organs, which lie for the most part in
the ventral half, below the axial rod. Here we find a large body-cavity
or visceral cavity in all the craniota. The spacious cavity that
encloses the greater part of the _viscera_ corresponds to only a part
of the original cœloma, which we considered in Chapter X; hence it nay
be called the _metacœloma._ As a rule, it is still briefly called the
cœloma; formerly it was known in anatomy as the pleuroperitoneal
cavity. In man and the other mammals (but only in these) this cœloma
divides, when fully developed, into two different cavities, which are
separated by a transverse partition—the muscular diaphragm. The fore or
pectoral cavity (pleura-cavity) contains the œsophagus (gullet), heart,
and lungs; the hind or peritoneal or abdominal cavity contains the
stomach, small and large intestines, liver, pancreas, kidneys, etc. But
in the vertebrate embryo, before the diaphragm is developed, the two
cavities form a single continuous body-cavity, and we find it thus in
all the lower vertebrates throughout life. This body-cavity is clothed
with a delicate layer of cells, the cœlom-epithelium. In the acrania
the cœlom is segmented both dorsally and ventrally, as their muscular
pouches and primitive genital organs plainly show (Fig. 102).

The chief of the viscera in the body-cavity is the alimentary canal,
the organ that represents the whole body in the gastrula. In all the
vertebrates it is a long tube, enclosed in the body-cavity and more or
less differentiated in length, and has two apertures—a mouth for taking
in food (Figs. 98, 100 _md_) and an anus for the ejection of unusable
matter or excrements (_af_). With the alimentary canal a number of
glands are connected which are of great importance for the vertebrate
body, and which all grow out of the canal. Glands of this kind are the
salivary glands, the lungs, the liver, and many smaller glands. Nearly
all these glands are wanting in the acrania; probably there were merely
a couple of simple hepatic tubes (Figs. 98, 100 _l_) in the vertebrate
stem-form. The wall of the alimentary canal and all its appendages
consists of two different layers; the inner, cellular clothing is the
gut-gland-layer, and the outer, fibrous envelope consists of the
gut-fibre-layer; it is mainly composed of muscular fibres which
accomplish the digestive movements of the canal, and of
connective-tissue fibres that form a firm envelope. We have a
continuation of it in the mesentery, a thin, bandage-like layer, by
means of which the alimentary canal is fastened to the ventral side of
the chorda, originally the dorsal partition of the two cœlom-pouches.
The alimentary canal is variously modified in the vertebrates both as a
whole and in its several sections, though the original structure is
always the same, and is very simple. As a rule, it is longer (often
several times longer) than the body, and therefore folded and winding
within the body-cavity, especially at the lower end. In man and the
higher vertebrates it is divided into several sections, often separated
by valves—the mouth, pharynx, œsophagus, stomach, small and large
intestine, and rectum. All these parts develop from a very simple
structure, which originally (throughout life in the amphioxus) runs
from end to end under the chorda in the shape of a straight cylindrical
canal.

As the alimentary canal may be regarded morphologically as the oldest
and most important organ in the body, it is interesting to understand
its essential features in the vertebrate more fully, and distinguish
them from unessential features. In this connection we must particularly
note that the alimentary canal of every vertebrate shows a very
characteristic division into two sections—a fore and a hind chamber.
The fore chamber is the head-gut or branchial gut (Figs. 98–100 _p,
k_), and is chiefly occupied with respiration. The hind section is the
trunk-gut or hepatic gut, which accomplishes digestion (_ma, d_). In
all vertebrates there are formed, at an early stage, to the right and
left in the fore-part of the head-gut, certain special clefts that have
an intimate connection with the original respiratory apparatus of
the vertebrate—the branchial (gill) clefts (_ks_). All the lower
vertebrates, the lancelets, lampreys, and fishes, are constantly taking
in water at the mouth, and letting it out again by the lateral clefts
of the gullet. This water serves for breathing. The oxygen contained in
it is inspired by the blood-canals, which spread out on the parts
between the gill-clefts, the gill-arches (_kg_). These very
characteristic branchial clefts and arches are found in the embryo of
man and all the higher vertebrates at an early stage of development,
just as we find them throughout life in the lower vertebrates. However,
these clefts and arches never act as respiratory organs in the mammals,
birds, and reptiles, but gradually develop into quite different parts.
Still, the fact that they are found at first in the same form as in the
fishes is one of the most interesting proofs of the descent of these
three higher classes from the fishes.

Not less interesting and important is an organ that develops from the
ventral wall in all vertebrates—the gill-groove or hypobranchial
groove. In the acrania and the ascidiæ it consists throughout life of a
glandular ciliated groove, which runs down from the mouth in the
ventral middle line of the gill-gut, and takes small particles of food
to the stomach (Fig. 101 _z_). But in the craniota the thyroid gland
(_thyreoidea_) is developed from it, the gland that lies in front of
the larynx, and which, when pathologically enlarged, forms goitre
(_struma_).

From the head-gut we get not only the gills, the organs of
water-breathing in the lower vertebrates, but also the lungs, the
organs of atmospheric breathing in the five higher classes. In these
cases a vesicular fold appears in the gullet of the embryo at an early
stage, and gradually takes the shape of two spacious sacs, which are
afterwards filled with air. These sacs are the two air-breathing lungs,
which take the place of the water-breathing gills. But the vesicular
invagination, from which the lungs arise, is merely the familiar
air-filled vesicle, which we call the floating-bladder of the fish, and
which alters its specific weight, acting as hydrostatic organ or
floating apparatus. This structure is not found in the lowest
vertebrate classes—the acrania and cyclostoma. We shall see more of it
in Volume II.

The second chief section of the vertebrate-gut, the trunk or liver-gut,
which accomplishes digestion, is of very simple construction in the
acrania. It consists of two different chambers. The first chamber,
immediately behind the gill-gut, is the expanded stomach (_ma_); the
second, narrower and longer chamber, is the straight small intestine
(_d_): it issues behind on the ventral side by the anus (_af_). Near
the limit of the two chambers in the visceral cavity we find the liver,
in the shape of a simple tube or blind sac (_l_); in the amphioxus it
is single; in the prospondylus it was probably double (Figs. 98, 100
_l_).

Closely related morphologically and physiologically to the alimentary
canal is the vascular system of the vertebrate, the chief sections of
which develop from the fibrous gut-layer. It consists of two different
but directly connected parts, the system of blood-vessels and that of
lymph-vessels. In the passages of the one we find red blood, and in the
other colourless lymph. To the lymphatic system belong, first of all,
the lymphatic canals proper or absorbent veins, which are distributed
among all the organs, and absorb the used-up juices from the tissues,
and conduct them into the venous blood; but besides these there are the
chyle-vessels, which absorb the white chyle, the milky fluid prepared
by the alimentary canal from the food, and conduct this also to the
blood.

The blood-vessel system of the vertebrate has a very elaborate
construction, but seems to have had a very simple form in the primitive
vertebrate, as we find it to-day permanently in the annelids (for
instance, earth-worms) and the amphioxus. We accordingly distinguish
first of all as essential, original parts of it two large single
blood-canals, which lie in the fibrous wall of the gut, and run along
the alimentary canal in the median plane of the body, one above and the
other underneath the canal. These principal canals give out numerous
branches to all parts of the body, and pass into each other by arches
before and behind; we will call them the primitive artery and the
primitive vein. The first corresponds to the dorsal vessel, the second
to the ventral vessel, of the worms. The primitive or principal artery,
usually called the aorta (Fig. 98 _a_), lies above the gut in the
middle line of its dorsal side, and conducts oxidised or arterial blood
from the gills to the body. The primitive or principal vein (Fig. 100
_v_) lies below the
gut, in the middle line of its ventral side, and is therefore also
called the vena subintestinalis; it conducts carbonised or venous blood
back from the body to the gills. At the branchial section of the gut in
front the two canals are connected by a number of branches, which rise
in arches between the gill-clefts. These “branchial vascular arches”
(_kg_) run along the gill-arches, and have a direct share in the work
of respiration. The anterior continuation of the principal vein which
runs on the ventral wall of the gill-gut, and gives off these vascular
arches upwards, is the branchial artery (_ka_). At the border of the
two sections of the ventral vessel it enlarges into a contractile
spindle-shaped tube (Figs. 98, 100 _h_). This is the first outline of
the heart, which afterwards becomes a four-chambered pump in the higher
vertebrates and man. There is no heart in the amphioxus, probably owing
to degeneration. In prospondylus the ventral gill-heart probably had
the simple form in which we still find it in the ascidia and the
embryos of the craniota (Figs. 98, 100 _h_).

The kidneys, which act as organs of excretion or urinary organs in all
vertebrates, have a very different and elaborate construction in the
various sections of this stem; we will consider them further in Chapter
2.29. Here I need only mention that in our hypothetical primitive
vertebrate they probably had the same form as in the actual
amphioxus—the primitive kidneys (_protonephra_). These are originally
made up of a double row of little canals, which directly convey the
used-up juices or the urine out of the body-cavity (Fig. 102 _n_). The
inner aperture of these pronephridial canals opens with a ciliated
funnel into the body-cavity; the external aperture opens in lateral
grooves of the epidermis, a couple of longitudinal grooves in the
lateral surface of the outer skin (Fig. 102 _b_). The pronephridial
duct is formed by the closing of this groove to the right and left at
the sides. In all the craniota it develops at an early stage in the
horny plate; in the amphioxus it seems to be converted into a wide
cavity, the atrium, or peribranchial space.

Next to the kidneys we have the sexual organs of the vertebrate. In
most of the members of this stem the two are united in a single
urogenital system; it is only in a few groups that the urinary and
sexual organs are separated (in the amphioxus, the cyclostoma, and some
sections of the fish-class). In man and all the higher vertebrates the
sexual apparatus is made up of various parts, which we will consider in
Chapter XXIX. But in the two lowest classes of our stem, the acrania
and cyclostoma, they consist merely of simple sexual glands or gonads,
the ovaries of the female sex and the testicles (_spermaria_) of the
male; the former provide the ova, the latter the sperm. In the craniota
we always find only one pair of gonads; in the amphioxus several pairs,
arranged in succession. They must have had the same form in our
hypothetical prospondylus (Figs. 98, 100 _s_). These segmental pairs of
gonads are the original ventral halves of the cœlom-pouches.

The organs which we have now enumerated in this general survey, and of
which we have noted the characteristic disposition, are those parts of
the organism that are found in all vertebrates without exception in the
same relation to each other, however much they may be modified. We have
chiefly had in view the transverse section of the body (Figs. 101,
102), because in this we see most clearly the distinctive arrangement
of them. But to complete our picture we must also consider the
segmentation or metamera-formation of them, which has yet been hardly
noticed, and which is seen best in the longitudinal section. In man and
all the more advanced vertebrates the body is made up of a series or
chain of similar members, which succeed each other in the long axis of
the body—the segments or metamera of the organism. In man these
homogeneous parts number thirty-three in the trunk, but they run to
several hundred in many of the vertebrates (such as serpents or eels).
As this internal articulation or metamerism is mainly found in the
vertebral column and the surrounding muscles, the sections or metamera
were formerly called pro-vertebræ. As a fact, the articulation is by no
means chiefly determined and caused by the skeleton, but by the
muscular system and the segmental arrangement of the kidneys and
gonads. However, the composition from these pro-vertebræ or internal
metamera is usually, and rightly, put forward as a prominent character
of the vertebrate, and the manifold division or differentiation of them
is of great importance in the various groups of the vertebrates. But as
far as our present
task—the derivation of the simple body of the primitive vertebrate from
the chordula—is concerned, the articulate parts or metamera are of
secondary interest, and we need not go into them just now.


Fig.103, A, B. C, D. Instances of redundant mammary glands and nipples
(hypermastism). Fig. 103 _A, B, C, D._—Instances of redundant mammary
glands and nipples (_hypermastism_). _A_ a pair of small redundant
breasts (with two nipples on the left) above the large normal ones;
from a 45-year-old Berlin woman, who had had children 17 times (twins
twice). (From _Hansemann._) _B_ the highest number: ten nipples (all
giving milk), three pairs above, one pair below, the large normal
breasts; from a 22-year-old servant at Warschau. (From _Neugebaur._)
_C_ three pairs of nipples: two pairs on the normal glands and one pair
above; from a 19-year-old Japanese girl. _D_ four pairs of nipples: one
pair above the normal and two pairs of small accessory nipples
underneath; from a 22-year-old Bavarian soldier. (From _Wiedersheim._)


The characteristic composition of the vertebrate body develops from the
embryonic structure in the same way in man as in all the other
vertebrates. As all competent experts now admit the monophyletic origin
of the vertebrates on the strength of this significant agreement, and
this “common descent of all the vertebrates from one original
stem-form” is admitted as an historical fact, we have found the answer
to “the question of questions.” We may, moreover, point out that this
answer is just as certain and precise in the case of the origin of man
from the mammals. This advanced vertebrate class is also monophyletic,
or has evolved from one common stem-group of lower vertebrates
(reptiles, and, earlier still, amphibia). This follows from the fact
that the mammals are clearly distinguished from the other classes of
the stem, not merely in one striking particular, but in a whole group
of distinctive characters.

It is only in the mammals that we find the skin covered with hair, the
breast-cavity separated from the abdominal cavity by a complete
diaphragm, and the larynx provided with an epiglottis. The
mammals alone have three small auscultory bones in the tympanic
cavity—a feature that is connected with the characteristic modification
of their maxillary joint. Their red blood-cells have no nucleus,
whereas this is retained in all other vertebrates. Finally, it is only
in the mammals that we find the remarkable function of the breast
structure which has given its name to the whole class—the feeding of
the young by the mother’s milk. The mammary glands which serve this
purpose are interesting in so many ways that we may devote a few lines
to them here.

As is well known, the lower mammals, especially those which beget a
number of young at a time, have several mammary glands at the breast.
Hedgehogs and sows have five pairs, mice four or five pairs, dogs and
squirrels four pairs, cats and bears three pairs, most of the ruminants
and many of the rodents two pairs, each provided with a teat or nipple
(_mastos_). In the various genera of the half-apes (lemurs) the number
varies a good deal. On the other hand, the bats and apes, which only
beget one young at a time as a rule, have only one pair of mammary
glands, and these are found at the breast, as in man.

These variations in the number or structure of the mammary apparatus
(_mammarium_) have become doubly interesting in the light of recent
research in comparative anatomy. It has been shown that in man and the
apes we often find redundant mammary glands (_hyper-mastism_) and
corresponding teats (_hyper-thelism_) in both sexes. Fig. 103 shows
four cases of this kind—_A, B,_ and _C_ of three women, and _D_ of a
man. They prove that all the above-mentioned numbers may be found
occasionally in man. Fig. 103 _A_ shows the breast of a Berlin woman
who had had children seventeen times, and who has a pair of small
accessory breasts (with two nipples on the left one) above the two
normal breasts; this is a common occurrence, and the small soft pad
above the breast is not infrequently represented in ancient statues of
Venus. In Fig. 103 _C_ we have the same phenomenon in a Japanese girl
of nineteen, who has two nipples on each breast besides (three pairs
altogether). Fig. 103 _D_ is a man of twenty-two with four pairs of
nipples (as in the dog), a small pair above and two small pairs beneath
the large normal teats. The maximum number of five pairs (as in the sow
and hedgehog) was found in a Polish servant of twenty-two who had had
several children; milk was given by each nipple; there were three pairs
of redundant nipples above and one pair underneath the normal and very
large breasts (Fig. 103 _B_).

A number of recent investigations (especially among recruits) have
shown that these things are not uncommon in the male as well as the
female sex. They can only be explained by evolution, which attributes
them to atavism and latent heredity. The earlier ancestors of all the
primates (including man) were lower placentals, which had, like the
hedgehog (one of the oldest forms of the living placentals), several
mammary glands (five or more pairs) in the abdominal skin. In the apes
and man only a couple of them are normally developed, but from time to
time we get a development of the atrophied structures. Special notice
should be taken of the arrangement of these accessory mammæ; they form,
as is clearly seen in Fig. 103 _B_ and _D,_ two long rows, which
diverge forward (towards the arm-pit), and converge behind in the
middle line (towards the loins). The milk-glands of the polymastic
lower placentals are arranged in similar lines.

The phylogenetic explanation of polymastism, as given in comparative
anatomy, has lately found considerable support in ontogeny. Hans
Strahl, E. Schmitt, and others, have found that there are always in the
human embryo at the sixth week (when it is three-fifths of an inch
long) the microscopic traces of five pairs of mammary glands, and that
they are arranged at regular distances in two lateral and divergent
lines, which correspond to the mammary lines. Only one pair of them—the
central pair—are normally developed, the others atrophying. Hence there
is for a time in the human embryo a normal hyperthelism, and this can
only be explained by the descent of man from lower primates (lemurs)
with several pairs.

But the milk-gland of the mammal has a great morphological interest
from another point of view. This organ for feeding the young in man and
the higher mammals is, as is known, found in both sexes. However, it is
usually active only in the female sex, and yields the valuable
“mother’s milk”; in the male sex it is
small and inactive, a real rudimentary organ of no physiological
interest. Nevertheless, in certain cases we find the breast as fully
developed in man as in woman, and it may give milk for feeding the
young.

We have a striking instance of this gynecomastism (large milk-giving
breasts in a male) in Fig. 104. I owe the photograph (taken from life)
to the kindness of Dr. Ornstein, of Athens, a German physician, who has
rendered service by a number of anthropological observations, (for
instance, in several cases of tailed men). The gynecomast in question
is a Greek recruit in his twentieth year, who has both normally
developed male organs and very pronounced female breasts. It is
noteworthy that the other features of his structure are in accord with
the softer forms of the female sex. It reminds us of the marble statues
of hermaphrodites which the ancient Greek and Roman sculptors often
produced. But the man would only be a real hermaphrodite if he had
ovaries internally besides the (externally visible) testicles.


Fig.104. A Greek gynecomast. Fig. 104—A Greek gynecomast.


I observed a very similar case during my stay in Ceylon (at Belligemma)
in 1881. A young Cinghalese in his twenty-fifth year was brought to me
as a curious hermaphrodite, half-man and half-woman. His large breasts
gave plenty of milk; he was employed as “male nurse” to suckle a
new-born infant whose mother had died at birth. The outline of his body
was softer and more feminine than in the Greek shown in Fig. 104. As
the Cinghalese are small of stature and of graceful build, and as the
men often resemble the women in clothing (upper part of the body naked,
female dress on the lower part) and the dressing of the hair (with a
comb), I first took the beardless youth to be a woman. The illusion was
greater, as in this remarkable case gynecomastism was associated with
_cryptorchism_—that is to say, the testicles had kept to their original
place in the visceral cavity, and had not travelled in the normal way
down into the scrotum. (Cf. Chapter XXIX.) Hence the latter was very
small, soft, and empty. Moreover, one could feel nothing of the
testicles in the inguinal canal. On the other hand, the male organ was
very small, but normally developed. It was
clear that this apparent hermaphrodite also was a real male.

Another case of practical gynecomastism has been described by Alexander
von Humboldt. In a South American forest he found a solitary settler
whose wife had died in child-birth. The man had laid the new-born child
on his own breast in despair; and the continuous stimulus of the
child’s sucking movements had revived the activity of the mammary
glands. It is possible that nervous suggestion had some share in it.
Similar cases have been often observed in recent years, even among
other male mammals (such as sheep and goats).

The great scientific interest of these facts is in their bearing on the
question of heredity. The stem-history of the mammarium rests partly on
its embryology (Chapter XXIV.) and partly on the facts of comparative
anatomy and physiology. As in the lower and higher mammals (the
monotremes, and most of the marsupials) the whole lactiferous apparatus
is only found in the female; and as there are traces of it in the male
only in a few younger marsupials, there can be no doubt that these
important organs were originally found only in the female mammal, and
that they were acquired by these through a special adaptation to habits
of life.

Later, these female organs were communicated to both sexes by heredity;
and they have been maintained in all persons of either sex, although
they are not physiologically active in the males. This normal
permanence of the female lactiferous organs in _both_ sexes of the
higher mammals and man is independent of any selection, and is a fine
instance of the much-disputed “inheritance of acquired characters.”



Chapter XII.
EMBRYONIC SHIELD AND GERMINATIVE AREA


The three higher classes of vertebrates which we call the amniotes—the
mammals, birds, and reptiles—are notably distinguished by a number of
peculiarities of their development from the five lower classes of the
stem—the animals without an amnion (the _anamnia_). All the amniotes
have a distinctive embryonic membrane known as the amnion (or
“water-membrane”), and a special embryonic appendage—the allantois.
They have, further, a large yelk-sac, which is filled with food-yelk in
the reptiles and birds, and with a corresponding clear fluid in the
mammals. In consequence of these later-acquired structures, the
original features of the development of the amniotes are so much
altered that it is very difficult to reduce them to the palingenetic
embryonic processes of the lower amnion-less vertebrates. The gastræa
theory shows us how to do this, by representing the embryology of the
lowest vertebrate, the skull-less amphioxus, as the original form, and
deducing from it, through a series of gradual modifications, the
gastrulation and cœlomation of the craniota.

It was somewhat fatal to the true conception of the chief embryonic
processes of the vertebrate that all the older embryologists, from
Malpighi (1687) and Wolff (1750) to Baer (1828) and Remak (1850),
always started from the investigation of the hen’s egg, and transferred
to man and the other vertebrates the impressions they gathered from
this. This classical object of embryological research is, as we have
seen, a source of dangerous errors. The large round food-yelk of the
bird’s egg causes, in the first place, a flat discoid expansion of the
small gastrula, and then so distinctive a development of this thin
round embryonic disk that the controversy as to its significance
occupies a large part of embryological literature.

One of the most unfortunate errors that this led to was the idea of an
original
antithesis of germ and yelk. The latter was regarded as a foreign body,
extrinsic to the real germ, whereas it is properly a part of it, an
embryonic organ of nutrition. Many authors said there was no trace of
the embryo until a later stage, and outside the yelk; sometimes the
two-layered embryonic disk itself, at other times only the central
portion of it (as distinguished from the germinative area, which we
will describe presently), was taken to be the first outline of the
embryo. In the light of the gastræa theory it is hardly necessary to
dwell on the defects of this earlier view and the erroneous conclusions
drawn from it. In reality, the first segmentation-cell, and even the
stem-cell itself and all that issues therefrom, belong to the embryo.
As the large original yelk-mass in the undivided egg of the bird only
represents an inclosure in the greatly enlarged ovum, so the later
contents of its embryonic yelk-sac (whether yet segmented or not) are
only a part of the entoderm which forms the primitive gut. This is
clearly shown by the ova of the amphibia and cyclostoma, which explain
the transition from the yelk-less ova of the amphioxus to the large
yelk-filled ova of the reptiles and birds.


Fig.105. Severance of the discoid mammal embryo from the yelk-sac, in
transverse section (diagrammatic). Fig. 105—Severance of the discoid
mammal embryo from the yelk-sac, in transverse section (diagrammatic).
_A_ The germinal disk (_h, hf_) lies flat on one side of the
branchial-gut vesicle (_kb_). _B_ In the middle of the germinal disk we
find the medullary groove (_mr_), and underneath it the chorda (_ch_).
_C_ The gut-fibre-layer (_df_) has been enclosed by the gut-gland-layer
(_dd_). _D_ The skin-fibre-layer (_hf_) and gut-fibre-layer (_df_)
divide at the periphery; the gut (_d_) begins to separate from the
yelk-sac or umbilical vesicle (_nb_). _ E_ The medullary tube (_mr_) is
closed; the body-cavity (_c_) begins to form. _F_ The provertebræ (_w_)
begin to grow round the medullary tube (_mr_) and the chorda (_ch_):
the gut (_d_) is cut off from the umbilical vesicle (_nb_). _H_ The
vertebræ (_w_) have grown round the medullary tube (_mr_) and chorda;
the body-cavity is closed, and the umbilical vesicle has disappeared.
The amnion and serous membrane are omitted. The letters have the same
meaning throughout: _h_ horn-plate, _ mr_ medullary tube, _hf_
skin-fibre-layer, _w_ provertebræ, _ch_ chorda, _c_ body-cavity or
cœloma, _df_ gut-fibre-layer, _dd_ gut-gland-layer, _d_ gut-cavity,
_nb_ umbilical vesicle.


It is precisely in the study of these difficult features that we see
the incalculable value of phylogenetic considerations in explaining
complex ontogenetic facts, and the need of separating cenogenetic
phenomena from palingenetic. This is particularly clear as regards the
comparative embryology of the vertebrates, because here the
phylogenetic unity of the stem has been already established by the
well-known facts of paleontology and comparative anatomy. If this unity
of the stem, on the basis of the amphioxus, were always borne in mind,
we should not have these errors constantly recurring.

In many cases the cenogenetic relation of the embryo to the food-yelk
has until now given rise to a quite wrong idea of
the first and most important embryonic processes in the higher
vertebrates, and has occasioned a number of false theories in
connection with them. Until thirty years ago the embryology of the
higher vertebrates always started from the position that the first
structure of the embryo is a flat, leaf-shaped disk; it was for this
reason that the cell-layers that compose this germinal disk (also
called germinative area) are called “germinal layers.” This flat
germinal disk, which is round at first and then oval, and which is
often described as the tread or cicatricula in the laid hen’s egg, is
found at a certain part of the surface of the large globular food-yelk.
I am convinced that it is nothing else than the discoid, flattened
gastrula of the birds. At the beginning of germination the flat
embryonic disk curves outwards, and separates on the inner side from
the underlying large yelk-ball. In this way the flat layers are
converted into tubes, their edges folding and joining together (Fig.
105). As the embryo grows at the expense of the food-yelk, the latter
becomes smaller and smaller; it is completely surrounded by the
germinal layers. Later still, the remainder of the food-yelk only forms
a small round sac, the yelk-sac or umbilical vesicle (Fig. 105 _nb_).
This is enclosed by the visceral layer, is connected by a thin stalk,
the yelk-duct, with the central part of the gut-tube, and is finally,
in most of the vertebrates, entirely absorbed by this (_H_). The point
at which this takes place, and where the gut finally closes, is the
visceral navel. In the mammals, in which the remainder of the yelk-sac
remains without and atrophies, the yelk-duct at length penetrates the
outer ventral wall. At birth the umbilical cord proceeds from here, and
the point of closure remains throughout life in the skin as the navel.

As the older embryology of the higher vertebrates was mainly based on
the chick, and regarded the antithesis of embryo (or formative-yelk)
and food-yelk (or yelk-sac) as original, it had also to look upon the
flat leaf-shaped structure of the germinal disk as the primitive
embryonic form, and emphasise the fact that hollow grooves were formed
of these flat layers by folding, and closed tubes by the joining
together of their edges.

This idea, which dominated the whole treatment of the embryology of the
higher vertebrates until thirty years ago, was totally false. The
gastræa theory, which has its chief application here, teaches us that
it is the very reverse of the truth. The cup-shaped gastrula, in the
body-wall of which the two primary germinal layers appear from the
first as closed tubes, is the original embryonic form of all the
vertebrates, and all the multicellular invertebrates; and the flat
germinal disk with its superficially expanded germinal layers is a
later, secondary form, due to the cenogenetic formation of the large
food-yelk and the gradual spread of the germ-layers over its surface.
Hence the actual folding of the germinal layers and their conversion
into tubes is not an original and primary, but a much later and
tertiary, evolutionary process. In the phylogeny of the vertebrate
embryonic process we may distinguish the following three stages:—

A. First stage: Primary
(palingenic) embryonic process.     B. Second stage: Secondary
(cenogenetic) embryonic process.     C. Third stage:
Tertiary (cenogenetic) embryonic process. The germinal layers form from
the first closed tubes, the one-layered blastula being converted into
the two-layered gastrula by invagination. No food-yelk.
    (_Amphioxus._)     The germinal layers spread out leaf-wise,
    food-yelk gathering in the ventral entoderm, and a large yelk-sac
    being formed from the middle of the gut-tube. (_Amphibia._)     The
    germinal layers form a flat germinal disk, the borders of which
    join together and form closed tubes, separating from the central
    yelk-sac. (_Amniotes._)

As this theory, a logical conclusion from the gastræa theory, has been
fully substantiated by the comparative study of gastrulation in the
last few decades, we must exactly reverse the hitherto prevalent mode
of treatment. The yelk-sac is not to be treated, as was done formerly,
as if it were originally antithetic to the embryo, but as an essential
part of it, a part of its visceral tube. The primitive gut of the
gastrula has, on this view, been divided into two parts in the higher
animals as a result of the cenogenetic formation of the food-yelk—the
permanent gut (_metagaster_), or permanent alimentary canal, and the
yelk-sac (_lecithoma_), or umbilical vesicle. This is very clearly
shown by the comparative ontogeny of the fishes and amphibia. In these
cases the whole yelk undergoes cleavage at first, and forms a
yelk-gland, composed of yelk-cells, in the ventral wall
of the primitive gut. But it afterwards becomes so large that a part of
the yelk does not divide, and is used up in the yelk-sac that is cut
off outside.

When we make a comparative study of the embryology of the amphioxus,
the frog, the chick, and the rabbit, there cannot, in my opinion, be
any further doubt as to the truth of this position, which I have held
for thirty years. Hence in the light of the gastræa theory we must
regard the features of the amphioxus as the only and real primitive
structure among all the vertebrates, departing very little from the
palingenetic embryonic form. In the cyclostoma and the frog these
features are, on the whole, not much altered cenogenetically, but they
are very much so in the chick, and most of all in the rabbit. In the
bell-gastrula of the amphioxus and in the hooded gastrula of the
lamprey and the frog the germinal layers are found to be closed tubes
or vesicles from the first. On the other hand, the chick-embryo (in the
new laid, but not yet hatched, egg) is a flat circular disk, and it was
not easy to recognise this as a real gastrula. Rauber and Goette have,
however, achieved this. As the discoid gastrula grows round the large
globular yelk, and the permanent gut then separates from the outlying
yelk-sac, we find all the processes which we have shown
(diagrammatically) in Figure 1.108—processes that were hitherto
regarded as principal acts, whereas they are merely secondary.


Figs. 106 and 107. The visceral embryonnic vesicle (blastocystis or
gastrocystis) of a rabbit. Fig. 106—The visceral embryonic vesicle
(_blastocystis_ or _gastrocystis_) of a rabbit (the “blastula” or
_vesicula blastodermica_ of other writers), _a_ outer envelope
(ovolemma), _b_ skin-layer or ectoderm, forming the entire wall of the
yelk-vesicle, _c_ groups of dark cells, representing the visceral layer
or entoderm. Fig. 107—The same in section. Letters as above. _ d_
cavity of the vesicle. (From _Bischoff._)


The oldest, oviparous mammals, the monotremes, behave in the same way
as the reptiles and birds. But the corresponding embryonic processes in
the viviparous mammals, the marsupials and placentals, are very
elaborate and distinctive. They were formerly quite misinterpreted; it
was not until the publication of the studies of Edward van Beneden
(1875) and the later research of Selenka, Kuppfer, Rabl, and others,
that light was thrown on them, and we were in a position to bring them
into line with the principles of the gastræa theory and trace them to
the embryonic forms of the lower vertebrates. Although there is no
independent food-yelk, apart from the formative yelk, in the mammal
ovum, and although its segmentation is total on that account,
nevertheless a large yelk-sac is formed in their embryos, and the
“embryo proper” spreads leaf-wise over its surface, as in the reptiles
and birds, which have a large food-yelk and partial segmentation. In
the mammals, as well as in the latter, the flat, leaf-shaped germinal
disk separates from the yelk-sac, and its edges join together and form
tubes.

How can we explain this curious anomaly? Only as a result of very
characteristic and peculiar cenogenetic modifications of the embryonic
process, the real causes of which must be sought in the change in the
rearing of the young on the part of the viviparous mammals. These are
clearly connected with the fact that the ancestors of the viviparous
mammals were oviparous amniotes like the present monotremes, and only
gradually became viviparous. This can no longer be questioned now that
it has been shown (1884) that the monotremes, the lowest and oldest of
the mammals, still lay eggs, and that these develop like the ova of the
reptiles and birds. Their nearest descendants, the marsupials, formed
the habit of retaining the eggs, and developing them in the
oviduct; the latter was thus converted into a womb (uterus). A
nutritive fluid that was secreted from its wall, and passed through the
wall of the blastula, now served to feed the embryo, and took the place
of the food-yelk. In this way the original food-yelk of the monotremes
gradually atrophied, and at last disappeared so completely that the
partial ovum-segmentation of their descendants, the rest of the
mammals, once more became total. From the _discogastrula_ of the former
was evolved the distinctive _epigastrula_ of the latter.

It is only by this phylogenetic explanation that we can understand the
formation and development of the peculiar, and hitherto totally
misunderstood, blastula of the mammal. The vesicular condition of the
mammal embryo was discovered 200 years ago (1677) by Regner de Graaf.
He found in the uterus of a rabbit four days after impregnation small,
round, loose, transparent vesicles, with a double envelope. However,
Graaf’s discovery passed without recognition. It was not until 1827
that these vesicles were rediscovered by Baer, and then more closely
studied in 1842 by Bischoff in the rabbit (Figs. 106, 107). They are
found in the womb of the rabbit, the dog, and other small mammals, a
few days after copulation. The mature ova of the mammal, when they have
left the ovary, are fertilised either here or in the oviduct
immediately afterwards by the invading sperm-cells.[25] (As to the womb
and oviduct see Chapter XXIX) The cleavage and formation of the
gastrula take place in the oviduct. Either here in the oviduct or after
the mammal gastrula has passed into the uterus it is converted into the
globular vesicle which is shown externally in Fig. 106, and in section
in Fig. 107. The thick, outer, structureless envelope that encloses it
is the original _ ovolemma_ or _zona pellucida,_ modified, and clothed
with a layer of albumin that has been deposited on the outside. From
this stage the envelope is called the external membrane, the _primary
chorion_ or prochorion (_a_). The real wall of the vesicle enclosed by
it consists of a simple layer of ectodermic cells (_b_), which are
flattened by mutual pressure, and generally hexagonal; a light nucleus
shines through their fine-grained protoplasm (Fig. 108). At one part
(_c_) inside this hollow ball we find a circular disc, formed of
darker, softer, and rounder cells, the dark-grained entodermic cells
(Fig. 109).

 [25] In man and the other mammals the fertilisation of the ova
 probably takes place, as a rule, in the oviduct; here the ova, which
 issue from the female ovary in the shape of the Graafian follicle, and
 enter the inner aperture of the oviduct, encounter the mobile
 sperm-cells of the male seed, which pass into the uterus at
 copulation, and from this into the external aperture of the oviduct.
 Impregnation rarely takes place in the ovary or in the womb.



Fig.108. Four entodermic cells from the vesicle of the rabbit. Fig.
109. Two entodermic cells from the embryonic vesicle of the rabbit.
Fig. 108—Four entodermic cells from the embryonic vesicle of the
rabbit. Fig. 109—Two entodermic cells from the embryonic vesicle of the
rabbit.


The characteristic embryonic form that the developing mammal now
exhibits has up to the present usually been called the “blastula”
(Bischoff), “sac-shaped embryo” (Baer), “vesicular embryo” (_vesicula
blastodermica,_ or, briefly, _blastosphæra_). The wall of the hollow
vesicle, which consists of a single layer of cells, was called the
“blastoderm,” and was supposed to be equivalent to the cell-layer of
the same name that forms the wall of the real blastula of the amphioxus
and many of the invertebrates (such as _Monoxenia,_ Fig. 29 _F, G_).
Formerly this real blastula was generally believed to be equivalent to
the embryonic vesicle of the mammal. However, this is by no means the
case. What is called the “blastula” of the mammal and the real blastula
of the amphioxus and many of the invertebrates are totally different
embryonic structures. The latter (blastula) is palingenetic, and
precedes the formation of the gastrula. The former (blastodermic
vesicle) is cenogenetic, and follows gastrulation. The globular wall of
the blastula is a real blastoderm, and consists of homogeneous
(blastodermic) cells; it is not yet differentiated into the two primary
germinal layers. But the globular wall of the mammal vesicle is the
differentiated ectoderm, and at one point in it we find a circular disk
of quite different cells—the entoderm. The round
cavity, filled with fluid, inside the real blastula is the
segmentation-cavity. But the similar cavity within the mammal vesicle
is the yelk-sac cavity, which is connected with the incipient
gut-cavity. This primitive gut-cavity passes directly into the
segmentation-cavity in the mammals, in consequence of the peculiar
cenogenetic changes in their gastrulation, which we have considered
previously (Chapter IX). For these reasons it is very necessary to
recognise the secondary embryonic vesicle in the mammal (_gastrocystis_
or _blastocystis_) as a characteristic structure peculiar to this
class, and distinguish it carefully from the primary blastula of the
amphioxus and the invertebrates.


Fig.110. Ovum of a rabbit from the uterus, one-sixth of an inch in
diameter. Fig. 111. The same ovum, seen in profile. Fig. 112. Ovum of a
rabbit from the uterus, one-fourth of an inch in diameter. Fig. 113.
The same ovum, seen in profile. Fig. 114. Ovum of a rabbit from the
uterus, one-third of an inch in diameter. Fig. 110—Ovum of a rabbit
from the uterus, one sixth of an inch in diameter. The embryonic
vesicle (_b_) has withdrawn a little from the smooth ovolemma (_a_). In
the middle of the ovolemma we see the round germinal disk
(blastodiscus, _c_), at the edge of which (at _d_) the inner layer of
the embryonic vesicle is already beginning to expand. (Figs. 110–114
from _ Bischoff._ Fig. 111—The same ovum, seen in profile. Letters as
in Fig. 110. Fig. 112—Ovum of a rabbit from the uterus, one-fourth of
an inch in diameter. The blastoderm is already for the most part
two-layered (_b_). The ovolemma, or outer envelope, is tufted (_a_).
Fig. 113—The same ovum, seen in profile. Letters as in Fig. 112. Fig.
114—Ovum of a rabbit from the uterus, one-third of an inch in diameter.
The embryonic vesicle is now nearly everywhere two-layered (_k_) only
remaining one-layered below (at _d_).


The small, circular, whitish, and opaque spot which the gastric disk
(Fig. 106) forms at a certain part of the surface of the clear and
transparent embryonic vesicle has long been known to science, and
compared to the germinal disk of the birds and reptiles. Sometimes it
has been called the germinal disk, sometimes the germinal spot, and
usually the germinative area. From the area the further development of
the embryo proceeds. However, the larger part of the embryonic vesicle
of the mammal is not directly used for building up the later body, but
for the construction of the temporary umbilical vesicle. The embryo
separates from this in proportion as it grows at its expense; the two
are only connected by the yelk-duct (the stalk of the yelk-sac), and
this maintains the direct communication between the cavity of the
umbilical vesicle and the forming visceral cavity (Fig. 105).


Fig.115. Round germinative area of the rabbit. Fig. 116. Oval area,
with the opaque whitish border of the dark area without. Fig. 115—Round
germinative area of the rabbit, divided into the central light area
(_area pellucida_) and the peripheral dark area (_area opaca_). The
light area seems darker on account of the dark ground appearing through
it. Fig. 116—Oval area, with the opaque whitish border of the dark area
without.


The germinative area or gastric disk of the animal consists at first
(like the germinal disk of birds and reptiles) merely of the two
primary germinal layers, the ectoderm and entoderm. But soon there
appears in the middle of the circular disk between the two a third
stratum of cells, the rudiment of the middle layer or fibrous layer
(_mesoderm_). This middle germinal layer consists from the first, as we
have seen in Chapter X, of two separate epithelial plates, the two
layers of the cœlom-pouches (parietal and visceral). However, in all
the amniotes (on account of the large formation of yelk) these thin
middle plates are so firmly pressed together that they seem to
represent a single layer. It is thus peculiar to the amniotes that the
middle of the germinative area is composed of four germinal layers, the
two limiting (or primary) layers and the middle layers between them
(Figs. 96, 97). These four secondary germinal layers can be clearly
distinguished as soon as what is called the sickle-groove (or
“embryonic sickle”) is seen at the hind border of the germinative area.
At the borders, however, the germinative area of the mammal only
consists of two layers. The rest of the wall of the embryonic vesicle
consists at first (but only for a short time in most of the mammals) of
a single layer, the outer germinal layer.

From this stage, however, the whole wall of the embryonic vesicle
becomes two-layered. The middle of the germinative area is much
thickened by the growth of the cells of the middle layers, and the
inner layer expands at the same time, and increases at the border of
the disk all round. Lying close on the outer layer throughout, it grows
over its inner surface at all points, covers first the upper and then
the lower hemisphere, and at last closes in the middle of the inner
layer (Figs. 110–114). The wall of the embryonic vesicle now consists
throughout of two layers of cells, the ectoderm without and the
entoderm within. It is only in the centre of the circular area, which
becomes thicker and thicker through the growth of the middle layers,
that it is made up of all four layers. At the same time, small
structureless tufts or warts are deposited on the surface of the outer
ovolemma or prochorion, which has been raised above the embryonic
vesicle (Figs. 112–114 _a_).

We may now disregard both the outer ovolemma and the greater part of
the vesicle, and concentrate our attention on the germinative area and
the four-layered embryonic disk. It is here alone that we find the
important changes which lead to the differentiation of the first
organs. It is immaterial whether we examine the germinative area of the
mammal (the rabbit, for instance) or the germinal disk of a bird or a
reptile (such as a lizard or tortoise). The embryonic processes we are
now going to consider are essentially the same in all members of the
three higher classes of vertebrates which we call the amniotes. Man is
found to agree in this respect with the rabbit, dog, ox, etc.; and in
all these animals the germinative area undergoes essentially the same
changes as in the birds and reptiles. They are most frequently and
accurately studied in the chick, because we can have incubated hens’
eggs in any quantity at any stage of development. Moreover, the round
germinal disk of the chick passes immediately after the beginning of
incubation (within a few hours) from the two-layered to the
four-layered stage, the two-layered mesoderm developing from the median
primitive groove between the ectoderm and entoderm (Figs. 82–95).


Fig.117. Oval germinal disk of the rabbit, magnified. Fig. 118.
Pear-shaped germinal shield of the rabbit (eight days old), magnified.
Fig. 117—Oval germinal disk of the rabbit, magnified. As the delicate,
half-transparent disk lies on a black ground, the pellucid area looks
like a dark ring, and the opaque area (lying outside it) like a white
ring. The oval shield in the centre also looks whitish, and in its axis
we see the dark medullary groove. (From _ Bischoff._) Fig.
118—Pear-shaped germinal shield of the rabbit (eight days old),
magnified. _rf_ medullary groove. _pr_ primitive groove (primitive
mouth). (From _ Kölliker._


The first change in the round germinal disk of the chick is that the
cells at its edges multiply more briskly, and form darker nuclei in
their protoplasm. This gives rise to a dark ring, more or less sharply
set off from the lighter centre of the germinal disk (Fig. 115). From
this point the latter takes the name of the “light area” (_area
pellucida_), and the darker ring is called the “dark area” (_area
opaca_). (In a strong light, as in Figs. 115–117, the light area seems
dark, because the dark ground is seen through it; and the dark area
seems whiter). The circular shape of the area now changes into
elliptic, and then immediately into oval (Figs. 116, 117). One end
seems to be broader and blunter, the other narrower and more pointed;
the former corresponds to the anterior and the latter to the posterior
section of the subsequent body. At the same time, we can already trace
the characteristic bilateral form of the body, the antithesis of right
and left, before and behind. This will be made clearer by the
“primitive streak,” which appears at the posterior end.

At an early stage an opaque spot is seen in the middle of the clear
germinative
area, and this also passes from a circular to an oval shape. At first
this shield-shaped marking is very delicate and barely perceptible; but
it soon becomes clearer, and now stands out as an oval shield,
surrounded by two rings or areas (Fig. 117). The inner and brighter
ring is the remainder of the pellucid area, and the dark outer ring the
remainder of the opaque area; the opaque shield-like spot itself is the
first rudiment of the dorsal part of the embryo. We give it briefly the
name of embryonic shield or dorsal shield. In most works this embryonic
shield is described as “the first rudiment or trace of the embryo,” or
“primitive embryo.” But this is wrong, though it rests on the authority
of Baer and Bischoff.


Fig.119. Median longitudinal section of the gastrula of four
vertebrates. Fig. 119—Median longitudinal section of the gastrula of
four vertebrates. (From _Rabl._) _A_ discogastrula of a shark
(_Pristiurus_). _B_ amphigastrula of a sturgeon (_Accipenser_). _C_
amphigastrula of an amphibium (_Triton_). _D_ epigastrula of an amniote
(diagram). _ a_ ventral, _b_ dorsal lip of the primitive mouth.


As a matter of fact, we already have the embryo in the stem-cell, the
gastrula, and all the subsequent stages. The embryonic shield is simply
the first rudiment of the dorsal part, which is the earliest to
develop. As the older names of “embryonic rudiment” and “germinative
area” are used in many different senses—and this has led to a fatal
confusion in embryonic literature—we must explain very clearly the real
significance of these important embryonic parts of the amniote. It will
be useful to do so in a series of formal principles:—

1. The so-called ”first trace of the embryo” in the amniotes, or the
embryonic shield, in the centre of the pellucid area, consists merely
of an early differentiation and formation of the middle dorsal parts.

2. Hence the best name for it is ”the dorsal shield,” as I proposed
long ago.

3. The germinative area, in which the first embryonic blood-vessels
appear at an early stage, is not opposed as an external area to the
”embryo proper,” but is a part of it.

4. In the same way, the yelk-sac or the umbilical vesicle is not a
foreign external
appendage of the embryo, but an outlying part of its primitive gut.

5. The dorsal shield gradually separates from the germinative area and
the yelk-sac, its edges growing downwards and folding together to form
ventral plates.

6. The yelk-sac and vessels of the germinative area, which soon spread
over its whole surface, are, therefore, real embryonic organs, or
temporary parts of the embryo, and have a transitory importance in
connection with the nutrition of the growing later body; the latter may
be called the ”permanent body” in contrast to them.

The relation of these cenogenetic features of the amniotes to the
palingenetic structures of the older non-amniotic vertebrates may be
expressed in the following theses: The original gastrula, which
completely passes into the embryonic body in the acrania, cyclostoma,
and amphibia, is early divided into two parts in the amniotes—the
embryonic shield, which represents the dorsal outline of the permanent
body; and the temporary embryonic organs of the germinative area and
its blood-vessels, which soon grow over the whole of the yelk-sac. The
differences which we find in the various classes of the vertebrate stem
in these important particulars can only be fully understood when we
bear in mind their phylogenetic relations on the one hand, and, on the
other, the cenogenetic modifications of structure that have been
brought about by changes in the rearing of the young and the variation
in the mass of the food-yelk.

We have already described in Chapter IX the changes which this increase
and decrease of the nutritive yelk causes in the form of the gastrula,
and especially in the situation and shape of the primitive mouth. The
primitive mouth or prostoma is originally a simple round aperture at
the lower pole of the long axis; its dorsal lip is above and ventral
lip below. In the amphioxus this primitive mouth is a little eccentric,
or shifted to the dorsal side (Fig. 39). The aperture increases with
the growth of the food-yelk in the cyclostoma and ganoids; in the
sturgeon it lies almost on the equator of the round ovum, the ventral
lip (_a_) in front and the dorsal lip (_b_) behind (Fig. 119 _b_). In
the wide-mouthed, circular discoid gastrula of the selachii or
primitive fishes, which spreads quite flat on the large food-yelk, the
anterior semi-circle of the border of the disk is the ventral, and the
posterior semicircle the dorsal lip (Fig. 119 _A_). The amphiblastic
amphibia are directly connected with their earlier fish-ancestors, the
dipneusts and ganoids, and further the oldest selachii (_Cestracion_);
they have retained their total unequal segmentation, and their small
primitive mouth (Fig. 119 _ C, ab_), blocked up by the yelk-stopper,
lies at the limit of the dorsal and ventral surface of the embryo (at
the lower pole of its equatorial axis), and there again has an upper
dorsal and a lower ventral lip (_a, b_). The formation of a large
food-yelk followed again in the stem-forms of the amniotes, the
protamniotes or proreptilia, descended from the amphibia (Fig. 119
_D_). But here the accumulation of the food-yelk took place only in the
ventral wall of the primitive-gut, so that the narrow primitive mouth
lying behind was forced upwards, and came to lie on the back of the
discoid ”epigastrula” in the shape of the ”primitive groove”; thus (in
contrast to the case of the selachii, Fig. 119 _A_) the dorsal lip
(_b_) had to be in front, and the ventral lip (_a_) behind (Fig. 119 _
D_). This feature was transmitted to all the amniotes, whether they
retained the large food-yelk (reptiles, birds, and monotremes), or lost
it by atrophy (the viviparous mammals).

This phylogenetic explanation of gastrulation and cœlomation, and the
comparative study of them in the various vertebrates, throw a clear and
full light on many ontogenetic phenomena, as to which the most obscure
and confused opinions were prevalent thirty years ago. In this we see
especially the high scientific value of the biogenetic law and the
careful separation of palingenetic from cenogenetic processes. To the
opponents of this law the real explanation of these remarkable
phenomena is impossible. Here, and in every other part of embryology,
the true key to the solution lies in phylogeny.



Chapter XIII.
DORSAL BODY AND VENTRAL BODY


The earliest stages of the human embryo are, for the reasons already
given, either quite unknown or only imperfectly known to us. But as the
subsequent embryonic forms in man behave and develop just as they do in
all the other mammals, there cannot be the slightest doubt that the
preceding stages also are similar. We have been able to see in the
cœlomula of the human embryo (Fig. 97), by transverse sections through
its primitive mouth, that its two cœlom-pouches are developed in just
the same way as in the rabbit (Fig. 96); moreover, the peculiar course
of the gastrulation is just the same.


Fig.120. Embryonic vesicle of a seven-days-old rabbit with oval
embryonic shield (ag). Fig. 120—Embryonic vesicle of a seven-days-old
rabbit with oval embryonic shield (_ag_). _A_ seen from above, _B_ from
the side. (From _Kölliker._) _ag_ dorsal shield or embryonic spot. In
_B_ the upper half of the vesicle is made up of the two primary
germinal layers, the lower (up to _ge_) only from the outer layer.


The germinative area forms in the human embryo in the same way as in
the other mammals, and in the middle part of this we have the embryonic
shield, the purport of which we considered in Chapter XII. The next
changes in the embryonic disk, or the “embryonic spot,” take place in
corresponding fashion. These are the changes we are now going to
consider more closely.

The chief part of the oval embryonic shield is at first the narrow
hinder end; it is in the middle line of this that the primitive streak
appears (Fig. 121 _ps_).The narrow longitudinal groove in it—the
so-called “primitive groove”—is, as we have seen, the primitive mouth
of the gastrula. In the gastrula-embryos of the mammals, which are much
modified cenogenetically, this cleft-shaped prostoma is lengthened so
much that it soon traverses the whole of the hinder half of the dorsal
shield; as we find in a rabbit embryo of six to eight days (Fig. 122
_pr_). The two swollen parallel borders that limit this median furrow
are the side lips of the primitive mouth, right and left. In this way
the bilateral-symmetrical type of the vertebrate becomes pronounced.
The subsequent head of the amniote is developed from the broader and
rounder fore-half of the dorsal shield.

In this fore-half of the dorsal shield a median furrow quickly makes
its appearance (Fig. 123 _rf_). This is the broader dorsal furrow or
medullary groove, the first beginning of the central nervous system.
The two parallel dorsal or medullary swellings that enclose it grow
together over it afterwards, and form the medullary tube. As is seen in
transverse sections, it is formed only of the outer germinal layer
(Figs. 95 and 136). The lips of the primitive mouth, however, lie, as
we know, at the important point where the outer layer bends over the
inner, and from which the two cœlom pouches grow between the primary
germinal layers.


Fig.121. Oval embryonic shield of the rabbit. Fig. 121—Oval embryonic
shield of the rabbit (_A_ of six days eighteen hours, _B_ of eight
days). (From _Kölliker._) _ps_ primitive streak, _pr_ primitive groove,
_arg_ area germinalis, _sw_ sickle-shaped germinal growth.


Fig.122. Dorsal shield (ag) and germinative area of a rabbit-embryo of
eight days. Fig. 123. Embryonic shield of a rabbit of eight days. Fig.
122—Dorsal shield (_ag_) and germinative area of a rabbit-embryo of
eight days. (From _Kölliker._) _pr_ primitive groove, _rf_ dorsal
furrow. Fig. 123.—Embryonic shield of a rabbit of eight days. (From
_Van Beneden._) _pr_ primitive groove, _cn_ canalis neurentericus, _nk_
nodus neurentericus (or “Hensen’s ganglion”), _kf_ head-process
(chorda).


Thus the median primitive furrow (_pr_)
in the hind-half and the median medullary furrow (_Rf_) in the
fore-half of the oval shield are totally different structures, although
the latter seems to a superficial observer to be merely the forward
continuation of the former. Hence they were formerly always confused.
This error was the more pardonable as immediately afterwards the two
grooves do actually pass into each other in a very remarkable way. The
point of transition is the remarkable neurenteric canal (Fig. 124
_cn_). But the direct connection which is thus established does not
last long; the two are soon definitely separated by a partition.


Fig.124. Longitudinal section of the coelomula of amphioxus. Fig.
124—Longitudinal section of the cœlomula of amphioxus (from the left).
_i_ entoderm, _d_ primitive gut, _cn_ medullary duct, _n_ nerve tube,
_m_ mesoderm, _s_ first primitive segment, _c_ cœlom-pouches. (From
_Hatschek._)


The enigmatic _neurenteric canal_ is a very old embryonic organ, and of
great phylogenetic interest, because it arises in the same way in all
the chordonia (both tunicates and vertebrates). In every case it
touches or embraces like an arch the posterior end of the chorda, which
has been developed here in front out of the middle line of the
primitive gut (between the two cœlom-folds of the sickle groove)
(“head-process,” Fig. 123 _kf_). These very ancient and strictly
hereditary structures, which have no physiological significance to-day,
deserve (as “rudimentary organs”) our closest attention. The tenacity
with which the useless neurenteric canal has been transmitted down to
man through the whole series of vertebrates is of equal interest for
the theory of descent in general, and the phylogeny of the chordonia in
particular.

The connection which the neurenteric canal (Fig. 123 _cn_) establishes
between the dorsal nerve-tube (_n_) and the ventral gut-tube (_d_) is
seen very plainly in the amphioxus in a longitudinal section of the
cœlomula, as soon as the primitive mouth is completely closed at its
hinder end. The medullary tube has still at this stage an opening at
the forward end, the neuroporus Fig. 83 _np_). This opening also is
afterwards closed. There are then two completely closed canals over
each other—the medullary tube above and the gastric tube below, the two
being separated by the chorda. The same features as in the acrania are
exhibited by the related tunicates, the ascidiæ.


Fig.125. Longitudinal section of the chordula of a frog. Fig.
125—Longitudinal section of the chordula of a frog. (From _Balfour._)
_nc_ nerve-tube, _x_ canalis neurentericus, _al_ alimentary canal, _yk_
yelk-cells, _m_ mesoderm.


Again, we find the neurenteric canal in just the same form and
situation in the amphibia. A longitudinal section of a young tadpole
(Fig. 125) shows how we may penetrate from the still open primitive
mouth (_x_) either into the wide primitive gut-cavity (_al_) or the
narrow overlying nerve-tube. A little later, when the primitive mouth
is closed, the narrow neurenteric canal (Fig. 126 _ne_) represents the
arched connection between the dorsal medullary canal (_mc_) and the
ventral gastric canal.

In the amniotes this original curved form of the neurenteric canal
cannot be found at first, because here the primitive mouth travels
completely over to the dorsal surface of the gastrula, and is converted
into the longitudinal furrow we call the primitive groove. Hence the
primitive groove (Fig. 128 _pr_), examined from above, appears to be
the straight
continuation of the fore-lying and younger medullary furrow (_me_). The
divergent hind legs of the latter embrace the anterior end of the
former. Afterwards we have the complete closing of the primitive mouth,
the dorsal swellings joining to form the medullary tube and growing
over it. The neurenteric canal then leads directly, in the shape of a
narrow arch-shaped tube (Fig. 129 _ne_), from the medullary tube (_sp_)
to the gastric tube (_pag_). Directly in front of it is the latter end
of the chorda (_cli_).


Fig.126. Longitudinal section of a frog-embryo. Fig. 126—Longitudinal
section of a frog-embryo. (From _Goette._) _m_ mouth, _l_ liver, _an_
anus, _ne_ canalis neurentericus, _mc_ medullary-tube, _pn_ pineal body
(epiphysis), _ch_ chorda.


While these important processes are taking place in the axial part of
the dorsal shield, its external form also is changing. The oval form
(Fig. 117) becomes like the sole of a shoe or sandal, lyre-shaped or
finger-biscuit shaped (Fig. 130). The middle third does not grow in
width as quickly as the posterior, and still less than the anterior
third; thus the shape of the permanent body becomes somewhat narrow at
the waist. At the same time, the oval form of the germinative area
returns to a circular shape, and the inner pellucid area separates more
clearly from the opaque outer area (Fig. 131 _a_). The completion of
the circle in the area marks the limit of the formation of
blood-vessels in the mesoderm.


Figs. 127 and 128. Dorsal shield of the chick. Figs. 127 and 128—Dorsal
shield of the chick. (From _Balfour._) The medullary furrow (_me_),
which is not yet visible in Fig. 130, encloses with its hinder end the
fore end of the primitive groove (_pr_) in Fig. 131.)


The characteristic sandal-shape of the dorsal shield, which is
determined by the narrowness of the middle part, and which is compared
to a violin, lyre, or shoe-sole, persists for a long time in all the
amniotes. All mammals, birds, and reptiles have substantially the same
construction at this stage, and even for a longer or shorter
period after the division of the primitive segments into the
cœlom-folds has begun (Fig. 132). The human embryonic shield assumes
the sandal-form in the second week of development; towards the end of
the week our sole-shaped embryo has a length of about one-twelfth of an
inch (Fig. 133).


Fig.129. Longitudinal section of the hinder end of a chick. Fig.
129—Longitudinal section of the hinder end of a chick. (From
_Balfour._) _sp_ medullary tube, connected with the terminal gut
(_pag_) by the neurenteric canal (_ne_), _ch_ chorda, _pr_ neurenteric
(or Hensen’s) ganglion, _al_ allantois, _ep_ ectoderm, _hy_ entoderm,
_so_ parietal layer, _sp_ visceral layer, _an_ anus-pit, _am_ amnion.


The complete bilateral symmetry of the vertebrate body is very early
indicated in the oval form of the embryonic shield (Fig. 117) by the
median primitive streak; in the sandal-form it is even more pronounced
(Figs. 131–135). In the lateral parts of the embryonic shield a darker
central and a lighter peripheral zone become more obvious; the former
is called the stem-zone (Fig. 134 _stz_), and the latter the parietal
zone (_pz_); from the first we get the dorsal and from the second the
ventral half of the body-wall. The stem-zone of the amniote embryo
would be called more appropriately the dorsal zone or dorsal shield;
from it develops the whole of the dorsal half of the later body (or
permanent body)—that is to say, the dorsal body (_episoma_). Again, it
would be better to call the “parietal zone” the ventral zone or ventral
shield; from it develop the ventral “lateral plates,” which afterwards
separate from the embryonic vesicle and form the ventral body
(_hyposoma_)—that is to say, the ventral half of the permanent body,
together with the body-cavity and the gastric canal that it encloses.


Fig.130. Germinal area or germinal disk of the rabbit, with sole-shaped
embryonic shield. Fig. 130—Germinal area or germinal disk of the
rabbit, with sole-shaped embryonic shield, magnified. The clear
circular field (_d_) is the opaque area. The pellucid area (_c_) is
lyre-shaped, like the embryonic shield itself (_b_). In its axis is
seen the dorsal furrow or medullary furrow (_a_). (From _Bischoff_.


The sole-shaped germinal shields of all the amniotes are still, at the
stage of construction which Fig. 134 illustrates in the rabbit and Fig.
135 in the opossum, so like each other that we can either not
distinguish them at all or only by means of quite subordinate
peculiarities in the size of the various parts. Moreover, the human
sandal-shaped embryo cannot at this stage be distinguished from those
of other mammals, and it particularly resembles that of the rabbit. On
the other hand, the outer form of these flat sandal-shaped embryos is
very different from the corresponding form of the lower animals,
especially the acrania (amphioxus). Nevertheless, the body is just the
same in the essential features of its structure as that we find in the
chordula of the latter (Figs. 83–86), and in the embryonic forms which
immediately develop from it. The striking external difference is here
again due to the fact that in the palingenetic embryos of the amphioxus
(Figs. 83, 84) and the amphibia (Figs. 85, 86) the gut-wall and
body-wall form closed tubes from the first, whereas in the cenogenetic
embryos of the amniotes they are forced to expand leaf-wise on the
surface owing to the great extension of the food-yelk.

It is all the more notable that the early separation of dorsal and
ventral halves takes place in the same rigidly hereditary fashion in
all the vertebrates. In both the acrania and the craniota the dorsal
body is about this period separated from the ventral body. In the
middle part of the body this division has already taken place by the
construction of the chorda between the dorsal nerve-tube and the
ventral canal. But in the outer or lateral part of the body it is only
brought about by the division of the coelom-pouches into two sections—a
dorsal _episomite_ (dorsal segment or provertebra) and a ventral
_hyposomite_ (or ventral segment) by a frontal constriction. In the
amphioxus each of the former makes a muscular pouch, and each of the
latter a sex-pouch or gonad.


Fig.131. Embryo of the opossum, sixty hours old. Fig. 132.
Sandal-shaped embryonic shield of a rabbit of eight days. Fig.
131—Embryo of the opossum, sixty hours old, one-sixth of an inch in
diameter. (From _Selenka_) _b_ the globular embryonic vesicle, _a_ the
round germinative area, _b_ limit of the ventral plates, _r_ dorsal
shield, _v_ its fore part, _u_ the first primitive segment, _ch_
chorda, _chr_ its fore-end, _pr_ primitive groove (or mouth). Fig.
132—Sandal-shaped embryonic shield of a rabbit of eight days, with the
fore part of the germinative area (_ao_ opaque, _ap_ pellucid area).
(From _Kölliker._) _rf_ dorsal furrow, in the middle of the medullary
plate, _h, pr_ primitive groove (mouth), _stz_ dorsal (stem) zone, _pz_
ventral (parietal) zone. In the narrow middle part the first three
primitive segments may be seen.


These important processes of differentiation in the mesoderm, which we
will consider more closely in the next chapter, proceed step by step
with interesting changes in the ectoderm, while the entoderm changes
little at first. We can study these processes best in transverse
sections, made vertically to the surface through the sole-shaped
embryonic shield. Such a transverse section of a chick embryo, at the
end of the first day of incubation, shows the gut-gland layer as a very
simple epithelium, which is spread like a leaf over the outer surface
of the food-yelk (Fig. 92). The chorda (_ch_) has separated from the
dorsal middle line of the entoderm; to the right and left of it are the
two halves of the mesoderm, or the two cœlom-folds. A narrow cleft in
the latter indicates the body-cavity (_uwh_); this separates the two
plates of the cœlom-pouches, the lower (visceral) and upper (parietal).
The broad dorsal furrow (_rf_) formed by the medullary plate (_m_) is
still wide open, but is divided from the lateral horn-plate
(_h_) by the parallel medullary swellings, which eventually close.


Fig.133. Human embryo at the sandal-stage. Fig. 134. Sandal-shaped
embryonic shield of a rabbit of nine days. Fig. 133—Human embryo at the
sandal-stage, one-twelfth of an inch long, from the end of the second
week, magnified. (From _Count Spee._) Fig. 134—Sandal-shaped embryonic
shield of a rabbit of nine days. (From _Kölliker._) (Back view from
above.) _stz_ stem-zone or dorsal shield (with eight pairs of primitive
segments), _pz_ parietal or ventral zone, _ap_ pellucid area, _af_
amnion-fold, _h_ heart, _ph_ pericardial cavity, _vo_
omphalo-mesenteric vein, _ab_ eye-vesicles, _vh_ fore brain, _mh_
middle brain, _hh_ hind brain, _uw_ primitive segments (or vertebræ).


During these processes important changes are taking place in the outer
germinal layer (the “skin-sense layer”). The continued rise and growth
of the dorsal swellings causes their higher parts to bend together at
their free borders, approach nearer and nearer (Fig. 136 _w_), and
finally unite. Thus in the end we get from the open dorsal furrow, the
upper cleft of which becomes narrower and narrower, a closed
cylindrical tube (Fig. 137 _mr_). This tube is of the utmost
importance; it is the beginning of the central nervous system, the
brain and spinal marrow, the _medullary tube._ This embryonic fact was
formerly looked upon as very mysterious. We shall see presently that in
the light of the theory of descent it is a thoroughly natural process.
The phylogenetic explanation of it is that the central nervous system
is the organ by means of which all intercourse with the outer world,
all psychic action and sense-perception, are accomplished; hence it was
bound to develop originally from the outer and upper surface of the
body, or from the outer skin. The medullary tube afterwards separates
completely from the outer germinal layer, and is surrounded by the
middle parts of the provertebræ and forced inwards (Fig. 146).The
remaining portion of the skin-sense layer (Fig. 93 _h_) is now called
the horn-plate or horn-layer, because from it is developed the whole of
the outer skin or epidermis, with all its horny appendages (nails,
hair, etc.).

A totally different organ, the _prorenal_
(primitive kidney) _duct_ (_ung_), is found to be developed at an early
stage from the ectoderm. This is originally a quite simple,
tube-shaped, lengthy duct, or straight canal, which runs from front to
rear at each side of the provertebræ (on the outer side, Fig. 93
_ung_). It originates, it seems, out of the horn-plate at the side of
the medullary tube, in the gap that we find between the provertebral
and the lateral plates. The prorenal duct is visible in this gap even
at the time of the severance of the medullary tube from the horn-plate.
Other observers think that the first trace of it does not come from the
skin-sense layer, but the skin-fibre layer.


Fig.135. Sandal-shaped embryonic shield of an opossum. Fig.
135—Sandal-shaped embryonic shield of an opossum (_Didelphys_), three
days old. (From _Selenka._) (Back view from above.) _stz_ stem-zone or
dorsal shield (with eight pairs of primitive segments), _pz_ parietal
or ventral zone, _ap_ pellucid area, _ao_ opaque area, _hh_ halves of
the heart, _v_ fore-end, _h_ hind-end. In the median line we see the
chorda (_ch_) through the transparent medullary tube (_m_). _u_
primitive segment, _pr_ primitive streak (or primitive mouth).


The inner germinal layer, or the gut-fibre layer (Fig. 93 _dd_),
remains unchanged during these processes. A little later, however, it
shows a quite flat, groove-like depression in the middle line of the
embryonic shield, directly under the chorda. This depression is called
the gastric groove or furrow. This at once indicates the future lot of
this germinal layer. As this ventral groove gradually deepens, and its
lower edges bend towards each other, it is formed into a closed tube,
the alimentary canal, in the same way as the medullary groove grows
into the medullary tube. The gut-fibre layer (Fig. 137 _f_), which lies
on the gut-gland layer (_d_), naturally follows it in its folding.
Moreover, the incipient gut-wall consists from the first of two layers,
internally the gut-gland layer and externally the gut-fibre layer.

The formation of the alimentary canal resembles that of the medullary
tube to this extent—in both cases a straight groove or furrow arises
first of all in the middle line of a flat layer. The edges of this
furrow then bend towards each other, and join to form a tube (Fig.
137). But the two processes are really very different. The medullary
tube closes in its whole length, and forms a cylindrical tube, whereas
the alimentary canal remains open in the middle, and its cavity
continues for a long time in connection with the cavity of the
embryonic vesicle. The open connection between the two cavities is only
closed at a very late stage, by the construction of the navel. The
closing of the medullary tube is effected from both sides, the edges of
the groove joining together from right and left. But the closing of the
alimentary canal is not only effected from right and left, but also
from front and rear, the edges of the ventral groove growing together
from every side towards the navel. Throughout the three higher classes
of vertebrates the whole of this process of the construction of the gut
is closely connected with the formation of the navel, or with the
separation of the embryo from the yelk-sac or umbilical vesicle.

In order to get a clear idea of this, we must understand carefully the
relation of the embryonic shield to the germinative area and the
embryonic vesicle. This is done best by a comparison of the five stages
which are shown in longitudinal section in Figs. 138–142. The embryonic
shield (_c_), which at first projects very slightly over the surface of
the germinative area, soon begins to rise higher above it, and to
separate from the embryonic vesicle. At this point the embryonic
shield, looked at from the dorsal surface, shows still the original
simple sandal-shape (Figs. 133–135). We do not yet see any trace of
articulation into head, neck, trunk, etc., or limbs. But the embryonic
shield has increased greatly in thickness, especially in the anterior
part. It now has the appearance of a thick, oval swelling, strongly
curved over the surface of the germinative area. It begins to sever
completely from the embryonic vesicle, with which it is connected at
the ventral surface. As this severance proceeds, the back bends more
and more; in proportion as the embryo grows the embryonic vesicle
decreases, and at last it merely hangs as a small vesicle from the
belly of the embryo (Fig. 142 _ds_). In consequence of the
growth-movements which cause this severance, a groove-shaped depression
is formed at the surface of the vesicle, the _limiting furrow,_ which
surrounds the vesicle in the shape of a pit, and a circular mound or
dam (Fig. 139 ks) is formed at the outside of this pit by the elevation
of the contiguous parts of the germinal vesicle.


Fig.136. Transverse section of the embryonic disk of a chick at the end
of the first day of incubation. Fig. 136—Transverse section of the
embryonic disk of a chick at the end of the first day of incubation,
magnified. The edges of the medullary plate (_m_), the medullary
swellings (_w_), which separate the medullary from the horn-plate
(_h_), are bending towards each other. At each side of the chorda
(_ch_) the primitive segment plates (_u_) have separated from the
lateral plates (_sp_). A gut-gland layer. (From _Remak._)


In order to understand clearly this important process, we may compare
the embryo to a fortress with its surrounding rampart and trench. The
ditch consists of the outer part of the germinative area, and comes to
an end at the point where the area passes into the vesicle. The
important fold of the middle germinal layer that brings about the
formation of the body-cavity spreads beyond the borders of the embryo
over the whole germinative area. At first this middle layer reaches as
far as the germinative area; the whole of the rest of the embryonic
vesicle consists in the beginning only of the two original limiting
layers, the outer and inner germinal layers. Hence, as far as the
germinative area extends the germinal layer splits into the two plates
we have already recognised in it, the outer skin-fibre layer and the
inner gut-fibre layer. These two plates diverge considerably, a clear
fluid gathering between them (Fig. 140 _am_). The inner plate, the
gut-fibre layer, remains on the inner layer of the embryonic vesicle
(on the gut-gland layer). The
outer plate, the skin-fibre layer, lies close on the outer layer of the
germinative area, or the skin-sense layer, and separates together with
this from the embryonic vesicle. From these two united outer plates is
formed a continuous membrane. This is the circular mound that rises
higher and higher round the whole embryo, and at last joins above it
(Figs. 139–142 _am_). To return to our illustration of the fortress, we
must imagine the circular rampart to be extraordinarily high and
towering far above the fortress. Its edges bend over like the combs of
an overhanging wall of rock that would enclose the fortress; they form
a deep hollow, and at last join together above. In the end the fortress
lies entirely within the hollow that has been formed by the growth of
the edges of this large rampart.


Fig.137. Three diagrammatic transverse sections of the embryonic disk
of the higher vertebrate, to show the origin of the tubular organs from
the bending germinal layers. Fig. 137—Three diagrammatic transverse
sections of the embryonic disk of the higher vertebrate, to show the
origin of the tubular organs from the bending germinal layers. In Fig.
_A_ the medullary tube (_n_) and the alimentary canal (_a_) are still
open grooves. In Fig. _B_ the medullary tube (_n_) and the dorsal wall
are closed, but the alimentary canal (_a_) and the ventral wall are
open; the prorenal ducts (_u_) are cut off from the horn-plate (_h_)
and internally connected with segmental prorenal canals. In Fig. _C_
both the medullary tube and the dorsal wall above and the alimentary
canal and ventral wall below are closed. All the open grooves have
become closed tubes; the primitive kidneys are directed inwards. The
letters have the same meaning in all three figures: _h_ skin-sense
layer, _n_ medullary tube, _u_ prorenal ducts, _x_ axial rod, _s_
primitive-vertebra, _r_ dorsal wall, _b_ ventral wall, _c_ body-cavity
or cœloma, _f_ gut-fibre layer, _t_ primitive artery (aorta), _v_
primitive vein (subintestinal vein), _d_ gut-fibre layer, _a_
alimentary canal.


As the two outer layers of the germinative area thus rise in a fold
about the embryo, and join above it, they come at last to form a
spacious sac-like membrane about it. This envelope takes the name of
the germinative membrane, or water-membrane, or _amnion_ (Fig. 142
_am_). The embryo floats in a watery fluid, which fills the space
between the embryo and the amnion, and is called the amniotic fluid
(Figs. 141, 142 _ah_). We will deal with this remarkable formation and
with the allantois later on (Chapter XV). In front of the allantois the
yelk-sac or umbilical vesicle (_ds_), the remainder of the original
embryonic vesicle, starts from the open belly of the embryo (Fig. 138
_kh_). In more advanced embryos, in which the gastric wall and the
ventral wall are nearly closed, it hangs out of the navel-opening in
the shape of a small vesicle with a stalk (Figs. 141, 142 _ds_). The
more the embryo grows, the smaller becomes the vitelline (yelk) sac. At
first the embryo looks like a small appendage of the large embryonic
vesicle. Afterwards it is the yelk-sac, or the remainder of the
embryonic vesicle, that seems a small pouch-like appendage of the
embryo (Fig. 142 _ds_). It ceases to have any significance in the end.
The very wide opening, through which the gastric cavity at first
communicates with the umbilical vesicle, becomes narrower and narrower,
and at last disappears altogether. The _navel,_ the small pit-like
depression that we find in the developed man in the middle of the
abdominal wall, is the spot at which the remainder of the embryonic
vesicle (the umbilical vesicle) originally entered into the ventral
cavity, and joined on to the growing gut.

The origin of the navel coincides with the complete closing of the
external ventral wall. In the amniotes the ventral wall originates in
the same way as the dorsal wall. Both are formed substantially from the
skin-fibre layer, and externally covered with the horn-plate, the
border section of the skin-sense layer. Both come into

existence by the conversion of the four flat germinal layers of the
embryonic shield into a double tube by folding from opposite
directions; above, at the back, we have the vertebral canal which
encloses the medullary tube, and below, at the belly, the wall of the
body-cavity which contains the alimentary canal (Fig. 137).


Figs. 138 to 142. Five diagrammatic longitudinal sections of the
maturing mammal embryo and its envelopes. Figs. 138–142—Five
diagrammatic longitudinal sections of the maturing mammal embryo and
its envelopes. In Figs. 138–141 the longitudinal section passes through
the sagittal or middle plane of the body, dividing the right and left
halves; in Fig. 142 the embryo is seen from the left side. In Fig. 138
the tufted it prochorion (_dd′_) encloses the germinal vesicle, the
wall of which consists of the two primary layers. Between the outer
(_a_) and inner (_i_) layer the middle layer (_m_) has been developed
in the region of the germinative area. In Fig. 139 the embryo (_e_)
begins to separate from the embryonic vesicle (_ds_), while the wall of
the amnion-fold rises about it (in front as head-sheath, _ks,_ behind
as tail-sheath, _ss_). In Fig. 140 the edges of the amniotic fold
(_am_) rise together over the back of the embryo, and form the amniotic
cavity (_ah_); as the embryo separates more completely from the
embryonic vesicle (_ds_) the alimentary canal (_dd_) is formed, from
the hinder end of which the allantois grows (_al_). In Fig. 141 the
allantois is larger; the yelk-sac (_ds_) smaller. In Fig. 142 the
embryo shows the gill-clefts and the outline of the two legs; the
chorion has formed branching villi (tufts.) In all four figures
_e_=embryo, _a_ outer germinal layer, _m_ middle germinal layer, _i_
inner germinal layer, _am_ amnion (_ks_ head-sheath, _ss_ tail-sheath),
_ah_ amniotic cavity, _as_ amniotic sheath of the umbilical cord, _kh_
embryonic vesicle, _ds_ yelk-sac (umbilical vesicle), _dg_ vitelline
duct, _df_ gut-fibre layer, _dd_ gut-gland layer, _al_ allantois,
_vl=hh_ place of heart, _d_ vitelline membrane (ovolemma or
prochorion), _d′_ tufts or villi of same, _sh_ serous membrane
(serolemma), _sz_ tufts of same, _ch_ chorion, _chz_ tufts or villi,
_st_ terminal vein, _r_ pericœlom or serocœlom (the space, filled with
fluid, between the amnion and chorion). (From _Kölliker._)


Figs. 143 and 144. Transverse sections of embryos (of chicks). Figs.
143–144—Transverse sections of embryos (of chicks). Fig. 143 of the
second, Fig. 144 of the third, Fig. 145 of the fourth, and Fig. 146 of
the fifth day of incubation. Fig. 143–145 from _Kölliker,_ magnified;
Fig. 146 from _Remak,_ magnified. _h_ horn-plate, _mr_ medullary tube,
_ung_ prorenal duct, _un_ prorenal vesicles, _hp_ skin-fibre layer,
_m=mu=mp_ muscle-plate, _uw_ provertebral plate (_wh_ cutaneous
rudiment of the body of the vertebra, _wb_ of the arch of the vertebra,
_wq_ the rib or transverse continuation), _uwh_ provertebral cavity,
_ch_ axial rod or chorda, _sh_ chorda-sheath, _bh_ ventral wall, _g_
hind and _v_ fore root of the spinal nerves, _a=af=am_ amniotic fold,
_p_ body-cavity or cœloma, _df_ gut-fibre layer, _ao_ primitive aortas,
_sa_ secondary aorta, _vc_ cardinal veins, _d=dd_ gut-gland layer, _dr_
gastric groove. In Fig. 143 the larger part of the right half, in Fig.
144 the larger part of the left half, of the section is omitted. Of the
yelk-sac or remainder of the embryonic vesicle only a small piece of
the wall is indicated below.


We will consider the formation of the dorsal wall first, and that of
the ventral wall afterwards (Figs. 143–147). In the middle of the
dorsal surface of the embryo there is originally, as we already know,
the medullary (_mr_) tube directly underneath the horn-plate (_h_),
from the middle part of which it has been developed. Later, however,
the provertebral plates (_uw_) grow over from the right and left
between these originally connected parts (Figs. 145, 146). The upper
and inner edges of the two provertebral plates push between the
horn-plate and medullary tube, force them away from each other, and
finally join between them in a seam that corresponds to the middle line
of the back. The coalescence of these two dorsal plates and the closing
in the middle of the dorsal wall take place in the same way as the
medullary tube, which is henceforth enclosed by the vertebral tube.
Thus is formed the dorsal wall, and the medullary tube takes up a
position inside the body. In the same way the provertebral mass grows
afterwards round the chorda, and forms the vertebral column. Below this
the inner and outer edge of the provertebral plate splits on each side
into two horizontal plates, of which the upper pushes between the
chorda and medullary tube, and the lower between the chorda and gastric
tube. As the plates meet from both sides above and below the chorda,
they completely enclose it, and so form the tubular, outer
chord-sheath, the sheath from which the vertebral column is formed
(_perichorda,_ Fig. 137 _C, s_; Figs. 145 _uwh,_ 146).

We find in the construction of the ventral wall precisely the same
processes
as in the formation of the dorsal wall (Fig. 137 _B,_ Fig. 144 _hp,_
Fig. 146 _bh_). It is formed on the flat embryonic shield of the
amniotes from the upper plates of the parietal zone. The right and left
parietal plates bend downwards towards each other, and grow round the
gut in the same way as the gut itself closes. The outer part of the
lateral plates forms the ventral wall or the lower wall of the body,
the two lateral plates bending considerably on the inner side of the
amniotic fold, and growing towards each other from right and left.
While the alimentary canal is closing, the body-wall also closes on all
sides. Hence the ventral wall, which encloses the whole ventral cavity
below, consists of two parts, two lateral plates that bend towards each
other. These approach each other all along, and at last meet at the
navel. We ought, therefore, really to distinguish two navels, an inner
and an outer one. The internal or intestinal navel is the definitive
point of the closing of the gut wall, which puts an end to the open
communication between the ventral cavity and the cavity of the yelk-sac
(Fig. 105). The external navel in the skin is the definitive point of
the closing of the ventral wall; this is visible in the developed body
as a small depression.


Figs. 145 and 146. Transverse sections of embryos (of chicks). Figs.
145–146—Transverse sections of embryos (of chicks). Fig. 143 of the
second, Fig. 144 of the third, Fig. 145 of the fourth, and Fig. 146 of
the fifth day of incubation. Fig. 143–145 from _Kölliker,_ magnified;
Fig. 146 from _Remak,_ magnified. _h_ horn-plate, _mr_ medullary tube,
_ung_ prorenal duct, _un_ prorenal vesicles, _hp_ skin-fibre layer,
_m=mu=mp_ muscle-plate, _uw_ provertebral plate (_wh_ cutaneous
rudiment of the body of the vertebra, _wb_ of the arch of the vertebra,
_wq_ the rib or transverse continuation), _uwh_ provertebral cavity,
_ch_ axial rod or chorda, _sh_ chorda-sheath, _bh_ ventral wall, _g_
hind and _v_ fore root of the spinal nerves, _a=af=am_ amniotic fold,
_p_ body-cavity or cœloma, _df_ gut-fibre layer, _ao_ primitive aortas,
_sa_ secondary aorta, _vc_ cardinal veins, _d=dd_ gut-gland layer, _dr_
gastric groove. In Fig. 143 the larger part of the right half, in Fig.
144 the larger part of the left half, of the section is omitted. Of the
yelk-sac or remainder of the embryonic vesicle only a small piece of
the wall is indicated below.


With the formation of the internal navel and the closing of the
alimentary canal is connected the formation of two cavities, which we
call the capital and the pelvic sections of the visceral cavity. As the
embryonic shield lies flat on the wall of the embryonic vesicle at
first, and only gradually separates from it, its fore and hind ends are
independent in the beginning; on the other hand, the middle part of the
ventral surface is connected with the yelk-sac by means of the
vitelline or umbilical duct (Fig. 147 _m_). This leads to a notable
curving of the dorsal surface; the head-end bends downwards towards the
breast and the tail-end towards the belly. We see this very clearly in
the excellent old diagrammatic illustration given by Baer (Fig. 147), a
median longitudinal section of the embryo of the chick, in which the
dorsal body or episoma is deeply shaded. The embryo seems to be trying
to roll up, like a hedgehog protecting itself from its pursuers. This
pronounced curve of the back is due to the more rapid growth of the
convex dorsal surface, and is directly connected with the severance of
the embryo from the yelk-sac. The further bending of the embryo leads
to the formation of the “head-cavity” of the gut (Fig. 148 above _D_)
and a similar one at the tail, known as its “pelvic cavity.”


Fig.147. Median longitudinal section of the embryo of a chick (fifth
day of incubation), seen from the right side (head to the right, tail
to the left). Fig. 147—Median longitudinal section of the embryo of a
chick (fifth day of incubation), seen from the right side (head to the
right, tail to the left). Dorsal body dark, with convex outline. _d_
gut, _o_ mouth, _a_ anus, _l_ lungs, _h_ liver, _g_ mesentery, _v_
auricle of the heart, _k_ ventricle of the heart, _b_ arch of the
arteries, _t_ aorta, _c_ yelk-sac, _m_ vitelline (yelk) duct, _u_
allantois, _r_ pedicle (stalk) of the allantois, _n_ amnion, _w_
amniotic cavity, _s_ serous membrane. (From _Baer._)


As a result of these processes the embryo attains a shape that may be
compared to a wooden shoe, or, better still, to an overturned canoe.
Imagine a canoe or boat with both ends rounded and a small covering
before and behind; if this canoe is turned upside down, so that the
curved keel is uppermost, we have a fair picture of the canoe-shaped
embryo (Fig. 147). The upturned convex keel corresponds to the middle
line of the back; the small chamber underneath the fore-deck represents
the capital cavity, and the small chamber under the rear-deck the
pelvic chamber of the gut (cf. Fig. 140).

The embryo now, as it were, presses into the outer surface of the
embryonic vesicle with its free ends, while it moves away from it with
its middle part. As a result of this change the yelk-sac becomes
henceforth only a pouch-like outer appendage at the middle of the
ventral wall. The ventral appendage, growing smaller and smaller, is
afterwards called the umbilical (navel) vesicle. The cavity of the
yelk-sac or umbilical vesicle communicates with the corresponding
visceral cavity by a wide opening, which gradually contracts into a
narrow and long canal, the vitelline (yelk) duct (_ductus vitellinus,_
Fig. 147 _m_). Hence, if we were to imagine ourselves in
the cavity of the yelk-sac, we could get from it through the yelk-duct
into the middle and still wide open part of the alimentary canal. If we
were to go forward from there into the head-part of the embryo, we
should reach the capital cavity of the gut, the fore-end of which is
closed up.

The reader will ask: “Where are the mouth and the anus?” These are not
at first present in the embryo. The whole of the primitive gut-cavity
is completely closed, and is merely connected in the middle by the
vitelline duct with the equally closed cavity of the embryonic vesicle
(Fig. 140). The two later apertures of the alimentary canal—the anus
and the mouth—are secondary constructions, formed from the outer skin.
In the horn-plate, at the spot where the mouth is found subsequently, a
pit-like depression is formed, and this grows deeper and deeper,
pushing towards the blind fore-end of the capital cavity; this is the
mouth-pit. In the same way, at the spot in the outer skin where the
anus is afterwards situated a pit-shaped depression appears, grows
deeper and deeper, and approaches the blind hind-end of the pelvic
cavity; this is the anus-pit. In the end these pits touch with their
deepest and innermost points the two blind ends of the primitive
alimentary canal, so that they are now only separated from them by thin
membranous partitions. This membrane finally disappears, and henceforth
the alimentary canal opens in front at the mouth and in the rear by the
anus (Figs. 141, 147). Hence at first, if we penetrate into these pits
from without, we find a partition cutting them off from the cavity of
the alimentary canal, which gradually disappears. The formation of
mouth and anus is secondary in all the vertebrates.


Fig.148. Longitudinal section of the fore half of a chick-embryo at the
end of the first day of incubation (seen from the left side). Fig.
148—Longitudinal section of the fore half of a chick-embryo at the end
of the first day of incubation (seen from the left side). _k_
head-plates, _ch_ chorda. Above it is the blind fore-end of the ventral
tube (_m_); below it the capital cavity of the gut. _d_ gut-gland
layer, _df_ gut-fibre layer, _h_ horn plate, _hh_ cavity of the heart,
_hk_ heart-capsule, _ks_ head-sheath, _kk_ head-capsule. (From
_Remak._)


During the important processes which lead to the formation of the
navel, and of the intestinal wall and ventral wall, we find a number of
other interesting changes taking place in the embryonic shield of the
amniotes. These relate chiefly to the prorenal ducts and the first
blood-vessels. The prorenal (primitive kidney) ducts, which at first
lie quite flat under the horn-plate or epiderm (Fig. 93 _ung_), soon
back towards each other in consequence of special growth movements
(Figs. 143–145 _ung_). They depart more and more from their point of
origin, and approach the gut-gland layer. In the end they lie deep in
the interior, on either side of the mesentery, underneath the chorda,
(Fig. 145 _ung_). At the same time, the two primitive aortas change
their position (cf. Figs. 138–145 _ao_); they travel inwards underneath
the chorda, and there coalesce at last to form a single secondary
aorta, which is found under the rudimentary vertebral column (Fig. 145
_ao_). The cardinal veins, the first venous blood-vessels, also back
towards each other, and eventually unite immediately above the
rudimentary kidneys (Figs. 145 _vc,_ 152 _cav_). In the same spot, at
the inner side of the fore-kidneys, we soon see the first trace of the
sexual organs. The most important part of this apparatus (apart from
all its appendages) is the ovary in the female and the testicle

in the male. Both develop from a small part of the cell-lining of the
body-cavity, at the spot where the skin-fibre layer and gut-fibre layer
touch. The connection of this embryonic gland with the prorenal ducts,
which lie close to it and assume most important relations to it, is
only secondary.


Fig.149. Longitudinal section of a human embryo of the fourth week,
one-fifth of an inch long. Fig. 149—Longitudinal section of a human
embryo of the fourth week, one-fifth of an inch long, magnified. (From
_Kollmann._)



Fig.150. Transverse section of a human embryo of fourteen days. Fig.
151. Transverse section of a shark-embryo (or young selachius). Fig.
150—Transverse section of a human embryo of fourteen days. _mr_
medullary tube, _ch_ chorda. _vu_ umbilical vein, _mt_ myotome, _mp_
middle plate, _ug_ prorenal duct, _lh_ body-cavity, _e_ ectoderm, _bh_
ventral skin, _hf_ skin-fibre layer, _df_ gut-fibre layer. (From
_Kollmann._)
Fig. 151—Transverse section of a shark-embryo (or young selachius).
_mr_ medullary tube, _ch_ chorda, _a_ aorta, _d_ gut, _vp_ principal
(or subintestinal) vein, _mt_ myotome, _mm_ muscular mass of the
provertebra, _mp_ middle plate, _ug_ prorenal duct, _lh_ body-cavity,
_e_ ectoderm of the rudimentary extremities, _mz_ mesenchymic cells,
_z_ point where the myotome and nephrotome separate. (From _H. E.
Ziegler._)


Fig.152. Transverse section of a duck-embryo with twenty-four primitive
segments. Fig. 152—Transverse section of a duck-embryo with twenty-four
primitive segments. (From _Balfour._) From a dorsal lateral joint of
the medullary tube (_spc_) the spinal ganglia (_spg_) grow out between
it and the horn-plate. _ch_ chorda, _ao_ double aorta, _hy_ gut-gland
layer, _sp_ gut-fibre layer, with blood-vessels in section, _ms_ muscle
plate, in the dorsal wall of the myocœl (episomite). Below the cardinal
vein (_cav_) is the prorenal duct (_wd_) and a segmental prorenal canal
(_st_). The skin-fibre layer of the body-wall (_so_) is continued in
the amniotic fold (_am_). Between the four secondary germinal layers
and the structures formed from them there is formed embryonic
connective matter with stellate cells and vascular structures
(Hertwig’s “mesenchym”).



Chapter XIV.
THE ARTICULATION OF THE BODY[26]


 [26] The term articulation is used in this chapter to denote both
 “segmentation” and “articulation” in the ordinary sense.—Translator.


The vertebrate stem, to which our race belongs as one of the latest and
most advanced outcomes of the natural development of life, is rightly
placed at the head of the animal kingdom. This privilege must be
accorded to it, not only because man does in point of fact soar far
above all other animals, and has been lifted to
the position of “lord of creation”; but also because the vertebrate
organism far surpasses all the other animal-stems in size, in
complexity of structure, and in the advanced character of its
functions. From the point of view of both anatomy and physiology, the
vertebrate stem outstrips all the other, or invertebrate, animals.

There is only one among the twelve stems of the animal kingdom that can
in many respects be compared with the vertebrates, and reaches an
equal, if not a greater, importance in many points. This is the stem of
the articulates, composed of three classes: 1, the annelids
(earth-worms, leeches, and cognate forms); 2, the crustacea (crabs,
etc.); 3, the tracheata (spiders, insects, etc.). The stem of the
articulates is superior not only to the vertebrates, but to all other
animal-stems, in variety of forms, number of species, elaborateness of
individuals, and general importance in the economy of nature.

When we have thus declared the vertebrates and the articulates to be
the most important and most advanced of the twelve stems of the animal
kingdom, the question arises whether this special position is accorded
to them on the ground of a peculiarity of organisation that is common
to the two. The answer is that this is really the case; it is their
segmental or transverse articulation, which we may briefly call
metamerism. In all the vertebrates and articulates the developed
individual consists of a series of successive members (segments or
metamera = “parts”); in the embryo these are called primitive segments
or somites. In each of these segments we have a certain group of organs
reproduced in the same arrangement, so that we may regard each segment
as an individual unity, or a special “individual” subordinated to the
entire personality.

The similarity of their segmentation, and the consequent physiological
advance in the two stems of the vertebrates and articulates, has led to
the assumption of a direct affinity between them, and an attempt to
derive the former directly from the latter. The annelids were supposed
to be the direct ancestors, not only of the crustacea and tracheata,
but also of the vertebrates. We shall see later (Chapter XX) that this
annelid theory of the vertebrates is entirely wrong, and ignores the
most important differences in the organisation of the two stems. The
internal articulation of the vertebrates is just as profoundly
different from the external metamerism of the articulates as are their
skeletal structure, nervous system, vascular system, and so on. The
articulation has been developed in a totally different way in the two
stems. The unarticulated chordula (Figs. 83–86), which we have
recognised as one of the chief palingenetic embryonic forms of the
vertebrate group, and from which we have inferred the existence of a
corresponding ancestral form for all the vertebrates and tunicates, is
quite unthinkable as the stem-form of the articulates.

All articulated animals came originally from unarticulated ones. This
phylogenetic principle is as firmly established as the ontogenetic fact
that every articulated animal-form develops from an unarticulated
embryo. But the organisation of the embryo is totally different in the
two stems. The chordula-embryo of all the vertebrates is characterised
by the dorsal medullary tube, the neurenteric canal, which passes at
the primitive mouth into the alimentary canal, and the axial chorda
between the two. None of the articulates, either annelids or arthropods
(crustacea and tracheata), show any trace of this type of organisation.
Moreover, the development of the chief systems of organs proceeds in
the opposite way in the two stems. Hence the segmentation must have
arisen independently in each. This is not at all surprising; we find
analogous cases in the stalk-articulation of the higher plants and in
several groups of other animal stems.

The characteristic internal articulation of the vertebrates and its
importance in the organisation of the stem are best seen in the study
of the skeleton. Its chief and central part, the cartilaginous or bony
vertebral column, affords an obvious instance of vertebrate metamerism;
it consists of a series of cartilaginous or bony pieces, which have
long been known as _vertebræ_ (or _spondyli_). Each vertebra is
directly connected with a special section of the muscular system, the
nervous system, the vascular system, etc. Thus most of the “animal
organs” take part in this vertebration. But we saw, when we were
considering our own vertebrate character (in Chapter XI), that the same
internal articulation is also found in the lowest primitive
vertebrates, the acrania, although here the whole skeleton consists
merely of the simple chorda, and is not at all articulated.
Hence the articulation does not proceed primarily from the skeleton,
but from the muscular system, and is clearly determined by the more
advanced swimming-movements of the primitive chordonia-ancestors.


Figs. 153-155. Sole-shaped embryonic disk of the chick, in three
successive stages of development, looked at from the dorsal surface,
magnified, somewhat diagrammatic. Figs. 153–155—Sole-shaped embryonic
disk of the chick, in three successive stages of development, looked at
from the dorsal surface, magnified, somewhat diagrammatic. Fig. 153
with six pairs of somites. Brain a simple vesicle (_hb_). Medullary
furrow still wide open from _x_; greatly widened at _z. mp_ medullary
plates, _sp_ lateral plates, _y_ limit of gullet-cavity (_sh_) and
fore-gut (_vd_). Fig. 154 with ten pairs of somites. Brain divided into
three vesicles: _v_ fore-brain, _m_ middle-brain, _h_ hind-brain, _c_
heart, _dv_ vitelline-veins. Medullary furrow still wide open behind
(_z_). _mp_ medullary plates. Fig. 155 with sixteen pairs of somites.
Brain divided into five vesicles: _v_ fore-brain, _z_
intermediate-brain, _m_ middle-brain, _h_ hind-brain, _n_ after-brain,
_a_ optic vesicles, _g_ auditory vesicles, _c_ heart, _ dv_ vitelline
veins, _mp_ medullary plate, _uw_ primitive vertebra.


It is, therefore, wrong to describe the first rudimentary segments in
the vertebrate embryo as primitive vertebræ or provertebræ; the fact
that they have been so called for some time has led to much error and
misunderstanding. Hence we shall give the name of “somites” or
primitive segments to these so-called “primitive vertebræ.” If the
latter name is retained at all, it should only be used of the
sclerotom—i.e., the small part of the somites from which the later
vertebra does actually develop.

Articulation begins in all vertebrates at a very early embryonic stage,
and this indicates the considerable phylogenetic age of the process.
When the chordula (Figs. 83–86) has completed its characteristic
composition, often even a little earlier, we find in the amniotes, in
the
middle of the sole-shaped embryonic shield, several pairs of dark
square spots, symmetrically distributed on both sides of the chorda
(Figs. 131–135).Transverse sections (Fig. 93 _uw_) show that they
belong to the stem-zone (episoma) of the mesoderm, and are separated
from the parietal zone (hyposoma) by the lateral folds; in section they
are still quadrangular, almost square, so that they look something like
dice. These pairs of “cubes” of the mesoderm are the first traces of
the primitive segments or somites, the so-called “protovertebræ.”
(Figs. 153–155 _uw_).


Fig.156. Embryo of the amphioxus, sixteen hours old, seen from the
back. Fig. 156—Embryo of the amphioxus, sixteen hours old, seen from
the back. (From _Hatschek._) _d_ primitive gut, _u_ primitive mouth,
_p_ polar cells of the mesoderm, _ c_ cœlom-pouches, _m_ their first
segment, _n_ medullary tube, _i_ entoderm, _e_ ectoderm, _s_ first
segment-fold.


Among the mammals the embryos of the marsupials have three pairs of
somites (Fig. 131) after sixty hours, and eight pairs after seventy-two
hours (Fig. 135). They develop more slowly in the embryo of the rabbit;
this has three somites on the eighth day (Fig. 132), and eight somites
a day later (Fig. 134). In the incubated hen’s egg the first somites
make their appearance thirty hours after incubation begins (Fig. 153).
At the end of the second day the number has risen to sixteen or
eighteen (Fig. 155). The articulation of the stem-zone, to which the
somites owe their origin, thus proceeds briskly from front to rear, new
transverse constrictions of the “protovertebral plates” forming
continuously and successively. The first segment, which is almost
half-way down in the embryonic shield of the amniote, is the foremost
of all; from this first somite is formed the first cervical vertebra
with its muscles and skeletal parts. It follows from this, firstly,
that the multiplication of the primitive segments proceeds backwards
from the front, with a constant lengthening of the hinder end of the
body; and, secondly, that at the beginning of segmentation nearly the
whole of the anterior half of the sole-shaped embryonic shield of the
amniote belongs to the later head, while the whole of the rest of the
body is formed from its hinder half. We are reminded that in the
amphioxus (and in our hypothetic primitive vertebrate, Figs. 98–102)
nearly the whole of the fore half corresponds to the head, and the hind
half to the trunk.


Fig.157. Embryo of the amphioxus, twenty hours old, with five somites.
Fig. 157—Embryo of the amphioxus, twenty hours old, with five somites.
(Right view; for left view see Fig. 124.) (From _Hatschek._) _ V_ fore
end, _H_ hind end. _ak, mk, ik_ outer, middle, and inner germinal
layers; _dh_ alimentary canal, _n_ neural tube, _cn_ canalis
neurentericus, _ush_ cœlom-pouches (or primitive-segment cavities),
_us1_ first (and foremost) primitive segment.


The number of the metamera, and of the embryonic somites or primitive
segments from which they develop, varies considerably in the
vertebrates, according as the hind part of the body is short or is
lengthened by a tail. In the developed man the trunk (including the
rudimentary tail) consists of thirty-three metamera, the solid centre
of which is formed by that number of vertebræ in the vertebral column
(seven cervical, twelve dorsal, five lumbar, five sacral, and four
caudal). To these we must add at least nine head-vertebræ, which
originally (in all the craniota) constitute the skull. Thus the total
number of the primitive segments of the human
body is raised to at least forty-two; it would reach forty-five to
forty-eight if (according to recent investigations) the number of the
original segments of the skull is put at twelve to fifteen. In the
tailless or anthropoid apes the number of metamera is much the same as
in man, only differing by one or two; but it is much larger in the
long-tailed apes and most of the other mammals. In long serpents and
fishes it reaches several hundred (sometimes 400).


Figs. 158-160. Embryo of the amphioxus, twenty four hours old, with
eight somites. Figs. 158–160—Embryo of the amphioxus, twenty four hours
old, with eight somites. (From _Hatschek._) Figs. 158 and 159 lateral
view (from left). Fig. 160 seen from back. In Fig. 158 only the
outlines of the eight primitive segments are indicated, in Fig. 159
their cavities and muscular walls. _V_ fore end, _H_ hind end, _d_ gut,
_du_ under and _ dd_ upper wall of the gut, _ne_ canalis neurentericus,
_ nv_ ventral, _nd_ dorsal wall of the neural tube, _np_ neuroporus,
_dv_ fore pouch of the gut, _ch_ chorda, _ mf_ mesodermic fold, _pm_
polar cells of the mesoderm (_ms_), _e_ ectoderm.


In order to understand properly the real nature and origin of
articulation in the human body and that of the higher vertebrates, it
is necessary to compare it with that of the lower vertebrates, and bear
in mind always the genetic connection of all the members of the stem.
In this the simple development of the invaluable amphioxus once more
furnishes the key to the complex and cenogenetically modified embryonic
processes of the craniota. The articulation of the amphioxus begins at
an early stage—earlier than in the craniotes. The two cœlom-pouches
have hardly grown out of the primitive gut (Fig. 156 _c_) when the
blind fore part of it (farthest away from the primitive mouth, _u_)
begins to separate by a transverse fold (_s_): this is the first
primitive segment. Immediately afterwards the hind part of the
cœlom-pouches begins to divide into a series of pieces by new
transverse folds (Fig. 157). The foremost of these primitive segments
(_us_1) is the first and oldest; in Figs. 124 and 157 there are already
five formed. They separate so rapidly, one behind the other, that eight
pairs are formed within twenty-four hours of the beginning of
development, and seventeen pairs twenty-four hours later. The number
increases as the embryo grows and extends
backwards, and new cells are formed constantly (at the primitive mouth)
from the two primitive mesodermic cells (Figs. 159–160).


Figs. 161 and 162. Transverse section of shark-embryos (through the
region of the kidneys). Figs. 161 and 162—Transverse section of
shark-embryos (through the region of the kidneys). (From _Wijhe_ and
_Hertwig._) In Fig. 162 the dorsal segment-cavities (_h_) are already
separated from the body-cavity (_lh_), but they are connected a little
earlier (Fig. 161), _nr_ neural tube, _ch_ chorda, _sch_ subchordal
string, _ao_ aorta, _sk_ skeletal-plate, _mp_ muscle-plate, _cp_
cutis-plate, _ w_ connection of latter (growth-zone), _vn_ primitive
kidneys, _ug_ prorenal duct, _uk_ prorenal canals, _ us_ point where
they are cut off, _tr_ prorenal funnel, _ mk_ middle germ-layer (_mk_1
parietal, _ mk_2 visceral), _ik_ inner germ-layer (gut-gland layer).


This typical articulation of the two cœlom-sacs begins very early in
the lancelet, before they are yet severed from the primitive gut, so
that at first each segment-cavity (_us_) still communicates by a narrow
opening with the gut, like an intestinal gland. But this opening soon
closes by complete severance, proceeding regularly backwards. The
closed segments then extend more, so that their upper half grows
upwards like a fold between the ectoderm (_ak_) and neural tube (_n_),
and the lower half between the ectoderm and alimentary canal (_ch_;
Fig. 82 _d,_ left half of the figure). Afterwards the two halves
completely separate, a lateral longitudinal fold cutting between them
(_mk,_ right half of Fig. 82). The dorsal segments (_sd_) provide the
muscles of the trunk the whole length of the body (159): this cavity
afterwards disappears. On the other hand, the ventral parts give rise,
from their uppermost section, to the pronephridia or primitive-kidney
canals, and from the lower to the segmental rudiments of the sexual
glands or gonads. The partitions of the muscular dorsal pieces
(_myotomes_) remain, and determine the permanent articulation of the
vertebrate organism. But the partitions of the large ventral pieces
(_gonotomes_) become thinner, and afterwards disappear in part, so that
their cavities run together to form the metacœl, or the simple
permanent body-cavity.

The articulation proceeds in substantially the same way in the other
vertebrates, the craniota, starting from the cœlom-pouches. But whereas
in the former case there is first a transverse division of the
cœlom-sacs (by vertical folds) and then the dorso-ventral division, the
procedure is reversed in the craniota; in their case each of the long
cœlom-pouches first divides into a dorsal (primitive segment plates)
and a ventral (lateral plates) section by a lateral longitudinal fold.
Only the former are then broken up into primitive segments by the
subsequent vertical folds; while the latter (segmented
for a time in the amphioxus) remain undivided, and, by the divergence
of their parietal and visceral plates, form a body-cavity that is
unified from the first. In this case, again, it is clear that we must
regard the features of the younger craniota as cenogenetically modified
processes that can be traced palingenetically to the older acrania.

We have an interesting intermediate stage between the acrania and the
fishes in these and many other respects in the cyclostoma (the hag and
the lamprey, cf. Chapter XXI).


Fig.163. Frontal (or horizontal-longitudinal) section of a
triton-embryo with three pairs of primitive segments. Fig. 163—Frontal
(or horizontal-longitudinal) section of a triton-embryo with three
pairs of primitive segments. _ch_ chorda, _us_ primitive segments,
_ush_ their cavity, _ ak_ horn plate.


Among the fishes the selachii, or primitive fishes, yield the most
important information on these and many other phylogenetic questions
(Figs. 161 and 162). The careful studies of Rückert, Van Wijhe, H. E.
Ziegler, and others, have given us most valuable results. The products
of the middle germinal layer are partly clear in these cases at the
period when the dorsal primitive segment cavities (or myocœls, _h_) are
still connected with the ventral body-cavity (_lh_; Fig. 161). In Fig.
162, a somewhat older embryo, these cavities are separated. The outer
or lateral wall of the dorsal segment yields the cutis-plate (_cp_),
the foundation of the connective corium. From its inner or median wall
are developed the muscle-plate (_mp,_ the rudiment of the
trunk-muscles) and the skeletal plate, the formative matter of the
vertebral column (_sk_).

In the amphibia, also, especially the water-salamander (_Triton_), we
can observe very clearly the articulation of the cœlom-pouches and the
rise of the primitive segments from their dorsal half (cf. Fig. 91, _A,
B, C_). A horizontal longitudinal section of the salamander-embryo
(Fig. 163) shows very clearly the series of pairs of these vesicular
dorsal segments, which have been cut off on each side from the ventral
side-plates, and lie to the right and left of the chorda.


Fig.164. The third cervical vertebra (human)> Fig. 165. The sixth
dorsal vertebra (human). Fig. 166. The second lumbar vertebra (human).
Fig. 164—The third cervical vertebra (human). Fig. 165—The sixth dorsal
vertebra (human).
Fig. 166—The second lumbar vertebra (human).


The metamerism of the amniotes agrees in all essential points with that
of the three lower classes of vertebrates we have considered; but it
varies considerably in detail, in consequence of cenogenetic
disturbances that are due in the first place (like the degeneration of
the cœlom-pouches) to the large development of the food-yelk. As the
pressure of this seems to force the two middle layers together from the
start, and as the solid structure of the mesoderm apparently belies the
original hollow character of the sacs, the two sections of the
mesoderm, which are at that time divided by the lateral fold—the dorsal
segment-plates and ventral side-plates—have the appearance at first of
solid layers of cells (Figs. 94–97). And when the articulation of the
somites begins in the sole-shaped embryonic shield, and a couple of
protovertebræ are developed in succession, constantly increasing in
number towards the rear, these cube-shaped somites (formerly called
protovertebræ, or primitive vertebræ) have the appearance of solid
dice, made up of mesodermic cells (Fig. 93). Nevertheless, there is for
a time a ventral cavity, or provertebral cavity, even in these solid
“protovertebræ” (Fig. 143 _uwh_). This vesicular condition of the
provertebra is of the greatest phylogenetic interest; we must,
according to the cœlom theory, regard it as an hereditary reproduction
of the hollow dorsal somites of the amphioxus (Figs. 156–160) and the
lower vertebrates (Fig. 161–163). This rudimentary “provertebral
cavity” has no physiological significance whatever in the
amniote-embryo; it soon disappears, being filled up with cells of the
muscular plate.


Fig.167. Head of a shark embryo. Fig. 167—Head of a shark embryo
(_Pristiurus_), one-third of an inch long, magnified. (From _Parker._)
Seen from the ventral side.


The innermost median part of the primitive segment plates, which lies
immediately on the chorda (Fig. 145 _ch_) and the medullary tube (_m_),
forms the vertebral column in all the higher vertebrates (it is wanting
in the lowest); hence it may be called the _skeleton_ plate. In each of
the provertebræ it is called the “sclerotome” (in opposition to the
outlying muscular plate, the “myotome”). From the phylogenetic point of
view the myotomes are much older than the sclerotomes. The lower or
ventral part of each sclerotome (the inner and lower edge of the
cube-shaped provertebra) divides into two plates, which grow round the
chorda, and thus form the foundation of the body of the vertebra
(_wh_). The upper plate presses between the chorda and the medullary
tube, the lower between the chorda and the alimentary canal (Fig. 137
_C_). As the plates of two opposite provertebral pieces unite from the
right and left, a circular sheath is formed round this part of the
chorda. From this develops the _body_ of a vertebra—that is to say, the
massive lower or ventral half of the bony ring, which is called the
“vertebra” proper and surrounds the medullary tube (Figs. 164–166). The
upper or dorsal half of this bony ring, the vertebral arch (Fig. 145
_wb_), arises in just the same way from the upper part of the skeletal
plate, and therefore from the inner and upper edge of the cube-shaped
primitive vertebra. As the upper edges of two opposing somites grow
together over the medullary tube from right and left, the vertebra-arch
becomes closed.

The whole of the secondary vertebra, which is thus formed from the
union of the skeletal plates of two provertebral pieces
and encloses a part of the chorda in its body, consists at first of a
rather soft mass of cells; this afterwards passes into a firmer,
cartilaginous stage, and finally into a third, permanent, bony stage.
These three stages can generally be distinguished in the greater part
of the skeleton of the higher vertebrates; at first most parts of the
skeleton are soft, tender, and membranous; they then become
cartilaginous in the course of their development, and finally bony.


Figs. 168 and 169. Head of a chick embryo, of the third day. Fig. 168
and 169—Head of a chick embryo, of the third day. Fig. 168 from the
front, Fig. 169 from the right. _n_ rudimentary nose (olfactory pit),
_l_ rudimentary eye (optic pit, lens-cavity), _g_ rudimentary ear
(auditory pit), _v_ fore-brain, _gl_ eye-cleft. Of the three pairs of
gill-arches the first has passed into a process of the upper jaw (_o_)
and of the lower jaw (_u_). (From _Kölliker._)


At the head part of the embryo in the amniotes there is not generally a
cleavage of the middle germinal layer into provertebral and lateral
plates, but the dorsal and ventral somites are blended from the first,
and form what are called the “head-plates” (Fig. 148 _k_). From these
are formed the skull, the bony case of the brain, and the muscles and
corium of the body. The skull develops in the same way as the
membranous vertebral column. The right and left halves of the head
curve over the cerebral vesicle, enclose the foremost part of the
chorda below, and thus finally form a simple, soft, membranous capsule
about the brain. This is afterwards converted into a cartilaginous
primitive skull, such as we find permanently in many of the fishes.
Much later this cartilaginous skull becomes the permanent bony skull
with its various parts. The bony skull in man and all the other
amniotes is more highly differentiated and modified than that of the
lower vertebrates, the amphibia and fishes. But as the one has arisen
phylogenetically from the other, we must assume that in the former no
less than the latter the skull was originally formed from the
sclerotomes of a number of (at least nine) head-somites.


Fig.170. Head of a dog embryo, seen from the front. Fig. 170—Head of a
dog embryo, seen from the front. _ a_ the two lateral halves of the
foremost cerebral vesicle, _ b_ rudimentary eye, _c_ middle cerebral
vesicle, _de_ first pair of gill-arches (_e_ upper-jaw process, _d_
lower-jaw process), _f, f′, f″,_ second, third, and fourth pairs of
gill-arches, _g h i k_ heart (_g_ right, _ h_ left auricle; _i_ left,
_k_ right ventricle), _ l_ origin of the aorta with three pairs of
arches, which go to the gill-arches. (From _Bischoff._)


While the articulation of the vertebrate body is always obvious in the
_episoma_ or dorsal body, and is clearly expressed in the segmentation
of the muscular plates and vertebræ, it is more latent in the
_hyposoma_ or ventral body. Nevertheless, the hyposomites of the
vegetal half of the body are not less important than the episomites of
the animal half. The segmentation in the ventral cavity affects the
following principal systems of organs: 1, the gonads or sex-glands
(gonotomes); 2, the nephridia or kidneys (nephrotomes); and 3, the
head-gut with its gill-clefts (branchiotomes).

The metamerism of the hyposoma is less conspicuous because in all the
craniotes the cavities of the ventral segments, in the walls of which
the sexual products are developed, have long since coalesced, and
formed a single large body-cavity, owing to the disappearance of the
partition. This cenogenetic process is so old that the cavity seems to
be unsegmented from the first in all the craniotes, and the rudiment of
the gonads also is almost always unsegmented. It is the more
interesting to learn that, according to the important discovery of
Rückert, this sexual structure is at first segmental even in the actual
selachii, and the several
gonotomes only blend into a simple sexual gland on either side
secondarily.

Amphioxus, the sole surviving representative of the acrania, once more
yields us most interesting information; in this case the sexual glands
remain segmented throughout life. The sexually mature lancelet has, on
the right and left of the gut, a series of metamerous sacs, which are
filled with ova in the female and sperm in the male. These segmental
gonads are originally nothing else than the real gonotomes, separate
body-cavities, formed from the hyposomites of the trunk.


Fig.171. Human embryo of the fourth week (twenty-six days old). Fig.
171—Human embryo of the fourth week (twenty-six days old), one-fourth
of an inch in length, magnified. (From _Moll._) The rudiments of the
cerebral nerves and the roots of the spinal nerves are especially
marked. Underneath the four gill-arches (left side) is the heart (with
auricle, _V,_ and ventricle, _K_), under this again the liver (_L_).


The gonads are the most important segmental organs of the hyposoma, in
the sense that they are phylogenetically the oldest. We find sexual
glands (as pouch-like appendages of the gastro-canal system) in most of
the lower animals, even in the medusæ, etc., which have no kidneys. The
latter appear first (as a pair of excretory tubes) in the platodes
(turbellaria), and have probably been inherited from these by the
articulates
(annelids) on the one hand and the unarticulated prochordonia on the
other, and from these passed to the articulated vertebrates. The oldest
form of the kidney system in this stem are the segmental pronephridia
or prorenal canals, in the same arrangement as Boveri found them in the
amphioxus. They are small canals that lie in the frontal plane, on each
side of the chorda, between the episoma and hyposoma (Fig. 102 _n_);
their internal funnel-shaped opening leads into the various
body-cavities, their outer opening is the lateral furrow of the
epidermis. Originally they must have had a double function, the
carrying away of the urine from the episomites and the release of the
sexual cells from the hyposomites.

The recent investigations of Ruckert and Van Wijhe on the mesodermic
segments of the trunk and the excretory system of the selachii show
that these “primitive fishes” are closely related to the amphioxus in
this further respect. The transverse section of the shark-embryo in
Fig. 161 shows this very clearly.

In other higher vertebrates, also, the kidneys develop (though very
differently formed later on) from similar structures, which have been
secondarily derived from the segmental pronephridia of the acrania. The
parts of the mesoderm at which the first traces of them are found are
usually called the middle or mesenteric plates. As the first traces of
the gonads make their appearance in the lining of these middle plates
nearer inward (or the middle) from the inner funnels of the
nephro-canals, it is better to count this part of the mesoderm with the
hyposoma.

The chief and oldest organ of the vertebrate hyposoma, the alimentary
canal, is generally described as an unsegmented organ. But we could
just as well say that it is the oldest of all the segmented organs of
the vertebrate; the double row of the cœlom-pouches grows out of the
dorsal wall of the gut, on either side of the chorda. In the brief
period during which these segmental cœlom-pouches are still openly
connected with the gut, they look just like a double chain of segmented
visceral glands. But apart from this, we have originally in all
vertebrates an important articulation of the fore-gut, that is wanting
in the lower gut, the segmentation of the branchial (gill) gut.


Fig.172. Transverse section of the shoulder and fore-limb (wing) of a
chick-embryo of the fourth day. Fig. 172—Transverse section of the
shoulder and fore-limb (wing) of a chick-embryo of the fourth day,
magnified about twenty times. Beside the medullary tube we can see on
each side three clear streaks in the dark dorsal wall, which advance
into the rudimentary fore-limb or wing (_e_). The uppermost of them is
the muscular plate; the middle is the hind and the lowest the fore root
of a spinal nerve. Under the chorda in the middle is the single aorta,
at each side of it a cardinal vein, and below these the primitive
kidneys. The gut is almost closed. The ventral wall advances into the
amnion, which encloses the embryo. (From _ Remak._)


The gill-clefts, which originally in the older acrania pierced the wall
of the fore-gut, and the gill-arches that separated them, were
presumably also segmental, and distributed among the various metamera
of the chain, like the gonads in the after-gut and the nephridia. In
the amphioxus, too, they are still segmentally formed. Probably there
was a division of labour of the hyposomites in the older (and long
extinct) acrania, in such wise that those of the fore-gut took over the
function of breathing and those of the after-gut that of reproduction.
The former developed into gill-pouches, the latter into sex-pouches.
There may have been primitive kidneys in both. Though the gills have
lost their function in the higher animals, certain parts of them have
been generally maintained in the embryo by a tenacious heredity. At a
very early stage we notice in the embryo of man and the other amniotes,
at each side of the head, the remarkable and important structures which
we call the gill-arches and gill-clefts (Figs. 167–170 _f_). They
belong to the characteristic and inalienable organs of the
amniote-embryo, and are found always in the same
spot and with the same arrangement and structure. There are formed to
the right and left in the lateral wall of the fore-gut cavity, in its
foremost part, first a pair and then several pairs of sac-shaped
inlets, that pierce the whole thickness of the lateral wall of the
head. They are thus converted into clefts, through which one can
penetrate freely from without into the gullet. The wall thickens
between these branchial folds, and changes into an arch-like or
sickle-shaped piece—the gill, or gullet-arch. In this the muscles and
skeletal parts of the branchial gut separate; a blood-vessel arch rises
afterwards on their inner side (Fig. 98 _ka_). The number of the
branchial arches and the clefts that alternate with them is four or
five on each side in the higher vertebrates (Fig. 170 _d, f, f′, f″_).
In some of the fishes (selachii) and in the cyclostoma we find six or
seven of them permanently.


Fig.173. Transverse section of the pelvic region and hind legs of a
chick-embryo of the fourth day. Fig. 173—Transverse section of the
pelvic region and hind legs of a chick-embryo of the fourth day,
magnified. _h_ horn-plate, _w_ medullary tube, _n_ canal of the tube,
_u_ primitive kidneys, _x_ chorda, _e_ hind legs, _b_ allantoic canal
in the ventral wall, _t_ aorta, _ v_ cardinal veins, _a_ gut, _d_
gut-gland layer, _ f_ gut-fibre layer, _g_ embryonic epithelium, _r_
dorsal muscles, _c_ body-cavity or cœloma. (From _ Waldeyer._)


These remarkable structures had originally the function of respiratory
organs—gills. In the fishes the water that serves for breathing, and is
taken in at the mouth, still always passes out by the branchial clefts
at the sides of the gullet. In the higher vertebrates they afterwards
disappear. The branchial arches are converted partly into the jaws,
partly into the bones of the tongue and the ear. From the first
gill-cleft is formed the tympanic cavity of the ear.

There are few parts of the vertebrate organism that, like the outer
covering or integument of the body, are not subject to metamerism. The
outer skin (_epidermis_) is unsegmented from the first, and proceeds
from the continuous horny plate. Moreover, the underlying _cutis_ is
also not metamerous, although it develops from the segmental structure
of the cutis-plates (Figs. 161, 162 _cp_). The vertebrates are
strikingly and profoundly different from the articulates in these
respects also.

Further, most of the vertebrates still have a number of unarticulated
organs, which have arisen locally, by adaptation of particular parts of
the body to certain special functions. Of this character are the
sense-organs in the episoma, and the limbs, the heart, the spleen, and
the large visceral glands—lungs, liver, pancreas, etc.—in the hyposoma.
The heart is originally only a local spindle-shaped enlargement of the
large ventral blood-vessel or principal vein, at the point where the
subintestinal passes into the branchial artery, at the limit of the
head and trunk (Figs. 170, 171). The three higher sense-organs—nose,
eye, and ear—were originally developed in the same form in all the
craniotes, as three pairs of small depressions in the skin at the side
of the head.

The organ of smell, the nose, has the appearance of a pair of small
pits above the mouth-aperture, in front of the head (Fig. 169 _n_). The
organ of sight, the eye, is found at the side of the head, also in the
shape of a depression (Figs. 169 _l_, 170 _b_), to which corresponds a
large outgrowth of the foremost cerebral vesicle on each side. Farther
behind, at each side of the head, there is a third depression, the
first trace of the organ of hearing (Fig. 169 _g_). As yet we can see
nothing of the later elaborate structure of these organs, nor of the
characteristic build of the face.

When the human embryo has reached
When the human embryo has reached this stage of development, it can
still scarcely be distinguished from that of any other higher
vertebrate. All the chief parts of the body are now laid down: the head
with the primitive skull, the rudiments of the three higher
sense-organs and the five cerebral vesicles, and the gill-arches and
clefts; the trunk with the spinal cord, the rudiment of the vertebral
column, the chain of metamera, the heart and chief blood-vessels, and
the kidneys. At this stage man is a higher vertebrate, but shows no
essential morphological difference from the embryos of the mammals, the
birds, the reptiles, etc. This is an ontogenetic fact of the utmost
significance. From it we can gather the most important phylogenetic
conclusions.


Fig.174. Development of the lizard’s legs. Fig. 174—Development of the
lizard’s legs (_Lacerta agilis_), with special relation to their
blood-vessels. _1, 3, 5, 7, 9, 11_ right fore-leg; _13, 15_ left
fore-leg; _2, 4, 6, 8, 10, 12_ right hind-leg; _ 14, 16_ left hind-leg;
_SRV_ lateral veins of the trunk, _VU_ umbilical vein. (From _F.
Hochstetter._)


There is still no trace of the limbs. Although head and trunk are
separated and all the principal internal organs are laid down, there is
no indication whatever of the “extremities” at this stage; they are
formed later on. Here again we have a fact of the utmost interest. It
proves that the older vertebrates had no feet, as we find to be the
case in the lowest living vertebrates (amphioxus and the cyclostoma).
The descendants of these ancient footless vertebrates only acquired
extremities—two fore-legs and two hind-legs—at a much later stage of
development.
These were at first all alike, though they afterwards vary considerably
in structure—becoming fins (of breast and belly) in the fishes, wings
and legs in the birds, fore and hind legs in the creeping animals, arms
and legs in the apes and man. All these parts develop from the same
simple original structure, which forms secondarily from the trunk-wall
(Figs. 172, 173). They have always the appearance of two pairs of small
buds, which represent at first simple roundish knobs or plates.
Gradually each of these plates becomes a large projection, in which we
can distinguish a small inner part and a broader outer part. The latter
is the rudiment of the foot or hand, the former that of the leg or arm.
The similarity of the original rudiment of the limbs in different
groups of vertebrates is very striking.


Fig.175. Human embryo, five weeks old, half an inch long, seen from the
right. Fig. 175—Human embryo, five weeks old, half an inch long, seen
from the right, magnified. (From _Russel Bardeen_ and _Harmon Lewis._)
In the undissected head we see the eye, mouth, and ear. In the trunk
the skin and part of the muscles have been removed, so that the
cartilaginous vertebral column is free; the dorsal root of a spinal
nerve goes out from each vertebra (towards the skin of the back). In
the middle of the lower half of the figure part of the ribs and
intercostal muscles are visible. The skin and muscles have also been
removed from the right limbs; the internal rudiments of the five
fingers of the hand, and five toes of the foot, are clearly seen within
the fin-shaped plate, and also the strong network of nerves that goes
from the spinal cord to the extremities. The tail projects under the
foot, and to the right of it is the first part of the umbilical cord.


How the five fingers or toes with their
blood-vessels gradually differentiate within the simple fin-like
structure of the limbs can be seen in the instance of the lizard in
Fig. 174. They are formed in just the same way in man: in the human
embryo of five weeks the five fingers can clearly be distinguished
within the fin-plate (Fig. 175).

The careful study and comparison of human embryos with those of other
vertebrates at this stage of development is very instructive, and
reveals more mysteries to the impartial student than all the religions
in the world put together. For instance, if we compare attentively the
three successive stages of development that are represented, in twenty
different amniotes we find a remarkable likeness. When we see that as a
fact twenty different amniotes of such divergent characters develop
from the same embryonic form, we can easily understand that they may
all descend from a common ancestor.


Figs. 176-178. Embryos of the bat (Vespertilio murinus) at three
different stages. Figs. 176–178—Embryos of the bat (_Vespertilio
murinus_) at three different stages. (From _Oscar Schultze._) Fig. 176:
Rudimentary limbs (_v_ fore-leg, _ h_ hind-leg). _l_ lenticular
depression, _r_ olfactory pit, _ok_ upper jaw, _uk_ lower jaw, _ k_2,
_k_3, _k_4 first, second and third gill-arches, _a_ amnion, _n_
umbilical vessel, _d_ yelk-sac. Fig. 177: Rudiment of flying membrane,
membranous fold between fore and hind leg. _n_ umbilical vessel, _o_
ear-opening, _f_ flying membrane. Fig. 178: The flying membrane
developed and stretched across the fingers of the hands, which cover
the face.


In the first stage of development, in which the head with the five
cerebral vesicles is already clearly indicated, but there are no limbs,
the embryos of all the vertebrates, from the fish to man, are only
incidentally or not at all different from each other. In the second
stage, which shows the limbs, we begin to see differences between the
embryos of the lower and higher vertebrates; but the human embryo is
still hardly distinguishable from that of the higher mammals. In the
third stage, in which the gill-arches have disappeared and the face is
formed, the differences become more pronounced. These are facts of a
significance that cannot be exaggerated.[27]

 [27] Because they show how the most diverse structures may be
 developed from a common form. As we actually see this in the case of
 the embryos, we have a right to assume it in that of the stem-forms.
 Nevertheless, this resemblance, however great, is never a real
 identity. Even the embryos of the different individuals of one species
 are usually not really identical. If the reader can consult the
 complete edition of this work at a library, he will find six plates
 illustrating these twenty embryos.


If there is an intimate causal connection between the processes of
embryology and stem-history, as we must assume in virtue of the laws of
heredity, several important phylogenetic conclusions follow at once
from these ontogenetic facts. The profound and remarkable similarity in
the embryonic development of man and the other vertebrates can only be
explained when we admit their descent from a common ancestor. As a
fact, this common descent is now accepted by all competent scientists;
they have substituted the natural evolution for the supernatural
creation of organisms.



Chapter XV.
FŒTAL MEMBRANES AND CIRCULATION


Among the many interesting phenomena that we have encountered in the
course of human embryology, there is an especial importance in the fact
that the development of the human body follows from the beginning just
the same lines as that of the other viviparous mammals. As a fact, all
the embryonic peculiarities that distinguish the mammals from other
animals are found also in man; even the ovum with its distinctive
membrane (_zona pellucida,_ Fig. 14) shows the same typical
structure in all mammals (apart from the older oviparous monotremes).
It has long since been deduced from the structure of the developed man
that his natural place in the animal kingdom is among the mammals.
Linné (1735) placed him in this class with the apes, in one and the
same order (_primates_), in his _Systema Naturæ._ This position is
fully confirmed by comparative embryology. We see that man entirely
resembles the higher mammals, and most of all the apes, in embryonic
development as well as in anatomic structure. And if we seek to
understand this ontogenetic agreement in the light of the biogenetic
law, we find that it proves clearly and necessarily the descent of man
from a series of other mammals, and proximately from the primates. The
common origin of man and the other mammals from a single ancient
stem-form can no longer be questioned; nor can the immediate
blood-relationship of man and the ape.


Fig.179. Human embryos from the second to the fifteenth week, seen from
the left. Fig. 179—Human embryos from the second to the fifteenth week,
seen from the left, the curved back turned towards the right. (Mostly
from _ Ecker._) II of fourteen days. III of three weeks. IV of four
weeks. V of five weeks. VI of six weeks. VII of seven weeks. VIII of
eight weeks. XII of twelve weeks. XV of fifteen weeks.


The essential agreement in the whole bodily form and inner structure is
still visible in the embryo of man and the other mammals at the late
stage of development at which the mammal-body can be recognised as
such. But at a somewhat earlier stage, in which the limbs, gill-arches,
sense-organs, etc., are already outlined, we cannot yet recognise the
mammal embryos as such, or distinguish them from those of birds and
reptiles. When we consider still earlier stages of development, we are
unable to discover any essential difference in bodily structure between
the embryos of these higher vertebrates and those of the lower, the
amphibia and fishes. If, in fine, we go back to the construction of the
body out of the four germinal layers, we are astonished to perceive
that these four layers are the same in all vertebrates, and everywhere
take a similar part in the building-up of the fundamental organs of the
body. If we inquire as to the origin of these four secondary layers, we
learn that they always arise in the same way from the two primary
layers; and the latter have the same significance in all the metazoa
(_i.e.,_ all animals except the unicellulars). Finally, we see that the
cells which make up the primary germinal layers owe their origin in
every case to the repeated cleavage of a single simple cell, the
stem-cell or fertilised ovum.


Fig.180. Very young human embryo of the fourth week, one-fourth of an
inch long. Fig. 180—Very young human embryo of the fourth week,
one-fourth of an inch long (taken from the womb of a suicide eight
hours after death). (From _Rabl._) _n_ nasal pits, _ a_ eye, _u_ lower
jaw, _z_ arch of hyoid bone, _ k3_ and _k4_ third and fourth gill-arch,
_h_ heart; _s_ primitive segments, _vg_ fore-limb (arm), _hg_ hind-limb
(leg), between the two the ventral pedicle.


It is impossible to lay too much stress on this remarkable agreement in
the chief embryonic features in man and the other animals. We shall
make use of it later on for our monophyletic theory of descent—the
hypothesis of a common descent of man and all the metazoa from the
gastræa. The first rudiments of the principal parts of the body,
especially the oldest organ, the alimentary canal, are the same
everywhere; they have always the same extremely simple form. All the
peculiarities that distinguish the various groups of animals from each
other only appear gradually in the course of embryonic development; and
the closer the relation of the various groups, the later they are
found. We may formulate this phenomenon in a definite law, which may in
a sense be regarded as an appendix to our biogenetic law. This is the
law of the ontogenetic connection of related animal forms. It runs: The
closer the
relation of two fully-developed animals in respect of their whole
bodily structure, and the nearer they are connected in the
classification of the animal kingdom, the longer do their embryonic
forms retain their identity, and the longer is it impossible (or only
possible on the ground of subordinate features) to distinguish between
their embryos. This law applies to all animals whose embryonic
development is, in the main, an hereditary summary of their ancestral
history, or in which the original form of development has been
faithfully preserved by heredity. When, on the other hand, it has been
altered by cenogenesis, or disturbance of development, we find a
limitation of the law, which increases in proportion to the
introduction of new features by adaptation (cf. Chapter I, pp. 4–6).
Thus the apparent exceptions to the law can always be traced to
cenogenesis.


Fig.181. Human embryo of the middle of the fifth week, one-third of an
inch long. Fig. 181—Human embryo of the middle of the fifth week,
one-third of an inch long. (From _Rabl._) Letters as in Fig. 180,
except _sk_ curve of skull, _ok_ upper jaw, _ hb_ neck-indentation.


When we apply to man this law of the ontogenetic connection of related
forms, and run rapidly over the earliest stages of human development
with an eye to it, we notice first of all the structural identity of
the ovum in man and the other mammals at the very beginning (Figs. 1,
14). The human ovum possesses all the distinctive features of the ovum
of the viviparous mammals, especially the characteristic formation of
its membrane (_zona pellucida_), which clearly distinguishes it from
the ovum of all other animals. When the human fœtus has attained the
age of fourteen days, it forms a round vesicle (or “embryonic vesicle”)
about a quarter of an inch in diameter. A thicker part of its border
forms a simple sole-shaped embryonic shield one-twelfth of an inch long
(Fig. 133). On its dorsal side we find in the middle line the straight
medullary furrow, bordered by the two parallel dorsal or medullary
swellings. Behind, it passes by the neurenteric canal into the
primitive gut or primitive groove. From this the folding of the two
cœlom-pouches proceeds in the same way as in the other mammals (cf.
Fig. 96, 97). In the middle of the sole-shaped embryonic shield the
first primitive segments immediately begin to make their appearance. At
this age the human embryo cannot be distinguished from that of other
mammals, such as the hare or dog.

A week later (or after the twenty-first day) the human embryo has
doubled its length; it is now about one-fifth of an inch long, and,
when seen from the side, shows the characteristic bend of the back, the
swelling of the head-end, the first outline of the three higher
sense-organs, and the rudiments of the gill-clefts, which pierce the
sides of the neck (Fig. 179, III). The allantois has grown out of the
gut behind. The embryo is already entirely enclosed in the amnion, and
is only connected in the middle of the belly by the vitelline duct with
the embryonic vesicle, which changes into the yelk-sac. There are no
extremities or limbs at this stage, no trace of arms or legs. The
head-end has been strongly differentiated from the tail-end; and the
first outlines of the cerebral vesicles in front, and the heart below,
under the fore-arm, are already more or less clearly seen. There is as
yet no real face. Moreover, we seek in vain at this stage a special
character that may distinguish the human embryo from that of other
mammals.

A week later (after the fourth week, on the twenty-eighth to thirtieth
day of development) the human embryo has
reached a length of about one-third of an inch (Fig 179 IV). We can now
clearly distinguish the head with its various parts; inside it the five
primitive cerebral vesicles (fore-brain, middle-brain,
intermediate-brain, hind-brain, and after-brain); under the head the
gill-arches, which divide the gill-clefts; at the sides of the head the
rudiments of the eyes, a couple of pits in the outer skin, with a pair
of corresponding simple vesicles growing out of the lateral wall of the
fore-brain (Figs. 180, 181 _a_). Far behind the eyes, over the last
gill-arches, we see a vesicular rudiment of the auscultory organ. The
rudimentary limbs are now clearly outlined—four simple buds of the
shape of round plates, a pair of fore (_vg_) and a pair of hind legs
(_hg_), the former a little larger than the latter. The large head
bends over the trunk, almost at a right angle. The latter is still
connected in the middle of its ventral side with the embryonic vesicle;
but the embryo has still further severed itself from it, so that it
already hangs out as the yelk-sac. The hind part of the body is also
very much curved, so that the pointed tail-end is directed towards the
head. The head and face-part are sunk entirely on the still open
breast. The bend soon increases so much that the tail almost touches
the forehead (Fig. 179 V.; Fig. 181). We may then distinguish three or
four special curves on the round dorsal surface—namely, a skull-curve
in the region of the second cerebral vesicle, a neck-curve at the
beginning of the spinal cord, and a tail-curve at the fore-end. This
pronounced curve is only shared by man and
the higher classes of vertebrates (the amniotes); it is much slighter,
or not found at all, in the lower vertebrates. At this age (four weeks)
man has a considerable tail, twice as long as his legs. A vertical
longitudinal section through the middle plane of this tail (Fig. 182)
shows that the hinder end of the spinal marrow extends to the point of
the tail, as also does the underlying chorda (_ch_), the terminal
continuation of the vertebral column. Of the latter, the rudiments of
the seven coccygeal (or lowest) vertebræ are visible—thirty-two
indicates the third and thirty-six the seventh of these. Under the
vertebral column we see the hindmost ends of the two large
blood-vessels of the tail, the principal artery (_aorta caudalis_ or
_arteria sacralis media, Ao_), and the principal vein (_vena caudalis_
or _sacralis media_). Underneath is the opening of the anus (_an_) and
the urogenital sinus (_S.ug_). From this anatomic structure of the
human tail it is perfectly clear that it is the rudiment of an
ape-tail, the last hereditary relic of a long hairy tail, which has
been handed down from our tertiary primate ancestors to the present
day.


Fig.182. Median longitudinal section of the tail of a human embryo,
two-thirds of an inch long. Fig. 182—Median longitudinal section of the
tail of a human embryo, two-thirds of an inch long. (From _Ross
Granville Harrison._) _Med_ medullary tube, _Ca.fil_ caudal filament,
_ch_ chorda, _ao_ caudal artery, _V.c.i_ caudal vein, _an_ anus, _S.ug_
sinus urogenitalis.


Fig.183. Human embryo, four weeks old, opened on the ventral side. Fig.
183—Human embryo, four weeks old, opened on the ventral side. Ventral
and dorsal walls are cut away, so as to show the contents of the
pectoral and abdominal cavities. All the appendages are also removed
(amnion, allantois, yelk-sac), and the middle part of the gut. _n_ eye,
_3_ nose, _4_ upper jaw, _5_ lower jaw, _6_ second, _6″_ third
gill-arch, _ov_ heart (_o_ right, _o′_ left auricle; _v_ right, _v′_
left ventricle), _b_ origin of the aorta, _f_ liver (_u_ umbilical
vein), _e_ gut (with vitelline artery, cut off at _a′_), _j′_ vitelline
vein, _m_ primitive kidneys, _t_ rudimentary sexual glands, _ r_
terminal gut (cut off at the mesentery _z_), _n_ umbilical artery, _u_
umbilical vein, _9_ fore-leg, _ 9′_ hind-leg. (From _Coste._)


Human embryo, five weeks old, opened from the ventral side. Fig.
184—Human embryo, five weeks old, opened from the ventral side (as in
Fig. 183). Breast and belly-wall and liver are removed. _3_ outer nasal
process, _4_ upper jaw, _5_ lower jaw, _z_ tongue, _v_ right, _v′_ left
ventricle of heart, _o′_ left auricle, _b_ origin of aorta, _b′, b″,
b‴_ first, second, and third aorta-arches, _c, c′, c″_ vena cava, _ae_
lungs (_y_ pulmonary artery), _e_ stomach, _m_ primitive kidneys (_j_
left vitelline vein, _s_ cystic vein, _a_ right vitelline artery, _n_
umbilical artery, _u_ umbilical vein), _ x_ vitelline duct, _i_ rectum,
_8_ tail, _9_ fore-leg, _9′_ hind-leg. (From _Coste._)


It sometimes happens that we find even external relics of this tail
growing. According to the illustrated works of
Surgeon-General Bernhard Ornstein, of Greece, these tailed men are not
uncommon; it is not impossible that they gave rise to the ancient
fables of the satyrs. A great number of such cases are given by Max
Bartels in his essay on “Tailed Men” (1884, in the _Archiv für
Anthropologie,_ Band XV), and critically examined. These atavistic
human tails are often mobile; sometimes they contain only muscles and
fat, sometimes also rudiments of caudal vertebræ. They have a length of
eight to ten inches and more. Granville Harrison has very carefully
studied one of these cases of “pigtail,” which he removed by operation
from a six months old child in 1901. The tail moved briskly when the
child cried or was excited, and was drawn up when at rest.


Fig.185. The head of Miss Julia Pastrana. Fig. 185—The head of Miss
Julia Pastrana. (From a photograph by _Hintze._)


Human ovum of twelve to thirteen days. Fig. 186—Human ovum of twelve to
thirteen days (?). (From _Allen Thomson._) 1. Not opened. 2. Opened and
magnified. Within the outer chorion the tiny curved fœtus lies on the
large embryonic vesicle, to the left above.


Fig.187. Human ovum of ten days. Fig. 188. Human foetus of ten days,
taken from the preceding ovum, magnified. Fig. 187—Human ovum of ten
days. (From _Allen Thomson._) Opened; the small fœtus in the right
half, above.
Fig. 188—Human fœtus of ten days, taken from the preceding ovum,
magnified, _a_ yelk-sac, _b_ neck (the medullary groove already
closed), _ c_ head (with open medullary groove), _d_ hind part (with
open medullary groove), _e_ a shred of the amnion.


Fig.189. Human ovum of twenty to twenty-two days. Fig. 190. Human
foetus of twenty to twenty-two days, taken from the preceding ovum,
magnified. Fig. 189—Human ovum of twenty to twenty-two days. (From
_Allen Thomson._) Opened. The chorion forms a spacious vesicle, to the
inner wall of which the small fœtus (to the right above) is attached by
a short umbilical cord.
Fig. 190—Human fœtus of twenty to twenty-two days, taken from the
preceding ovum, magnified. _a_ amnion, _b_ yelk-sac, _c_ lower-jaw
process of the first gill-arch, _d_ upper-jaw process of same, _e_
second gill-arch (two smaller ones behind). Three gill-clefts are
clearly seen. _f_ rudimentary fore-leg, _ g_ auditory vesicle, _h_ eye,
_i_ heart.


In the opinion of some travellers and anthropologists, the atavistic
tail-formation is hereditary in certain isolated tribes (especially in
south-eastern Asia and the archipelago), so that we might speak of a
special race or “species” of tailed men
(_Homo caudatus_). Bartels has “no doubt that these tailed men will be
discovered in the advance of our geographical and ethnographical
knowledge of the lands in question” (_Archiv für Anthropologie,_ Band
XV, p. 129).


Fig.191. Human embryo of sixteen to eighteen days. Fig. 191—Human
embryo of sixteen to eighteen days. (From _Coste._) Magnified. The
embryo is surrounded by the amnion, (_a_), and lies free with this in
the opened embryonic vesicle. The belly is drawn up by the large
yelk-sac (_d_), and fastened to the inner wall of the embryonic
membrane by the short and thick pedicle (_b_). Hence the normal convex
curve of the back (Fig. 190) is here changed into an abnormal concave
surface. _h_ heart, _ m_ parietal mesoderm. The spots on the outer wall
of the serolemma are the roots of the branching chorion-villi, which
are free at the border.


When we open a human embryo of one month  (Fig. 183), we find the
alimentary canal formed in the body-cavity, and for the most part cut
off from the embryonic vesicle. There are both mouth and anus
apertures. But the mouth-cavity is not yet separated from the nasal
cavity, and the face not yet shaped. The heart shows all its four
sections; it is very large, and almost fills the whole of the pectoral
cavity (Fig. 183 _ov_). Behind it are the very small rudimentary lungs.
The primitive kidneys (_m_) are very large; they fill the greater part
of the abdominal cavity, and extend from the liver (_f_) to the pelvic
gut. Thus at the end of the first month all the chief organs are
already outlined. But there are at this stage no features by which the
human embryo materially differs from that of the dog, the hare, the ox,
or the horse—in a word, of any other higher mammal. All these embryos
have the same, or at least a very similar, form; they can at the most
be
distinguished from the human embryo by the total size of the body or
some other insignificant difference in size. Thus, for instance, in man
the head is larger in proportion to the trunk than in the ox. The tail
is rather longer in the dog than in man. These are all negligible
differences. On the other hand, the whole internal organisation and the
form and arrangement of the various organs are essentially the same in
the human embryo of four weeks as in the embryos of the other mammals
at corresponding stages.


Fig.192. Human embryo of the fourth week, one-third of an inch long,
lying in the dissected chorion. Fig. 192—Human embryo of the fourth
week, one-third of an inch long, lying in the dissected chorion.


Fig.193. Human embryo of the fourth week, with its membranes, like Fig.
192, but a little older. Fig. 193—Human embryo of the fourth week, with
its membranes, like Fig. 192, but a little older. The yelk-sac is
rather smaller, the amnion and chorion larger.


It is otherwise in the second month of human development. Fig. 179
represents a human embryo of six weeks (VI), one of seven weeks (VII),
and one of eight weeks (VIII), at natural size. The differences which
mark off the human embryo from that of the dog and the lower mammals
now begin to be more pronounced. We can see important differences at
the sixth, and still more at the eighth week, especially in the
formation of the head. The size of the various sections of the brain is
greater in man, and the tail is shorter.

Other differences between man and the lower mammals are found in the
relative size of the internal organs. But even at this stage the human
embryo differs very little from that of the nearest related mammals—the
apes, especially the anthropomorphic apes.

The features by means of which we distinguish between them are not
clear until later on. Even at a much more advanced stage of
development, when we can distinguish the human fœtus from that of the
ungulates at a glance, it still closely resembles that of the higher
apes. At last we get the distinctive features, and
we can distinguish the human embryo confidently at the first glance
from that of all other mammals during the last four months of fœtal
life—from the sixth to the ninth month of pregnancy. Then we begin to
find also the differences between the various races of men, especially
in regard to the formation of the skull and the face. (Cf. Chapter
XXIII.)


Fig.194. Human embryo with its membranes, six weeks old. Fig. 194—Human
embryo with its membranes, six weeks old. The outer envelope of the
whole ovum is the chorion, thickly covered with its branching villi, a
product of the serous membrane. The embryo is enclosed in the delicate
amnion-sac. The yelk-sac is reduced to a small pear-shaped umbilical
vesicle; its thin pedicle, the long vitelline duct, is enclosed in the
umbilical cord. In the latter, behind the vitelline duct, is the much
shorter pedicle of the allantois, the inner lamina of which (the
gut-gland layer) forms a large vesicle in most of the mammals, while
the outer lamina is attached to the inner wall of the outer embryonic
coat, and forms the placenta there. (Half diagrammatic.)


The striking resemblance that persists so long between the embryo of
man and of the higher apes disappears much earlier in the lower apes.
It naturally remains longest in the large anthropomorphic apes
(gorilla, chimpanzee, orang, and gibbon). The physiognomic similarity
of these animals, which we find so great in their earlier years,
lessens with the increase of age. On the other hand, it remains
throughout life in the remarkable long-nosed ape of Borneo (_Nasalis
larvatus_). Its finely-shaped nose would be regarded with envy by many
a man who has too little of that organ. If we compare the face of the
long-nosed ape with that of abnormally ape-like human beings (such as
the famous Miss Julia Pastrana, Fig. 185), it will be admitted to
represent a higher stage of development. There are still people among
us who look especially to the face for the “image of God in man.” The
long-nosed ape would have more claim to this than some of the
stumpy-nosed human individuals one meets.

This progressive divergence of the human from the animal form, which is
based on the law of the ontogenetic connection between related forms,
is found in the structure of the internal organs as well as in external
form. It is also expressed in the construction of the envelopes and
appendages that we find surrounding the fœtus externally, and that we
will now consider more closely. Two of these appendages—the amnion and
the allantois—are only found in the three higher classes of
vertebrates, while the third, the yelk-sac, is found in most of the
vertebrates. This is a circumstance of great importance, and it gives
us valuable data for constructing man’s genealogical tree.

As regards the external membrane that encloses the ovum in the mammal
womb,
we find it just the same in man as in the higher mammals. The ovum is,
the reader will remember, first surrounded by the transparent
structureless _ovolemma_ or _zona pellucida_ (Figs. 1, 14). But very
soon, even in the first week of development, this is replaced by the
permanent chorion. This is formed from the external layer of the
amnion, the _serolemma,_ or “serous membrane,” the formation of which
we shall consider presently; it surrounds the fœtus and its appendages
as a broad, completely closed sac; the space between the two, filled
with clear watery fluid, is the _serocœlom,_ or interamniotic cavity
(“extra-embryonic body-cavity”). But the smooth surface of the sac is
quickly covered with numbers of tiny tufts, which are really hollow
outgrowths like the fingers of a glove (Figs. 186, 191, 198 _chz_).
They ramify and push into the corresponding depressions that are formed
by the tubular glands of the mucous membrane of the maternal womb.
Thus, the ovum secures its permanent seat (Fig. 186–194).


Fig.195. Diagram of the embryonic organs of the mammal (foetal
membranes and appendages). Fig. 195—Diagram of the embryonic organs of
the mammal (fœtal membranes and appendages). (From _Turner._) _E, M, H_
outer, middle, and inner germ layer of the embryonic shield, which is
figured in median longitudinal section, seen from the left. _am_
amnion. _AC_ amniotic cavity, _UV_ yelk-sac or umbilical vesicle, _ALC_
allantois, _al_ pericœlom or serocœlom (inter-amniotic cavity), _ sz_
serolemma (or serous membrane), _pc_ prochorion (with villi).)


In human ova of eight to twelve days this external membrane, the
chorion, is already covered with small tufts or villi, and forms a ball
or spheroid of one-fourth to one-third of an inch in diameter (Figs.
186–188). As a large quantity of fluid gathers inside it, the chorion
expands more and more, so that the embryo only occupies a small part of
the space within the vesicle. The villi of the chorion grow larger and
more numerous. They branch out more and more. At first the villi cover
the whole surface, but they afterwards disappear from the greater part
of it; they then develop with proportionately greater vigour at a spot
where the placenta is formed from the allantois.

When we open the chorion of a human embryo of three weeks, we find on
the ventral side of the fœtus a large round sac, filled with fluid.
This is the yelk-sac, or “umbilical vesicle,” the origin of which we
have considered previously. The larger the embryo becomes the smaller
we find the yelk-sac. In the end we find the remainder of it in the
shape of a small pear-shaped vesicle, fastened to a long thin stalk (or
pedicle), and hanging from the open belly of the fœtus (Fig. 194). This
pedicle is the vitelline duct, and is separated from the body at the
closing of the navel.

Behind the yelk-sac a second appendage,
of much greater importance, is formed at an early stage at the belly of
the mammal embryo. This is the allantois or “primitive urinary sac,” an
important embryonic organ, only found in the three higher classes of
vertebrates. In all the amniotes the allantois quickly appears at the
hinder end of the alimentary canal, growing out of the cavity of the
pelvic gut (Fig. 147 _r, u,_ Fig. 195 _ ALC_).

The further development of the allantois varies considerably in the
three sub-classes of the mammals. The two lower sub-classes, monotremes
and marsupials, retain the simpler structure of their ancestors, the
reptiles. The wall of the allantois and the enveloping serolemma
remains smooth and without villi, as in the birds. But in the third
sub-class of the mammals the serolemma forms, by invagination at its
outer surface, a number of hollow tufts or villi, from which it takes
the name of the _chorion_ or _mallochorion._ The gut-fibre layer of the
allantois, richly supplied with branches of the umbilical vessel,
presses into these tufts of the primary chorion, and forms the
“secondary chorion.” Its embryonic blood-vessels are closely correlated
to the contiguous maternal blood-vessels of the environing womb, and
thus is formed the important nutritive apparatus of the embryo which we
call the placenta.


Fig.196. Diagrammatic frontal section of the pregnant human womb. Fig.
196—Diagrammatic frontal section of the pregnant human womb. (From
_Longet._) The embryo hangs by the umbilical cord, which encloses the
pedicle of the allantois (_al_). _ nb_ umbilical vessel, _am_ amnion,
_ch_ chorion, _ ds_ decidua serotina, _dv_ decidua vera, _dr_ decidua
reflexa, _z_ villi of the placenta, _c_ cervix uteri, _ u_ uterus.)


The pedicle of the allantois, which connects the embryo with the
placenta and conducts the strong umbilical vessels from the former to
the latter, is covered by the amnion, and, with this amniotic sheath
and the pedicle of the yelk-sac, forms what is called the _umbilical
cord_ (Fig. 196 _al_). As the large and blood-filled vascular network
of the fœtal allantois attaches itself closely to the mucous lining of
the maternal womb, and the partition between the blood-vessels of
mother and child becomes much thinner, we get that remarkable nutritive
apparatus of the fœtal body which is characteristic of the placentalia
(or choriata). We shall return afterwards to the closer consideration
of this (cf. Chapter XXIII).

In the various orders of mammals the placenta undergoes many
modifications, and these are in part of great evolutionary importance
and useful in classification. There is only one of these that need be
specially mentioned—the important fact, established by Selenka in 1890,
that the distinctive human placentation is confined to the anthropoids.
In this most advanced group of the mammals the allantois is very small,
soon loses its cavity, and then, in common with the amnion, undergoes
certain peculiar changes. The umbilical cord develops in this case from
what is called the “ventral pedicle.” Until very recently this was
regarded as a structure peculiar to man. We now know from Selenka that
the much-discussed ventral pedicle is merely the pedicle of the
allantois, combined with the pedicle of the amnion and the rudimentary
pedicle of the yelk-sac. It has just the same structure in the orang
and gibbon (Fig. 197) and very probably in the chimpanzee and gorilla,
as in man; it is, therefore, not a _disproof,_ but a striking fresh
proof, of the blood-relationship of man and the anthropoid apes.

We find only in the anthropoid apes—the gibbon and orang of Asia and
the chimpanzee and gorilla of Africa—the peculiar and elaborate
formation of the placenta that characterises man (Fig. 198).
In this case there is at an early stage an intimate blending of the
chorion of the embryo and the part of the mucous lining of the womb to
which it attaches. The villi of the chorion with the blood-vessels they
contain grow so completely into the tissue of the uterus, which is rich
in blood, that it becomes impossible to separate them, and they form
together a sort of cake. This comes away as the “afterbirth” at
parturition; at the same time, the part of the mucous lining of the
womb that has united inseparably with the chorion is torn away; hence
it is called the _decidua_ (“falling-away membrane”), and also the
“sieve-membrane,” because it is perforated like a sieve. We find a
decidua of this kind in most of the higher placentals; but it is only
in man and the anthropoid apes that it divides into three parts—the
outer, inner, and placental decidua. The external or true decidua (Fig.
196 _du,_ Fig. 199 _g_) is the part of the mucous lining of the womb
that clothes the inner surface of the uterine cavity wherever it is not
connected with the placenta. The placental or spongy decidua
(_placentalis_ or _serotina,_ Fig. 196 _ds,_ Fig. 199 _d_) is really
the placenta itself, or the maternal part of it (_placenta
uterina_)—namely, that part of the mucous lining of the womb which
unites intimately with the chorion-villi of the fœtal placenta. The
internal or false decidua (_interna_ or _reflexa,_ Fig. 196 _dr,_ Fig.
199 _f_) is that part of the mucous lining of the womb which encloses
the remaining surface of the ovum, the smooth chorion (_chorion læve_),
in the shape of a special thin membrane. The origin of these three
different deciduous membranes, in regard to which quite erroneous views
(still retained in their names) formerly prevailed, is now quite clear,
The external _ decidua vera_ is the specially modified and subsequently
detachable superficial stratum of the original mucous lining of the
womb. The placental _decidua serotina_ is that part of the preceding
which is completely transformed by the ingrowth of the chorion-villi,
and is used for constructing the placenta. The inner _decidua reflexa_
is formed by the rise of a circular fold of the mucous lining (at the
border of the _decidua vera_ and _ serotina_), which grows over the
fœtus (like the anmnion) to the end.


Fig.197. Male embryo of the Siamang-gibbon (Hylobates siamanga) of
Sumatra. Fig. 197—Male embryo of the Siamang-gibbon (_Hylobates
siamanga_) of Sumatra; to the left the dissected uterus, of which only
the dorsal half is given. The embryo has been taken out, and the limbs
folded together; it is still connected by the umbilical cord with the
centre of the circular placenta which is attached to the inside of the
womb. This embryo takes the head-position in the womb, and this is
normal in man also.


The peculiar anatomic features that characterise the human fœtal
membranes are found in just the same way in the higher
apes. Until recently it was thought that the human embryo was
distinguished by its peculiar construction of a solid allantois and a
special ventral pedicle, and that the umbilical cord developed from
this in a different way than in the other mammals. The opponents of the
unwelcome “ape-theory” laid great stress on this, and thought they had
at last discovered an important indication that separated man from all
the other placentals. But the remarkable discoveries published by the
distinguished zoologist Selenka in 1890 proved that man shares these
peculiarities of placentation with the anthropoid apes, though they are
not found in the other apes. Thus the very feature which was advanced
by our critics as a disproof became a most important piece of evidence
in favour of our pithecoid origin.)


Fig.198. Frontal section of the pregnant human womb. Fig. 198—Frontal
section of the pregnant human womb. (From _Turner._) The embryo (a
month old) hangs in the middle of the amniotic cavity by the ventral
pedicle or umbilical cord, which connects it with the placenta (above).


Of the three vesicular appendages of the amniote embryo which we have
now described the amnion has no blood-vessels at any moment of its
existence. But the other two vesicles, the yelk-sac and the allantois,
are equipped with large blood-vessels, and these effect the nourishment
of the embryonic body. We may take the opportunity to make a few
general observations on the first circulation in the embryo and its
central organ, the heart. The first blood-vessels, the heart, and the
first blood itself, are formed from the gut-fibre layer. Hence it was
called by earlier embryologists the “vascular layer.” In a sense the
term is quite correct. But it must not be understood as if all the
blood-vessels in the body came from this layer, or as if the whole of
this layer were taken up only with the formation of blood-vessels.
Neither of these suppositions is true. Blood-vessels may be formed
independently in other parts, especially in the various products of the
skin-fibre layer.


Fig.199. Human foetus, twelve weeks old, with its membranes. Fig.
199—Human fœtus, twelve weeks old, with its membranes. The umbilical
cord goes from its navel to the placenta. _b_ amnion, _c_ chorion, _d_
placenta, _d_ apostrophe, relics of villi on smooth chorion, _f_
internal or reflex decidua, _g_ external or true decidua. (From _B.
Schultze._)


Fig.200. Mature human foetus (at the end of the pregnancy, in its
natural position, taken out of the uterine cavity). Fig. 200—Mature
human fœtus (at the end of pregnancy, in its natural position, taken
out of the uterine cavity). On the inner surface of the latter (to the
left) is the placenta, which is connected by the umbilical cord with
the child’s navel. (From _Bernhard Schultze._)


The first blood-vessels of the mammal embryo have been considered by us
previously, and we shall study the development of the heart in the
second volume.

In every vertebrate it lies at first in the ventral wall of the
fore-gut, or in the ventral (or cardiac) mesentery, by which it is
connected for a time with the wall of the body. But it soon severs
itself from the place of its origin, and lies freely in a cavity—the
cardiac cavity. For a short time it is still connected with the former
by the thin plate of the mesocardium. Afterwards it lies quite free in
the cardiac cavity, and is only directly connected with the gut-wall by
the vessels which issue from it.


Fig.201. Vitelline vessels in the germinative area of a chick-embryo,
at the close of the third day of incubation. Fig. 201—Vitelline vessels
in the germinative area of a chick-embryo, at the close of the third
day of incubation. (From _Balfour._) The detached germinative area is
seen from the ventral side: the arteries are dark, the veins light. _H_
heart, _AA_ aorta-arches, _Ao_ aorta, _R.of.A_ right omphalo-mesenteric
artery, _S.T._ sinus terminalis, _ L.Of_ and _R.Of_ right and left
omphalo-mesenteric veins, _S.V._ sinus venosus, _D.C._ ductus Cuvieri,
_ S.Ca.V._ and _V.Ca._ fore and hind cardinal veins.


The fore-end of the spindle-shaped tube, which soon bends into an
S-shape (Figure 1.202), divides into a right and left branch. These
tubes are bent upwards arch-wise, and represent the first arches of the
aorta. They rise in the wall of the fore-gut, which they enclose in a
sense, and then unite above, in the upper wall of the fore gut-cavity,
to form a large single artery, that runs backward immediately under the
chorda, and is called the aorta (Fig. 201 _Ao_). The first pair of
aorta-arches rise on the inner wall of the first pair of gill-arches,
and so lie between the first gill-arch (_k_) and the fore-gut (_d_),
just as we find them throughout life in the fishes. The single aorta,
which results from the conjunction of these two first vascular arches,
divides again immediately into two parallel branches, which run
backwards on either side of the chorda. These are the primitive aortas
which we have already mentioned; they are also called the posterior
vertebral arteries. These two arteries now give off at each side,
behind, at right angles, four or five branches, and these pass from the
embryonic body to the germinative area, they
are called omphalo-mesenteric or vitelline arteries. They represent the
first beginning of a fœtal circulation. Thus, the first blood-vessels
pass over the embryonic body and reach as far as the edge of the
germinative area. At first they are confined to the dark or “vascular”
area. But they afterwards extend over the whole surface of the
embryonic vesicle. In the end, the whole of the yelk-sac is covered
with a vascular net-work. These vessels have to gather food from the
contents of the yelk-sac and convey it to the embryonic body. This is
done by the veins, which pass first from the germinative area, and
afterwards from the yelk-sac, to the farther end of the heart. They are
called vitelline, or, frequently, omphalo-mesenteric, veins.

These vessels naturally atrophy with the degeneration of the umbilical
vesicle, and the vitelline circulation is replaced by a second, that of
the allantois. Large blood-vessels are developed in the wall of the
urinary sac or the allantois, as before, from the gut-fibre layer.
These vessels grow larger and larger, and are very closely connected
with the vessels that develop in the body of the embryo itself. Thus,
the secondary, allantoic circulation gradually takes the place of the
original vitelline circulation. When the allantois has attached itself
to the inner wall of the chorion and been converted into the placenta,
its blood-vessels alone effect the nourishment of the embryo. They are
called umbilical vessels, and are originally double—a pair of umbilical
arteries and a pair of umbilical veins. The two umbilical veins (Fig.
183 _u_), which convey blood from the placenta to the heart, open it
first into the united vitelline veins. The latter then disappear, and
the right umbilical vein goes with them, so that henceforth a single
large vein, the left umbilical vein, conducts all the blood from the
placenta to the heart of the embryo. The two arteries of the allantois,
or the umbilical arteries (Figs. 183 _n_, 184 _n_), are merely the
ultimate terminations of the primitive aortas, which are strongly
developed afterwards. This umbilical circulation is retained until the
nine months of embryonic life are over, and the human embryo enters
into the world as the independent individual. The umbilical cord (Fig.
196 _al_), in which these large blood-vessels pass from the embryo to
the placenta, comes away, together with the latter, in the after-birth,
and with the use of the lungs begins an entirely new form of
circulation, which is confined to the body of the infant.


Fig.202. Boat-shaped embryo of the dog, from the ventral side,
magnified. Fig. 202—Boat-shaped embryo of the dog, from the ventral
side, magnified. In front under the forehead we can see the first pair
of gill-arches; underneath is the S-shaped heart, at the sides of which
are the auditory vesicles. The heart divides behind into the two
vitelline veins, which expand in the germinative area (which is torn
off all round). On the floor of the open belly lie, between the
protovertebræ, the primitive aortas, from which five pairs of vitelline
arteries are given off. (From _ Bischoff._)


There is a great phylogenetic significance in the perfect agreement
which we find between man and the anthropoid apes in these important
features of embryonic circulation, and the special construction of the
placenta and the umbilical cord. We must infer from it a close
blood-relationship of man and the anthropomorphic apes—a common descent
of them from one and the same extinct group of lower apes. Huxley’s
“pithecometra-principle” applies to these ontogenetic features as much
as to any other morphological relations: “The differences in
construction of any part of the body are less between man and the
anthropoid apes than between the latter and the lower apes.”

This important Huxleian law, the chief consequence of which is “the
descent of man from the ape,” has lately been confirmed in an
interesting and unexpected way from the side of the experimental
physiology of the blood. The experiments of Hans Friedenthal at Berlin
have shown that human blood, mixed with the blood of lower apes, has a
poisonous effect on the latter; the serum of the one destroys the
blood-cells of the other. But this does not happen when human blood is
mixed with that of the anthropoid ape. As we know from many other
experiments that the mixture of two different kinds of blood is only
possible without injury in the case of two closely related animals of
the same family, we have another proof of the close blood-relationship,
in the literal sense of the word, of man and the anthropoid ape.


Fig.203. Lar or white-handed gibbon (Hylobates lar or albimanus), from
the Indian mainland. Fig. 203—Lar or white-handed gibbon (_Hylobates
lar_ or _albimanus_), from the Indian mainland (From _Brehm._)


Fig.204. Young orang (Satyrus orang), asleep. Fig. 204—Young orang
(_Satyrus orang_), asleep.


The existing anthropoid apes are only a small remnant of a large family
of eastern apes (or _Catarrhinæ_), from which man was evolved about the
end of the Tertiary period. They fall into two geographical groups—the
Asiatic and the African anthropoids. In each group we can distinguish
two genera. The oldest of these four genera is the gibbon _Hylobates,_
Fig. 203); there are from eight to twelve species of it in the East
Indies. I made observations of four of them during my voyage in the
East Indies (1901), and had a specimen of the ash-grey gibbon
(_Hylobates leuciscus_) living for several months in the garden of my
house in Java. I have described the interesting habits of this ape
(regarded by the Malays as the wild descendant of men who had lost
their way) in my _Malayischen_
_Reisebriefen_ (chap. xi). Psychologically, he showed a good deal of
resemblance to the children of my Malay hosts, with whom he played and
formed a very close friendship.


Fig.205. Wild orang (Dyssatyrus auritus). Fig. 205—Wild orang
(_Dyssatyrus auritius_). (From _R. Fick_ and _Leutemann._).


The second, larger and stronger, genus of Asiatic anthropoid ape is the
orang (_Satyrus_); he is now found only in the islands of Borneo and
Sumatra. Selenka, who has published a very thorough _Study of the
Development and Cranial Structure of the Anthropoid Apes_ (1899),
distinguishes ten races of the orang, which may, however, also be
regarded as “local varieties or species.” They fall into two sub-genera
or genera: one group, _Dyssatyrus_ (orang-bentang, Fig. 205), is
distinguished for the strength of its limbs, and the formation of very
peculiar and salient cheek-pads in the elderly male; these are wanting
in the other group, the ordinary orang-outang (_Eusatyrus_).

Several species have lately been distinguished in the two genera of the
black African anthropoid apes (chimpanzee and gorilla). In the genus
_Anthropithecus_ (or _Anthropopithecus,_ formerly _Troglodytes_), the
bald-headed chimpanzee, _A. calvus_ (Fig. 206), and the gorilla-like
_A. mafuca_ differ very strikingly from the ordinary _Anthropithecus
niger_ (Fig. 207), not only in the size and proportion of many parts of
the body, but also in the peculiar shape of the head, especially the
ears and lips, and in the hair and colour. The controversy that still
continues as to whether these different forms of

chimpanzee and orang are “merely local varieties” or “true species” is
an idle one; as in all such disputes of classifiers there is an utter
absence of clear ideas as to what a species really is.


Fig.206. The bald-headed chimpanzee (Anthropithecus calvus). Female.
Fig. 206—The bald-headed chimpanzee (_Anthropithecus calvus_). Female.
This fresh species, described by Frank Beddard in 1897 as Troglodytes
calvus, differs considerably from the ordinary _A. niger_ Fig. 207) in
the structure of the head, the colouring, and the absence of hair in
parts.


Of the largest and most famous of all the anthropoid apes, the gorilla,
Paschen has lately discovered a giant-form in the interior of the
Cameroons, which seems to differ from the ordinary species (_Gorilla
gina_ Fig. 208), not only by its unusual size and strength, but also by
a special formation of the skull. This giant gorilla (_Gorilla gigas,_
Fig. 209) is six feet eight inches long; the span of its great arms is
about nine feet; its powerful chest is twice as broad as that of a
strong man.


Fig.207. Female chimpanzee (Anthropithecus niger). Fig. 207—Female
chimpanzee (_Anthropithecus niger_). (From _ Brehm._)


The whole structure of this huge anthropoid ape is not merely very
similar to that of man, but it is substantially the same. “The same 200
bones, arranged in the same way, form our internal skeleton; the same
300 muscles effect our movements; the same hair covers our skin; the
same groups of ganglionic cells compose the ingenious mechanism of our
brain; the same four-chambered heart is the central pump of our
circulation.” The really existing differences in the shape and size of
the various parts are explained by differences in their growth, due to
adaptation to different habits of life and unequal use of the various
organs. This of itself proves morphologically the descent of man from
the ape. We will return to the point in Chapter XXIII. But I wanted to
point already to this important solution of “the question of
questions,” because that agreement
in the formation of the embryonic membranes and in fœtal circulation
which I have described affords a particularly weighty proof of it. It
is the more instructive as even cenogenetic structures may in certain
circumstances have a high phylogenetic value. In conjunction with the
other facts, it affords a striking confirmation of our biogenetic law.


Fig.208. Female gorilla. Fig. 208—Female gorilla. (From _Brehm_).


Fig.209. Male giant-gorilla (Gorilla gigas), from Yaunde, in the
interior of the Cameroons. Killed by H. Paschen, stuffed by Umlauff.
Fig. 209—Male giant-gorilla (_Gorilla gigas_), from Yaunde, in the
interior of the Cameroons. Killed by H. Paschen, stuffed by Umlauff.



Chapter XVI.
STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT


In turning from the embryology to the phylogeny of man—from the
development of the individual to that of the species—we must bear in
mind the direct causal connection that exists between these two main
branches of the science of human evolution. This important causal nexus
finds its simplest expression in “the fundamental law of organic
development,” the content and purport of which we have fully considered
in the first chapter. According to this biogenetic law, ontogeny is a
brief and condensed recapitulation of phylogeny. If this compendious
reproduction were complete in all cases, it would be very easy to
construct the whole story of evolution on an embryonic basis. When we
wanted to know the ancestors of any higher organism, and, therefore, of
man—to know from what forms the race as a whole has been evolved we
should merely have to follow the series of forms in the development of
the individual from the ovum; we could then regard each of the
successive forms as the representative of an extinct ancestral form.
However, this direct application of ontogenetic facts to phylogenetic
ideas is possible, without limitations, only in a very small section of
the animal kingdom. There are, it is true, still a number of lower
invertebrates (for instance, some of the Zoophyta and Vermalia) in
which we are justified in recognising at once each embryonic form as
the historical reproduction, or silhouette, as it were, of an extinct
ancestor. But in the great majority of the animals, and in the case of
man, this is impossible, because the embryonic forms themselves have
been modified through the change of the conditions of existence, and
have lost their original character to some extent. During the
immeasurable course of organic history, the many millions of years
during which life was developing on our planet, secondary changes of
the embryonic forms have taken place in most animals. The young of
animals (not only detached larvæ, but also the embryos enclosed in the
womb) may be modified by the influence of the environment, just as well
as the mature organisms are by adaptation to the conditions of life;
even species are altered during the embryonic development. Moreover, it
is an advantage for all higher organisms (and the advantage is greater
the more advanced they are) to curtail and simplify the original course
of development, and thus to obliterate the traces of their ancestors.
The higher the individual organism is in the animal kingdom, the less
completely does it reproduce in its embryonic development the series of
its ancestors, for reasons that are as yet only partly known to us. The
fact is easily proved by comparing the different developments of higher
and lower animals in any single stem.

In order to appreciate this important feature, we have distributed the
embryological phenomena in two groups, _ palingenetic_ and
_cenogenetic._ Under palingenesis we count those facts of embryology
that we can directly regard as a faithful synopsis of the corresponding
stem-history. By cenogenesis we understand those embryonic processes
which we cannot directly correlate with corresponding evolutionary
processes, but must regard as modifications or falsifications of them.
With this careful discrimination between palingenetic and cenogenetic
phenomena, our biogenetic law assumes the following more precise
shape:—The rapid and brief development of the individual (ontogeny) is
a condensed synopsis of the long and slow history of the stem
(phylogeny): this synopsis is the more faithful and complete in
proportion as the original features have been preserved by heredity,
and modifications have not been introduced by adaptation.


In order to distinguish correctly between palingenetic and cenogenetic
phenomena in embryology, and deduce sound conclusions in connection
with stem-history, we must especially make a comparative study of the
former. In doing this it is best to employ the methods that have long
been used by geologists for the purpose of establishing the succession
of the sedimentary rocks in the crust of the earth. This solid crust,
which encloses the glowing central mass like a thin shell, is composed
of different kinds of rocks: there are, firstly, the volcanic rocks
which were formed directly by the cooling at the surface of the molten
mass of the earth; secondly, there are the sedimentary rocks, that have
been made out of the former by the action of water, and have been laid
in successive strata at the bottom of the sea. Each of these
sedimentary strata was at first a soft layer of mud; but in the course
of thousands of years it condensed into a solid, hard mass of stone
(sandstone, limestone, marl, etc.), and at the same time permanently
preserved the solid and imperishable bodies that had chanced to fall
into the soft mud. Among these bodies, which were either fossilised or
left characteristic impressions of their forms in the soft slime, we
have especially the more solid parts of the animals and plants that
lived and died during the deposit of the slimy strata.

Hence each of the sedimentary strata has its characteristic fossils,
the remains of the animals and plants that lived during that particular
period of the earth’s history. When we make a comparative study of
these strata, we can survey the whole series of such periods. All
geologists are now agreed that we can demonstrate a definite historical
succession in the strata, and that the lowest of them were deposited in
very remote, and the uppermost in comparatively recent, times. However,
there is no part of the earth where we find the series of strata in its
entirety, or even approximately complete. The succession of strata and
of corresponding historical periods generally given in geology is an
ideal construction, formed by piecing together the various partial
discoveries of the succession of strata that have been made at
different points of the earth’s surface (cf. Chapter XVIII).

We must act in this way in constructing the phylogeny of man. We must
try to piece together a fairly complete picture of the series of our
ancestors from the various phylogenetic fragments that we find in the
different groups of the animal kingdom. We shall see that we are really
in a position to form an approximate picture of the evolution of man
and the mammals by a proper comparison of the embryology of very
different animals—a picture that we could never have framed from the
ontogeny of the mammals alone. As a result of the above-mentioned
cenogenetic processes—those of disturbed and curtailed heredity—whole
series of lower stages have dropped out in the embryonic development of
man and the other mammals especially from the earliest periods, or been
falsified by modification. But we find these lower stages in their
original purity in the lower vertebrates and their invertebrate
ancestors. Especially in the lowest of all the vertebrates, the
lancelet or Amphioxus, we have the oldest stem-forms completely
preserved in the embryonic development. We also find important evidence
in the fishes, which stand between the lower and higher vertebrates,
and throw further light on the course of evolution in certain periods.
Next to the fishes come the amphibia, from the embryology of which we
can also draw instructive conclusions. They represent the transition to
the higher vertebrates, in which the middle and older stages of
ancestral development have been either distorted or curtailed, but in
which we find the more recent stages of the phylogenetic process well
preserved in ontogeny. We are thus in a position to form a fairly
complete idea of the past development of man’s ancestors within the
vertebrate stem by putting together and comparing the embryological
developments of the various groups of vertebrates. And when we go below
the lowest vertebrates and compare their embryology with that of their
invertebrate relatives, we can follow the genealogical tree of our
animal ancestors much farther, down to the very lowest groups of
animals.

In entering the obscure paths of this phylogenetic labyrinth, clinging
to the Ariadne-thread of the biogenetic law and guided by the light of
comparative anatomy, we will first, in accordance with the methods we
have adopted, discover and arrange those fragments from the manifold
embryonic developments of very different animals from which the
stem-history of man can be composed. I would call attention
particularly to the fact that
we can employ this method with the same confidence and right as the
geologist. No geologist has ever had ocular proof that the vast rocks
that compose our Carboniferous or Jurassic or Cretaceous strata were
really deposited in water. Yet no one doubts the fact. Further, no
geologist has ever learned by direct observation that these various
sedimentary formations were deposited in a certain order; yet all are
agreed as to this order. This is because the nature and origin of these
rocks cannot be rationally understood unless we assume that they were
so deposited. These hypotheses are universally received as safe and
indispensable “geological theories,” because they alone give a rational
explanation of the strata.

Our evolutionary hypotheses can claim the same value, for the same
reasons. In formulating them we are acting on the same inductive and
deductive methods, and with almost equal confidence, as the geologist.
We hold them to be correct, and claim the status of “biological
theories” for them, because we cannot understand the nature and origin
of man and the other organisms without them, and because they alone
satisfy our demand for a knowledge of causes. And just as the
geological hypotheses that were ridiculed as dreams at the beginning of
the nineteenth century are now universally admitted, so our
phylogenetic hypotheses, which are still regarded as fantastic in
certain quarters, will sooner or later be generally received. It is
true that, as will soon appear, our task is not so simple as that of
the geologist. It is just as much more difficult and complex as man’s
organisation is more elaborate than the structure of the rocks.

When we approach this task, we find an auxiliary of the utmost
importance in the comparative anatomy and embryology of two lower
animal-forms. One of these animals is the lancelet (_Amphioxus_), the
other the sea-squirt (_Ascidia_). Both of these animals are very
instructive. Both are at the border between the two chief divisions of
the animal kingdom—the vertebrates and invertebrates. The vertebrates
comprise the already mentioned classes, from the Amphioxus to man
(acrania, lampreys, fishes, dipneusts, amphibia, reptiles, birds, and
mammals). Following the example of Lamarck, it is usual to put all the
other animals together under the head of invertebrates. But, as I have
often mentioned already, the group is composed of a number of very
different stems. Of these we have no interest just now in the
echinoderms, molluscs, and articulates, as they are independent
branches of the animal-tree, and have nothing to do with the
vertebrates. On the other hand, we are greatly concerned with a very
interesting group that has only recently been carefully studied, and
that has a most important relation to the ancestral tree of the
vertebrates. This is the stem of the Tunicates. One member of this
group, the sea-squirt, very closely approaches the lowest vertebrate,
the Amphioxus, in its essential internal structure and embryonic
development. Until 1866 no one had any idea of the close connection of
these apparently very different animals; it was a very fortunate
accident that the embryology of these related forms was discovered just
at the time when the question of the descent of the vertebrates from
the invertebrates came to the front. In order to understand it
properly, we must first consider these remarkable animals in their
fully-developed forms and compare their anatomy.

We begin with the lancelet—after man the most important and interesting
of all animals. Man is at the highest summit, the lancelet at the
lowest root, of the vertebrate stem.

It lives on the flat, sandy parts of the Mediterranean coast, partly
buried in the sand, and is apparently found in a number of seas.[28] It
has been found in the North Sea (on the British and Scandinavian coasts
and in Heligoland), and at various places on the Mediterranean (for
instance, at Nice, Naples, and Messina). It is also found on the coast
of Brazil and in the most distant parts of the Pacific Ocean (the coast
of Peru, Borneo, China, Australia, etc.). Recently eight to ten species
of the amphioxus have been determined, distributed in two or three
genera.

 [28] See the ample monograph by Arthur Willey, _ Amphioxus and the
 Ancestry of the Vertebrates_; Boston, 1894.


Johannes Müller classed the lancelet with the fishes, although he
pointed out that the differences between this simple vertebrate and the
lowest fishes are much greater than between the fishes and the
amphibia. But this was far from expressing the real significance of the
animal. We may confidently lay down the following principle: The
Amphioxus differs more from the fishes than the fishes do from
man and the other vertebrates. As a matter of fact, it is so different
from all the other vertebrates in its whole organisation that the laws
of logical classification compel us to distinguish two divisions of
this stem: 1, the Acrania (Amphioxus and its extinct relatives); and 2,
the Craniota (man and the other vertebrates). The first and lower
division comprises the vertebrates that have no vertebræ or skull
(_cranium_). Of these the only living representatives are the Amphioxus
and Paramphioxus, though there must have been a number of different
species at an early period of the earth’s history.


Fig.210. The lancelet (Amphioxus lanceolatus), left view. Fig. 211.
Transverse section of the head of the Amphioxus. Fig. 210—The lancelet
(_Amphioxus lanceolatus_), left view. The long axis is vertical; the
mouth-end is above, the tail-end below; _a_ mouth, surrounded by
threads of beard; _b_ anus, _c_ gill-opening (_porus branchialis_), _d_
gill-crate, _ e_ stomach, _f_ liver, _g_ small intestine, _h_ branchial
cavity, _i_ chorda (axial rod), underneath it the aorta; _k_ aortic
arches, _l_ trunk of the branchial artery, _m_ swellings on its
branches, _n_ vena cava, _ o_ visceral vein.
Fig. 211—Transverse section of the head of the Amphioxus. (From
_Boveri._) Above the branchial gut (_kd_) is the chorda, above this the
neural tube (in which we can distinguish the inner grey and the outer
white matter); above again is the dorsal fin (_fh_). To the right and
left above (in the episoma) are the thick muscular plates (_m_); below
(in the hyposoma) the gonads (_g_). _ ao_ aorta (here double), _c_
corium, _ec_ endostyl, _f_ fascie, _gl_ glomerulus of the kidneys, _k_
branchial vessel, _ld_ partition between the cœloma (_sc_) and atrium
(_p_), _mt_ transverse ventral muscle, _n_ renal canals, of upper and
_uf_ lower canals in the mantle-folds, _p_ peribranchial cavity,
(atrium), _ sc_ cœloma (subchordal body-cavity), _si_ principal (or
subintestinal) vein, _sk_ perichorda (skeletal layer).


Opposed to the Acrania is the second division of the vertebrates, which
comprises all the other members of the stem, from the fishes up to man.
All these vertebrates have a head quite distinct from the trunk, with a
skull (_cranium_) and brain; all have a centralised heart, fully-formed
kidneys, etc. Hence they are called the _Craniota._ These Craniotes
are, however, without a skull in their earlier period. As we already
know from embryology, even man, like every other mammal, passes in the
earlier course of his development through the important stage which we
call the chordula; at this lower stage the animal has neither vertebræ
nor skull nor limbs (Figs. 83–86). And even after the formation of the
primitive vertebræ has begun, the segmented fœtus of the amniotes still
has for a long time the simple form of a lyre-shaped disk or a sandal,
without limbs or extremities. When we compare this embryonic condition,
the sandal-shaped fœtus, with the developed lancelet, we may say that
the amphioxus is, in a certain sense, a permanent sandal-embryo, or a
permanent embryonic form of the Acrania; it never rises above a low
grade of development which we have long since passed.

The fully-developed lancelet (Fig. 210) is about two inches long, is
colourless or of a light red tint, and has the shape of a narrow
lancet-formed leaf. The body is pointed at both ends, but much
compressed at the sides. There is no trace of limbs. The outer skin is
very thin and delicate, naked, transparent, and composed of two
different layers, a simple external stratum of cells, the epidermis,
and a thin underlying cutis-layer. Along the middle line of the back
runs a narrow fin-fringe which expands behind into an oval tail-fin,
and is continued below in a short anus-fin. The fin-fringe is supported
by a number of square elastic fin-plates.

In the middle of the body we find a thin string of cartilage, which
goes the whole length of the body from front to back, and is pointed at
both ends (Fig. 210 _i_). This straight, cylindrical rod (somewhat
compressed for a time) is the axial rod or the _chorda dorsalis_; in
the lancelet this is the only trace of a vertebral column. The chorda
develops no further, but retains its original simplicity throughout
life. It is enclosed by a firm membrane, the chorda-sheath or
_perichorda._ The real features of this and of its dependent formations
are best seen in the transverse section of the Amphioxus (Fig. 211).
The perichorda forms a cylindrical tube immediately over the chorda,
and the central nervous system, the medullary tube, is enclosed in it.
This important psychic organ also remains in its simplest shape
throughout life, as a cylindrical tube, terminating with almost equal
plainness at either end, and enclosing a narrow canal in its thick
wall. However, the fore end is a little rounder, and contains a small,
almost imperceptible bulbous swelling of the canal. This must be
regarded as the beginning of a rudimentary brain. At the foremost end
of it there is a small black pigment-spot, a rudimentary eye; and a
narrow canal leads to a superficial sense-organ. In the vicinity of
this optic spot we find at the left side a small ciliated depression,
the single olfactory organ. There is no organ of hearing. This
defective development of the higher sense-organs is probably, in the
main, not an original feature, but a result of degeneration.

Underneath the axial rod or chorda runs a very simple alimentary canal,
a tube that opens on the ventral side of the animal by a mouth in front
and anus behind. The oval mouth is surrounded by a ring of cartilage,
on which there are twenty to thirty cartilaginous threads (organs of
touch, Fig. 210 _a_). The alimentary canal divides into sections of
about equal length by a constriction in the middle. The fore section,
or head-gut, serves for respiration; the hind section, or trunk-gut,
for digestion. The limit of the two alimentary regions is also the
limit of the two parts of the body, the head and the trunk. The
head-gut or branchial gut forms a broad gill-crate, the grilled wall of
which is pierced by numbers of gill-clefts (Fig. 210 _d_). The fine
bars of the gill-crate between the clefts are strengthened with firm
parallel rods, and these are connected in pairs by cross-rods. The
water that enters the mouth of the Amphioxus passes through these
clefts into the large surrounding branchial cavity or _ atrium,_ and
then pours out behind through a hole in it, the respiratory pore
(_porus branchialis,_ Fig. 210 _c_). Below, on the ventral side of the
gill-crate, there is in the middle
line a ciliated groove with a glandular wall (the hypobranchial
groove), which is also found in the Ascidia and the larvæ of the
Cyclostoma. It is interesting because the thyroid gland in the larynx
of the higher vertebrates (underneath the “Adam’s apple”) has been
developed from it.

Behind the respiratory part of the gut we have the digestive section,
the trunk or liver (hepatic) gut. The small particles that the
Amphioxus takes in with the water—infusoria, diatoms, particles of
decomposed plants and animals, etc.—pass from the gill-crate into the
digestive part of the canal, and are used up as food. From a somewhat
enlarged portion, that corresponds to the stomach (Fig. 210 _e_), a
long, pouch-like blind sac proceeds straight forward (_f_); it lies
underneath on the left side of the gill-crate, and ends blindly about
the middle of it. This is the liver of the Amphioxus, the simplest kind
of liver that we meet in any vertebrate. In man also the liver
develops, as we shall see, in the shape of a pouch-like blind sac, that
forms out of the alimentary canal behind the stomach.


Fig.212. Transverse section of an Amphioxus-larva, with five
gill-clefts, through the middle of the body. Fig. 213. Diagram of the
preceding. Fig. 212—Transverse section of an Amphioxus-larva, with five
gill-clefts, through the middle of the body. Fig. 213—Diagram of the
preceding. (From _ Hatschek._) _A_ epidermis, _B_ medullary tube, _ C_
chorda, _C_1 inner chorda-sheath, _D_ visceral epithelium, _E_
sub-intestinal vein. _1_ cutis, _2_ muscle-plate (myotome), _3_
skeletal plate (sclerotome), _4_ cœloseptum (partition between dorsal
and ventral cœloma), _5_ skin-fibre layer, _6_ gut-fibre layer, _I_
myocœl (dorsal body-cavity), _ II_ splanchnocœl (ventral body-cavity).)


The formation of the circulatory system in this animal is not less
interesting. All the other vertebrates have a compressed, thick,
pouch-shaped heart, which develops from the wall of the gut at the
throat, and from which the blood-vessels proceed; in the Amphioxus
there is no special centralised heart, driving the blood by its
pulsations. This movement is effected, as in the annelids, by the thin
blood-vessels themselves, which discharge the function of the heart,
contracting and pulsating in their whole length, and thus driving the
colourless blood through the entire body. On the under-side of the
gill-crate, in the middle line, there is the trunk of a large vessel
that corresponds to the heart of the other vertebrates and the trunk of
the branchial artery that proceeds from it; this drives the blood into
the gills (Fig. 210 _l_). A number of small vascular arches arise on
each side from this branchial artery, and form little heart-shaped
swellings or _ bulbilla_ (_m_) at their points of departure; they
advance along the branchial arches, between the gill-clefts and the
fore-gut, and unite, as branchial veins, above the gill-crate in a
large trunk blood-vessel that runs under the chorda dorsalis. This is
the principal artery or primitive aorta (Fig. 214 _D_). The branches
which it gives off to all parts of the body unite again in a larger
venous vessel at the underside of the gut, called the subintestinal
vein (Figs. 210 _o,_ 212 _E_). This single main vessel of the Amphioxus
goes like a closed circular water-conduit along the alimentary canal
through the whole body, and pulsates in its whole length above and
below. When the upper tube contracts the lower one is filled with
blood, and _vice versa._ In the upper tube the blood flows from front
to rear, then back from rear to front in the lower vessel. The whole of
the long tube that runs along the ventral side of the alimentary canal
and contains venous blood may be called the “principal vein,” and may
be compared to the ventral vessel in the worms. On the other hand, the
long
straight vessel that runs along the dorsal line of the gut above,
between it and the chorda, and contains arterial blood, is clearly
identical with the aorta or principal artery of the other vertebrates;
and on the other side it may be compared to the dorsal vessel in the
worms.

The cœloma or body-cavity has some very important and distinctive
features in the Amphioxus. The embryology of it is most instructive in
connection with the stem-history of the body-cavity in man and the
other vertebrates. As we have already seen (Chapter X), in these the
two cœlom-pouches are divided at an early stage by transverse
constrictions into a double row of primitive segments (Fig. 124), and
each of these subdivides, by a frontal or lateral constriction, into an
upper (dorsal) and lower (ventral) pouch.


Fig.214. Transverse section of a young Amphioxus, immediately after
metamorphosis. Fig. 215. Diagram of preceding. Fig. 214—Transverse
section of a young Amphioxus, immediately after metamorphosis, through
the hindermost third (between the atrium-cavity and the anus). Fig.
215—Diagram of preceding. (From _Hatschek._) _A_ epidermis, _B_
medullary tube, _C_ chorda, _ D_ aorta, _E_ visceral epithelium, _F_
subintestinal vein. _1_ corium-plate, _2_ muscle-plate, _3_
fascie-plate, _4_ outer chorda-sheath, _5_ myoseptum, _ 6_ skin-fibre
plate, _7_ gut-fibre plate, _I_ myocœl, _II_ splanchnocœl, _I_1 dorsal
fin, _I_2 anus-fin.)


These important structures are seen very clearly in the trunk of the
amphioxus (the latter third, Figs. 212–215), but it is otherwise in the
head, the foremost third (Fig. 216). Here we find a number of
complicated structures that cannot be understood until we have studied
them on the embryological side in the next chapter (cf. Fig. 81). The
branchial gut lies free in a spacious cavity filled with water, which
was wrongly thought formerly to be the body-cavity (Fig. 216 _A_). As a
matter of fact, this atrium (commonly called the peribranchial cavity)
is a secondary structure formed by the development of a couple of
lateral mantle-folds or gill-covers (_M_1, _U_). The real body-cavity
(_Lh_) is very narrow and entirely closed, lined with epithelium. The
peribranchial cavity (_A_) is full of water, and its walls are lined
with the skin-sense layer; it opens outwards in the rear through the
respiratory pore (Fig. 210 _ c_).

On the inner surface of these mantle-folds (_M_1), in the ventral half
of the wide mantle cavity (atrium), we find the sex-organs of the
Amphioxus. At each side of the branchial gut there are between twenty
and thirty roundish four-cornered sacs, which can clearly be seen from
without with the naked eye, as they shine through the thin transparent
body-wall. These sacs are the sexual glands they are the same size and
shape in both sexes, only differing in contents. In the female they
contain a quantity of simple ova (Fig. 219 _g_); in the male a number
of much smaller cells that change into mobile ciliated cells
(sperm-cells). Both sacs lie on the inner wall of the atrium, and have
no special outlets. When the ova of the female and the sperm of the
male are ripe, they fall into the atrium, pass through the gill-clefts
into the
fore-gut, and are ejected through the mouth.


Fig.216. Transverse section of the lancelet, in the fore half. Fig.
216—Transverse section of the lancelet, in the fore half. (From
_Ralph._) The outer covering is the simple cell-layer of the epidermis
(_E_). Under this is the thin corium, the subcutaneous tissue of which
is thickened; it sends connective-tissue partitions between the muscles
(_M_1) and to the chorda-sheath. (_N_ medullary tube, _Ch_ chorda, _Lh_
body-cavity, _A_ atrium, _L_ upper wall of same, _E_1 inner wall, _E_2
outer wall, _Lh_1 ventral remnant of same, _Kst_ gill-reds, _M_ ventral
muscles, _R_ seam of the joining of the ventral folds (gill-covers),
_G_ sexual glands.


Above the sexual glands, at the dorsal angle of the atrium, we find the
kidneys. These important excretory organs could not be found in the
Amphioxus for a long time, on account of their remote position and
their smallness; they were discovered in 1890 by Theodor Boveri (Fig.
217 _x_). They are short segmented canals; corresponding to the
primitive kidneys of the other vertebrates (Fig. 218 _B_). Their
internal aperture (Fig. 217 _B_) opens into the body-cavity; their
outer aperture into the atrium (_C_). The prorenal canals lie in the
middle of the line of the head, outwards from the uppermost section of
the gill-arches, and have important relations to the branchial vessels
(_H_). For this reason, and in their whole arrangement, the primitive
kidneys of the Amphioxus
show clearly that they are equivalent to the prorenal canals of the
Craniotes (Fig. 218 _B_). The prorenal duct of the latter (Fig. 218
_C_) corresponds to the branchial cavity or atrium of the former (Fig.
217 _C_).


Fig.217. Transverse section through the middle of the Amphioxus. Fig.
218. Transverse section of a primitive fish embryo. Fig. 217—Transverse
section through the middle of the Amphioxus. (From _Boveri._) On the
left a gill-rod has been struck, and on the right a gill-cleft;
consequently on the left we see the whole of a prorenal canal (_x_), on
the right only the section of its fore-leg. _A_ genital chamber
(ventral section of the gonocœl), _x_ pronephridium, _B_ its
cœlom-aperture, _C_ atrium, _D_ body-cavity, _E_ visceral cavity, _F_
subintestinal vein, _G_ aorta (the left branch connected by a branchial
vessel with the subintestinal vein), _H_ renal vessel. Fig.
218—Transverse section of a primitive fish embryo (Selachii-embryo,
from _Boveri._). To the left pronephridia (_B_), the right primitive
kidneys (_A_). The dotted lines on the right indicate the later opening
of the primitive kidney canals (_A_) into the prorenal duct (_C_). _ D_
body-cavity, _E_ visceral cavity, _F_ subintestinal vein, _G_ aorta,
_H_ renal vessel.


If we sum up the results of our anatomic study of the Amphioxus, and
compare them with the familiar organisation of man, we shall find an
immense distance between the two. As a fact, the highest summit of the
vertebrate organisation which man represents is in every respect so far
above the lowest stage, at which the lancelet remains, that one would
at first scarcely believe it possible to class both animals in the same
division of the animal kingdom. Nevertheless, this classification is
indisputably just. Man is only a more advanced stage of the vertebral
type that we find unmistakably in the Amphioxus in its characteristic
features. We need only recall the picture of the ideal Primitive
Vertebrate given in a former chapter, and compare it with the lower
stages of human embryonic development, to convince ourselves of our
close relationship to the lancelet. (Cf. Chapter XI)

It is true that the Amphioxus is far below all other living
vertebrates. It is true that it has no separate head, no developed
brain or skull, the characteristic feature of the other vertebrates.
It is (probably as a result of degeneration) without the auscultory
organ and the centralised heart that all the others have; and it has no
fully-formed kidneys. Every single organ in it is simpler and less
advanced than in any of the others. Yet the characteristic connection
and arrangement of all the organs is just the same as in the other
vertebrates. All these, moreover, pass, during their embryonic
development, through a stage in which their whole organisation is no
higher than that of the Amphioxus, but is substantially identical with
it.


Fig.219. Transverse section of the head of the Amphioxus. Fig.
219—Transverse section of the head of the Amphioxus (at the limit of
the first and second third of the body). (From _Boveri_) _a_ aorta
(here double), _b_ atrium, _c_ chorda, _co_ umlaut cœloma
(body-cavity), _e_ endostyl (hypobranchial groove), _g_ gonads
(ovaries), _kb_ gill-arches, _kd_ branchial gut, _l_ liver-tube (on the
right, one-sided), _m_ muscles, _n_ renal canals, _r_ spinal cord, _sn_
spinal nerves, _sp_ gill-clefts.


In order to see this quite clearly, it is particularly useful to
compare the Amphioxus with the youthful forms of those vertebrates that
are classified next to it. This is the class of the Cyclostoma. There
are to-day only a few species of this once extensive class, and these
may be distributed in two groups. One group comprises the hag-fishes or
Myxinoides. The other group are the Petromyzontes, or lampreys, which
are a familiar delicacy in their marine form. These Cyclostoma are
usually classified with the fishes. But they are far below the true
fishes, and form a very interesting connecting-group between them and
the lancelet. One can see how closely they approach the latter by
comparing a young lamprey with the Amphioxus. The chorda is of the same
simple character in both; also the medullary tube, that lies above the
chorda, and the alimentary canal below it. However, in the lamprey the
spinal cord swells in front into a simple pear-shaped cerebral vesicle,
and at each side of it there are a very simple eye and a rudimentary
auditory vesicle. The nose is a single pit, as in the Amphioxus. The
two sections of the gut are also just the same and very rudimentary in
the lamprey. On the other hand, we see a great advance in the structure
of the heart, which is found underneath the gills in the shape of a
centralised muscular tube, and is divided into an auricle and a
ventricle. Later on the lamprey advances still further, and gets a
skull, five cerebral vesicles, a series of independent gill-pouches,
etc. This makes all the more interesting the striking resemblance of
its immature larva to the developed and sexually mature Amphioxus.

While the Amphioxus is thus connected through the Cyclostoma with the
fishes, and so with the series of the higher vertebrates, it is, on the
other hand, very closely related to a lowly invertebrate marine animal,
from which it seems to be entirely remote at first glance. This
remarkable animal is the sea-squirt or Ascidia, which was formerly
thought to be closely related to the mussel, and so classed in the
molluscs. But since the remarkable embryology of these animals was
discovered in 1866, there can be no question that they have nothing to
do with the molluscs. To the great astonishment of zoologists, they
were found, in their whole individual development, to be closely
related to the vertebrates. When fully developed the Ascidiæ are
shapeless lumps that would not, at first sight, be taken for animals at
all. The oval body, frequently studded with knobs or uneven and lumpy,
in which we can discover no special external organs, is attached at one
end to marine plants, rocks, or the floor of the sea. Many species look
like potatoes, others like melon-cacti, others like prunes. Many of the
Ascidiæ form transparent crusts or
deposits on stones and marine plants. Some of the larger species are
eaten like oysters. Fishermen, who know them very well, think they are
not animals, but plants. They are sold in the fish markets of many of
the Italian coast-towns with other lower marine animals under the name
of “sea-fruit” (_frutti di mare_). There is nothing about them to show
that they are animals. When they are taken out of the water with the
net the most one can perceive is a slight contraction of the body that
causes water to spout out in two places. The bulk of the Ascidiæ are
very small, at the most a few inches long. A few species are a foot or
more in length. There are many species of them, and they are found in
every sea. As in the case of the Acrania, we have no fossilised remains
of the class, because they have no hard and fossilisable parts.
However, they must be of great antiquity, and must go back to the
primordial epoch.

The name of “Tunicates” is given to the whole class to which the
Ascidiæ belong, because the body is enclosed in a thick and stiff
covering like a mantle (_tunica_). This mantle—sometimes soft like
jelly, sometimes as tough as leather, and sometimes as stiff as
cartilage—has a number of peculiarities. The most remarkable of them is
that it consists of a woody matter, cellulose—the same vegetal
substance that forms the stiff envelopes of the plant-cells, the
substance of the wood. The tunicates are the only class of animals that
have a real cellulose or woody coat. Sometimes the cellulose mantle is
brightly coloured, at other times colourless. Not infrequently it is
set with needles or hairs, like a cactus. Often we find a mass of
foreign bodies—stone, sand, fragments of mussel-shells, etc.—worked
into the mantle. This has earned for the Ascidia the name of “the
microcosm.”


Fig.220. Organisation of an Ascidia (left view). Fig. 220—Organisation
of an Ascidia (left view); the dorsal side is turned to the right and
the ventral side to the left, the mouth (_o_) above; the ascidia is
attached at the tail end. The branchial gut (_br_), which is pierced by
a number of clefts, continues below in the visceral gut. The rectum
opens through the anus (_a_) into the atrium (_cl_), from which the
excrements are ejected with the respiratory water through the
mantle-hole or cloaca (_a_); _m_ mantle. (From _ Gegenbaur._


The hind end, which corresponds to the tail of the Amphioxus, is
usually attached, often by means of regular roots. The dorsal and
ventral sides differ a good deal internally, but frequently cannot be
distinguished externally. If we open the thick tunic or mantle in order
to examine the internal organisation, we first find a spacious cavity
filled with water—the mantle-cavity or respiratory cavity (Fig. 220
_cl_). It is also called the branchial cavity and the cloaca, because
it receives the excrements and sexual products as well as the
respiratory water. The greater part of the respiratory cavity is
occupied by the large grated branchial sac (_br_). This is so like the
gill-crate of the Amphioxus in its whole arrangement that the
resemblance was pointed out by the English naturalist Goodsir, years
ago, before anything was known of the relationship of the two animals.
As a fact, even in the Ascidia the mouth (_o_) opens first into this
wide branchial sac. The respiratory water passes through the
lattice-work of the branchial sac into the branchial cavity, and is
ejected from this by the respiratory pore (_a_′). Along the ventral
side of the branchial sac runs a ciliated groove—the hypobranchial
groove which we have previously found at the same spot in the
Amphioxus. The food of the Ascidia also
consists of tiny organisms, infusoria, diatoms, parts of decomposed
marine plants and animals; etc. These pass with the water into the
gill-crate and the digestive part of the gut at the end of it, at first
into an enlargement of it that represents the stomach. The adjoining
small intestine usually forms a loop, bends forward, and opens by an
anus (Fig. 220 _a_), not directly outwards, but first into the mantle
cavity; from this the excrements are ejected by a common outlet (_a_′)
together with the used-up water and the sexual products. The outlet is
sometimes called the branchial pore, and sometimes the cloaca or
ejection-aperture. In many of the Ascidiæ a glandular mass opens into
the gut, and this represents the liver. In some there is another gland
besides the liver, and this is taken to represent the kidneys. The
body-cavity proper, or cœloma, which is filled with blood and encloses
the hepatic gut, is very narrow in the Ascidia, as in the Amphioxus,
and is here also usually confounded with the wide atrium, or
peribranchial cavity, full of water.


Organisation of an Ascidia (as in Fig. 220, seen from the left). Fig.
221—Organisation of an Ascidia (as in Fig. 220, seen from the left).
_sb_ branchial sac, _v_ stomach, _i_ small intestine, _c_ heart, _t_
testicle, _vd_ sperm-duct, _o_ ovary, _ o_′ ripe ova in the branchial
cavity. The two small arrows indicate the entrance and exit of the
water through the openings of the mantle. (From _Milne-Edwards._)


There is no trace in the fully-developed Ascidia of a chorda dorsalis,
or internal axial skeleton. It is the more interesting that the young
animal that emerges from the ovum _has_ a chorda, and that there is a
rudimentary medullary tube above it. The latter is wholly atrophied in
the developed Ascidia, and looks like a small nerve-ganglion in front
above the gill-crate. It corresponds to the upper “gullet-ganglion” or
“primitive brain” in other vermalia. Special sense-organs are either
wanting altogether or are only found in a very rudimentary form, as
simple optic spots and touch-corpuscles or tentacles that surround the
mouth. The muscular system is very slightly and irregularly developed.
Immediately under the thin corium, and closely connected with it, we
find a thin muscle tube, as in the worms. On the other hand, the
Ascidia has a centralised heart, and in this respect it seems to be
more advanced than the Amphioxus. On the ventral side of the gut, some
distance behind the gill-crate, there is a spindle-shaped heart. It
retains permanently the simple tubular form that we find temporarily as
the first structure of the heart in the vertebrates. This simple heart
of the Ascidia has, however, a remarkable peculiarity. It contracts in
alternate directions. In all other animals the beat of the heart is
always in the same direction (generally from rear to front); it changes
in the Ascidia to the reverse direction. The heart contracts first from
the rear to the front, stands still for a minute, and then begins to
beat the opposite way, now driving the blood from front to rear; the
two large vessels that start from either end of the heart act
alternately as arteries and veins. This feature is found in the
Tunicates alone.

Of the other chief organs we have still to mention the sexual glands,
which lie right behind in the body-cavity. All the Ascidiæ are
hermaphrodites. Each individual has a male and a female gland, and so
is able to fertilise itself. The ripe ova (Fig. 221 _o_′) fall directly
from the ovary (_o_) into the mantle-cavity. The male sperm is
conducted into this cavity from the testicle (_t_) by a special duct
(_vd_). Fertilisation is accomplished here, and in many of the Ascidiæ
developed embryos are found. These are then ejected
with the breathing-water through the cloaca (_q_), and so “born alive.”

If we now glance at the entire structure of the simple Ascidia
(especially _Phallusia, Cynthia,_ etc.) and compare it with that of the
Amphioxus, we shall find that the two have few points of contact. It is
true that the fully-developed Ascidia resembles the Amphioxus in
several important features of its internal structure, and especially in
the peculiar character of the gill-crate and gut. But in most other
features of organisation it is so far removed from it, and is so unlike
it in external appearance, that the really close relationship of the
two was not discovered until their embryology was studied. We will now
compare the embryonic development of the two animals, and find to our
great astonishment that the same embryonic form develops from the ovum
of the Amphioxus as from that of the Ascidia—a typical _ chordula._



Chapter XVII.
EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT


The structural features that distinguish the vertebrates from the
invertebrates are so prominent that there was the greatest difficulty
in the earlier stages of classification in determining the affinity of
these two great groups. When scientists began to speak of the affinity
of the various animal groups in more than a figurative—in a
genealogical—sense, this question came at once to the front, and seemed
to constitute one of the chief obstacles to the carrying-out of the
evolutionary theory. Even earlier, when they had studied the relations
of the chief groups, without any idea of real genealogical connection,
they believed they had found here and there among the invertebrates
points of contact with the vertebrates: some of the worms, especially,
seemed to approach the vertebrates in structure, such as the marine
arrow-worm (_Sagitta_). But on closer study the analogies proved
untenable. When Darwin gave an impulse to the construction of a real
stem-history of the animal kingdom by his reform of the theory of
evolution, the solution of this problem was found to be particularly
difficult. When I made the first attempt in my _General Morphology_
(1866) to work out the theory and apply it to classification, I found
no problem of phylogeny that gave me so much trouble as the linking of
the vertebrates with the invertebrates.

But just at this time the true link was discovered, and at a point
where it was least expected. Towards the end of 1866 two works of the
Russian zoologist, Kowalevsky, who had lived for some time at Naples,
and studied the embryology of the lower animals, were issued in the
publications of the St. Petersburg Academy. A fortunate accident had
directed the attention of this able observer almost simultaneously to
the embryology of the lowest vertebrate, the Amphioxus, and that of an
invertebrate, the close affinity of which to the Amphioxus had been
least suspected, the Ascidia. To the extreme astonishment of all
zoologists who were interested in this important question, there turned
out to be the utmost resemblance in structure from the commencement of
development between these two very different animals—the lowest
vertebrate and the mis-shaped, sessile invertebrate. With this
undeniable identity of ontogenesis, which can be demonstrated to an
astounding extent, we had, in virtue of the biogenetic law, discovered
the long-sought genealogical link, and definitely identified the
invertebrate group that represents the nearest blood-relatives of the
vertebrates.
The discovery was confirmed by other zoologists, and there can no
longer be any doubt that of all the classes of invertebrates that of
the Tunicates is most closely related to the vertebrates, and of the
Tunicates the nearest are the Ascidiæ. We cannot say that the
vertebrates are descended from the Ascidiæ—and still less the
reverse—but we can say that of all the invertebrates it is the
Tunicates, and, within this group, the Ascidiæ, that are the nearest
blood-relatives of the ancient stem-form of the vertebrates. We must
assume as the common ancestral group of both stems an extinct family of
the extensive vermalia-stem, the _Prochordonia_ or _Prochordata_
(“primitive chorda-animals”).

In order to appreciate fully this remarkable fact, and especially to
secure the sound basis we seek for the genealogical tree of the
vertebrates, it is necessary to study thoroughly the embryology of both
these animals, and compare the individual development of the Amphioxus
step by step with that of the Ascidia. We begin with the ontogeny of
the Amphioxus.

From the concordant observations of Kowalevsky at Naples and Hatschek
at Messina, it follows, firstly, that the ovum-segmentation and
gastrulation of the Amphioxus are of the simplest character. They take
place in the same way as we find them in many of the lower animals of
different invertebrate stems, which we have already described as
original or primordial; the development of the Ascidia is of the same
type. Sexually mature specimens of the Amphioxus, which are found in
great quantities at Messina from April or May onwards, begin as a rule
to eject their sexual products in the evening; if you catch them about
the middle of a warm night and put them in a glass vessel with
seawater, they immediately eject through the mouth their accumulated
sexual products, in consequence of the disturbance. The males give out
masses of sperm, and the females discharge ova in such quantity that
many of them stick to the fibrils about their mouths. Both kinds of
cells pass first into the mantle-cavity after the opening of the
gonads, proceed through the gill-clefts into the branchial gut, and are
discharged from this through the mouth.

The ova are simply round cells. They are only 1/250 of an inch in
diameter, and thus are only half the size of the mammal ova, and have
no distinctive features. The clear protoplasm of the mature ovum is
made so turbid by the numbers of dark granules of food-yelk or
deutoplasm scattered in it that it is difficult to follow the process
of fecundation and the behaviour of the two nuclei during it (p. 51).
The active elements of the male sperm, the cone-shaped spermatozoa, are
similar to those of most other animals (cf. Fig. 20). Fecundation takes
place when these lively ciliated cells of the sperm approach the ovum,
and seek to penetrate into the yelk-matter or the cellular substance of
the ovum with their head-part—the thicker part of the cell that
encloses the nucleus. Only one spermatozoon can bore its way into the
yelk at one pole of the ovum-axis; its head or nucleus coalesces with
the female nucleus, which remains after the extrusion of the directive
bodies from the germinal vesicle. Thus is formed the “stem-nucleus,” or
the nucleus of the “stem-cell” (cytula, Fig. 2). This now undergoes
total segmentation, dividing into two, four, eight, sixteen, thirty-two
cells, and so on. In this way we get the spherical, mulberry-shaped
body, which we call the _ morula._

The segmentation of the Amphioxus is not entirely regular, as was
supposed after the first observations of Kowalevsky (1866). It is not
completely equal, but a little unequal. As Hatschek afterwards found
(1879), the segmentation-cells only remain equal up to the
morula-stage, the spherical body of which consists of thirty-two cells.
Then, as always happens in unequal segmentation, the more sluggish
vegetal cells are outstripped in the cleavage. At the lower or vegetal
pole of the ovum a crown of eight large entodermic cells remains for a
long time unchanged, while the other cells divide, owing to the
formation of a series of horizontal circles, into an increasing number
of crowns of sixteen cells each. Afterwards the segmentation-cells get
more or less irregularly displaced, while the segmentation-cavity
enlarges in the centre of the morula; in the end the former all lie on
the surface of the latter, so that the fœtus attains the familiar
blastula shape and forms a hollow ball, the wall of which consists of a
single stratum of cells (Fig. 38 _ A–C_). This layer is the blastoderm,
the simple epithelium from the cells of which all the tissues of the
body proceed.


These important early embryonic processes take place so quickly in the
Amphioxus that four or five hours after fecundation, or about midnight,
the spherical blastula is completed. A pit-like depression is then
formed at the vegetal pole of it, and in consequence of this the hollow
sphere doubles on itself (Fig. 38 _D_). This pit becomes deeper and
deeper (Fig. 38 _E, F_); at last the invagination (or doubling) is
complete, and the inner or folded part of the blastula-wall lies on the
inside of the outer wall. We thus get a hollow hemisphere, the thin
wall of which is made up of two layers of cells (Fig. 38 _E_). From
hemispherical the body soon becomes almost spherical once more, and
then oval, the internal cavity enlarging considerably and its mouth
growing narrower (Fig. 213). The form which the Amphioxus-embryo has
thus reached is a real “cup-larva” or _gastrula,_ of the original
simple type that we have previously described as the “bell-gastrula” or
_archigastrula_ (Figs. 29–35).

As in all the other animals that form an archigastrula, the whole body
is nothing but a simple gastric sac or stomach; its internal cavity is
the primitive gut (_progaster_ or _ archenteron,_ Fig. 38 _g,_ 35 _d_),
and its aperture the primitive mouth (_prostoma_ or _blastoporus, o_).
The wall is at once gut-wall and body-wall. It is composed of two
simple cell-layers, the familiar primary germinal layers. The inner
layer or the invaginated part of the blastoderm, which immediately
encloses the gut-cavity is the entoderm, the inner or vegetal
germ-layer, from which develop the wall of the alimentary canal and all
its appendages, the cœlom-pouches, etc. (Figs. 35, 36 _ i_). The outer
stratum of cells, or the non-invaginated part of the blastoderm, is the
ectoderm, the outer or animal germ-layer, which provides the outer skin
(epidermis) and the nervous system (_e_). The cells of the entoderm are
much larger, darker, and more fatty than those of the ectoderm, which
are clearer and less rich in fatty particles. Hence before and during
invagination there is an increasing differentiation of the inner from
the outer layer. The animal cells of the outer layer soon develop
vibratory hairs; the vegetal cells of the inner layer do so much later.
A thread-like process grows out of each cell, and effects continuous
vibratory movements. By the vibrations of these slender hairs the
gastrula of the Amphioxus swims about in the sea, when it has pierced
the thin ovolemma, like the gastrula of many other animals (Fig. 36).
As in many other lower animals, the cells have only one whip-like hair
each, and so are called _flagellate_ (whip) cells (in contrast with the
_ciliated_ cells, which have a number of short lashes or cilia).

In the further course of its rapid development the roundish
bell-gastrula becomes elongated, and begins to flatten on one side,
parallel to the long axis. The flattened side is the subsequent dorsal
side; the opposite or ventral side remains curved. The latter grows
more quickly than the former, with the result that the primitive mouth
is forced to the dorsal side (Fig. 39). In the middle of the dorsal
surface a shallow longitudinal groove or furrow is formed (Fig. 79),
and the edges of the body rise up on each side of this groove in the
shape of two parallel swellings. This groove is, of course, the dorsal
furrow, and the swellings are the dorsal or medullary swellings; they
form the first structure of the central nervous system, the medullary
tube. The medullary swellings now rise higher; the groove between them
becomes deeper and deeper. The edges of the parallel swellings curve
towards each other, and at last unite, and the medullary tube is formed
(Figs. 83 _m,_ 84 _m_). Hence the formation of a medullary tube out of
the outer skin takes place in the naked dorsal surface of the
free-swimming larva of the Amphioxus in just the same way as we have
found in the embryo of man and the higher animals within the fœtal
membranes.

Simultaneously with the construction of the medullary tube we have in
the Amphioxus-embryo the formation of the chorda, the cœlom-pouches,
and the mesoderm proceeding from their wall. These processes also take
place with characteristic simplicity and clearness, so that they are
very instructive to compare with the vermalia on the one hand and with
the higher vertebrates on the other. While the medullary groove is
sinking in the middle line of the flat dorsal side of the oval embryo,
and its parallel edges unite to form the ectodermic neural tube, the
single chorda is formed directly underneath them, and on each side of
this a parallel longitudinal fold, from the dorsal wall of the
primitive gut. These longitudinal folds of the entoderm proceed from
the primitive mouth, or from its lower
and hinder edge. Here we see at an early stage a couple of large
entodermic cells, which are distinguished from all the others by their
great size, round form, and fine-grained protoplasm; they are the two
promesoblasts, or polar cells of the mesoderm (Fig. 83 _p_). They
indicate the original starting-point of the two cœlom-pouches, which
grow from this spot between the inner and outer germinal layers, sever
themselves from the primitive gut, and provide the cellular material
for the middle layer.

Immediately after their formation the two cœlom-pouches of the
Amphioxus are divided into several parts by longitudinal and transverse
folds. Each of the primary pouches is divided into an upper dorsal and
a lower ventral section by a couple of lateral longitudinal folds (Fig.
82). But these are again divided by several parallel transverse folds
into a number of successive sacs, the primitive segments or somites
(formerly called by the unsuitable name of “primitive vertebræ”). They
have a different future above and below. The upper or dorsal segments,
the _episomites,_ lose their cavity later on, and form with their cells
the muscular plates of the trunk. The lower or ventral segments, the
_hyposomites,_ corresponding to the lateral plates of the
craniote-embryo, fuse together in the upper part owing to the
disappearance of their lateral walls, and thus form the later
body-cavity (metacœl); in the lower part they remain separate, and
afterwards form the segmental gonads.

In the middle, between the two lateral cœlom-folds of the primitive
gut, a single central organ detaches from this at an early stage in the
middle line of its dorsal wall. This is the dorsal chorda (Figs. 83, 84
_ch_). This axial rod, which is the first foundation of the later
vertebral column in all the vertebrates, and is the only representative
of it in the Amphioxus, originates from the entoderm.

In consequence of these important folding-processes in the primitive
gut, the simple entodermic tube divides into four different sections:—
I, underneath, at the ventral side, the permanent alimentary canal or
permanent gut; II, above, at the dorsal side, the axial rod or chorda;
and III, the two cœlom-sacs, which immediately sub-divide into two
structures:—IIIA, above, on the dorsal side, the _episomites,_ the
double row of primitive or muscular segments; and IIIB, below, on each
side of the gut, the _hyposomites,_ the two lateral plates that give
rise to the sex-glands, and the cavities of which partly unite to form
the body-cavity. At the same time, the neural or medullary tube is
formed above the chorda, on the dorsal surface, by the closing of the
parallel medullary swellings. All these processes, which outline the
typical structure of the vertebrate, take place with astonishing
rapidity in the embryo of the Amphioxus; in the afternoon of the first
day, or twenty-four hours after fertilisation, the young vertebrate,
the typical embryo, is formed; it then has, as a rule, six to eight
somites.

The chief occurrence on the second day of development is the
construction of the two permanent openings of the gut—the mouth and
anus. In the earlier stages the alimentary tube is found to be entirely
closed, after the closing of the primitive mouth; it only communicates
behind by the neurenteric canal with the medullary tube. The permanent
mouth is a secondary formation, at the opposite end. Here, at the end
of the second day, we find a pit-like depression in the outer skin,
which penetrates inwards into the closed gut. The anus is formed behind
in the same way a few hours later (in the vicinity of the additional
gastrula-mouth). In man and the higher vertebrates also the mouth and
anus are formed, as we have seen, as flat pits in the outer skin; they
then penetrate inwards, gradually becoming connected with the blind
ends of the closed gut-tube. During the second day the Amphioxus-embryo
undergoes few other changes. The number of primitive segments
increases, and generally amounts to fourteen, some forty-eight to fifty
hours after impregnation.

Almost simultaneously with the formation of the mouth the first
gill-cleft breaks through in the fore section of the Amphioxus-embryo
(generally forty hours after the commencement of development). It now
begins to nourish itself independently, as the food material stored up
in the ovum is completely used up. The further development of the free
larvæ takes place very slowly, and extends over several months. The
body becomes much longer, and is compressed at the sides, the head-end
being broadened in a sort of triangle. Two rudimentary sense-organs are
developed in it. Inside we find the first blood-vessels, an upper or
dorsal vessel, corresponding to the aorta, between the gut and the
dorsal cord, and a lower or ventral
vessel, corresponding to the subintestinal vein, at the lower border of
the gut. Now, the gills or respiratory organs also are formed at the
fore-end of the alimentary canal. The whole of the anterior or
respiratory section of the gut is converted into a gill-crate, which is
pierced trellis-wise by numbers of branchial-holes, as in the ascidia.
This is done by the foremost part of the gut-wall joining star-wise
with the outer skin, and the formation of clefts at the point of
connection, piercing the wall and leading into the gut from without. At
first there are very few of these branchial clefts; but there are soon
a number of them—first in one, then in two, rows. The foremost
gill-cleft is the oldest. In the end we have a sort of lattice work of
fine gill-clefts, supported on a number of stiff branchial rods; these
are connected in pairs by transverse rods.


Figs. 222-224. Transverse sections of young Amphioxus-larvae. Figs.
222–224—Transverse sections of young Amphioxus-larvæ (diagrammatic,
from _ Ralph._) (Cf. also Fig. 216.) In Fig. 222 there is free
communication from without with the gut-cavity (_D_) through the
gill-clefts (_K_). In Fig. 223 the lateral folds of the body-wall, or
the gill-covers, which grow downwards, are formed. In Fig. 224 these
lateral folds have united underneath and joined their edges in the
middle line of the ventral side (_R_ seam). The respiratory water now
passes from the gut-cavity (_D_) into the mantle-cavity (_A_). The
letters have the same meaning throughout: _N_ medullary tube, _Ch_
chorda, _ M_ lateral muscles, _Lh_ body-cavity, _G_ part of the
body-cavity in which the sexual organs are subsequently formed. _ D_
gut-cavity, clothed with the gut-gland layer (_a_). A mantle-cavity,
_K_ gill-clefts, _b_=_E_ epidermis, _E_1 the same as visceral
epithelium of the mantle-cavity, _E_2 as parietal epithelium of the
mantle-cavity.


At an early stage of embryonic development the structure of the
Amphioxus-larva is substantially the same as the ideal picture we have
previously formed of the “Primitive Vertebrate” (Figs. 98–102). But the
body afterwards undergoes various modifications, especially in the
fore-part. These modifications do not concern us, as they depend on
special adaptations, and do not affect the hereditary vertebrate type.
When the free-swimming Amphioxus-larva is three months old, it abandons
its pelagic habits and changes into the young animal that lives in the
sand. In spite of its smallness (one-eighth of an inch), it has
substantially the same structure as the adult. As regards the remaining
organs of the Amphioxus, we need only mention that the gonads or sexual
glands are developed very late, immediately out of the inner cell-layer
of the
body-cavity. Although we can find afterwards no continuation of the
body-cavity (Fig. 216 _U_) in the lateral walls of the mantle-cavity,
in the gill-covers or mantle-folds (Fig. 224 _U_), there is one present
in the beginning (Fig. 224 _Lh_). The sexual cells are formed below, at
the bottom of this continuation (Fig. 224 _S_). For the rest, the
subsequent development into the adult Amphioxus of the larva we have
followed is so simple that we need not go further into it here.

We may now turn to the embryology of the Ascidia, an animal that seems
to stand so much lower and to be so much more simply organised,
remaining for the greater part of its life attached to the bottom of
the sea like a shapeless lump. It was a fortunate accident that
Kowalevsky first examined just those larger specimens of the Ascidiæ
that show most clearly the relationship of the vertebrates to the
invertebrates, and the larvæ of which behave exactly like those of the
Amphioxus in the first stages of development. This resemblance is so
close in the main features that we have only to repeat what we have
already said of the ontogenesis of the Amphioxus.

The ovum of the larger Ascidia (_Phallusia, Cynthia,_ etc.) is a simple
round cell of 1/250 to 1/125 of an inch in diameter. In the thick
fine-grained yelk we find a clear round germinal vesicle of about 1/750
of an inch in diameter, and this encloses a small embryonic spot or
nucleolus. Inside the membrane that surrounds the ovum, the stem-cell
of the Ascidia, after fecundation, passes through just the same
metamorphoses as the stem-cell of the Amphioxus. It undergoes total
segmentation; it divides into two, four, eight, sixteen, thirty-two
cells, and so on. By continued total cleavage the morula, or
mulberry-shaped cluster of cells, is formed. Fluid gathers inside it,
and thus we get once more a globular vesicle (the blastula); the wall
of this is a single stratum of cells, the blastoderm. A real gastrula
(a simple bell-gastrula) is formed from the blastula by invagination,
in the same way as in the amphioxus.

Up to this there is no definite ground in the embryology of the Ascidiæ
for bringing them into close relationship with the Vertebrates; the
same gastrula is formed in the same way in many other animals of
different stems. But we now find an embryonic process that is peculiar
to the Vertebrates, and that proves irrefragably the affinity of the
Ascidiæ to the Vertebrates. From the epidermis of the gastrula a
_medullary tube_ is formed on the dorsal side, and, between this and
the primitive gut, a _chorda_; these are the organs that are otherwise
only found in Vertebrates. The formation of these very important organs
takes place in the Ascidia-gastrula in precisely the same way as in
that of the Amphioxus. In the Ascidia (as in the other case) the oval
gastrula is first flattened on one side—the subsequent dorsal side. A
groove or furrow (the medullary groove) is sunk in the middle line of
the flat surface, and two parallel longitudinal swellings arise on
either side from the skin layer. These medullary swellings join
together over the furrow, and form a tube; in this case, again, the
neural or medullary tube is at first open in front, and connected with
the primitive gut behind by the neurenteric canal. Further, in the
Ascidia-larva also the two permanent apertures of the alimentary canal
only appear later, as independent and new formations. The permanent
mouth does not develop from the primitive mouth of the gastrula; this
primitive mouth closes up, and the later anus is formed near it by
invagination from without, on the hinder end of the body, opposite to
the aperture of the medullary tube.

During these important processes, that take place in just the same way
in the Amphioxus, a tail-like projection grows out of the posterior end
of the larva-body, and the larva folds itself up within the round
ovolemma in such a way that the dorsal side is curved and the tail is
forced on to the ventral side. In this tail is developed—starting from
the primitive gut—a cylindrical string of cells, the fore end of which
pushes into the body of the larva, between the alimentary canal and the
neural canal, and is no other than the chorda dorsalis. This important
organ had hitherto been found only in the Vertebrates, not a single
trace of it being discoverable in the Invertebrates. At first the
chorda only consists of a single row of large entodermic cells. It is
afterwards composed of several rows of cells. In the Ascidia-larva,
also, the chorda develops from the dorsal middle part of the primitive
gut, while the two cœlom-pouches detach themselves from it on both
sides. The simple body-cavity is formed by the coalescence of the two.

When the Ascidia-larva has attained
this stage of development it begins to move about in the ovolemma. This
causes the membrane to burst. The larva emerges from it, and swims
about in the sea by means of its oar-like tail. These free-swimming
larvæ of the Ascidia have been known for a long time. They were first
observed by Darwin during his voyage round the world in 1833. They
resemble tadpoles in outward appearance, and use their tails as oars,
as the tadpoles do. However, this lively and highly-developed condition
does not last long. At first there is a progressive development; the
foremost part of the medullary tube enlarges into a brain, and inside
this two single sense-organs are developed, a dorsal auditory vesicle
and a ventral eye. Then a heart is formed on the ventral side of the
animal, or the lower wall of the gut, in the same simple form and at
the same spot at which the heart is developed in man and all the other
vertebrates. In the lower muscular wall of the gut we find a weal-like
thickening, a solid, spindle-shaped string of cells, which becomes
hollow in the centre; it begins to contract in different directions,
now forward and now backward, as is the case with the adult Ascidia. In
this way the sanguineous fluid accumulated in the hollow muscular tube
is driven in alternate directions into the blood-vessels, which develop
at both ends of the cardiac tube. One principal vessel runs along the
dorsal side of the gut, another along its ventral side. The former
corresponds to the aorta and the dorsal vessel in the worms. The other
corresponds to the subintestinal vein and the ventral vessel of the
worms.


Fig.225. An Appendicaria (Copelata), seen from the left. Fig. 225—An
Appendicaria (Copelata), seen from the left. _m_ mouth, _k_ branchial
gut, _o_ gullet, _ v_ stomach, _a_ anus, _n_ brain (ganglion above the
gullet), _g_ auditory vesicle, _f_ ciliated groove under the gills, _h_
heart, _t_ testicles, _e_ ovary, _ c_ chorda, _s_ tail.


With the formation of these organs the progressive development of the
Ascidia comes to an end, and degeneration sets in. The free-swimming
larva sinks to the floor of the sea, abandons its locomotive habits,
and attaches itself to stones, marine plants, mussel-shells, corals,
and other objects; this is done with the part of the body that was
foremost in movement. The attachment is effected by a number of
out-growths, usually three, which can be seen even in the free-swimming
larva. The tail is lost, as there is no further use for it. It
undergoes a fatty degeneration, and disappears with the chorda
dorsalis. The tailless body changes into an unshapely tube, and, by the
atrophy of some parts and the modification of others, gradually assumes
the appearance we have already described.

Among the living Tunicates there is a very interesting group of small
animals that remain throughout life at the stage of development of the
tailed, free Ascidia-larva, and swim about briskly in the sea by means
of their broad oar-tail. These are the remarkable Copelata
(_Appendicaria_ and _Vexillaria,_ Fig. 225). They are the only living
Vertebrates that have throughout life a chorda dorsalis and a neural
string above it; the latter must be regarded as the prolongation of the
cerebral ganglion and the equivalent of the medullary tube. Their
branchial gut also opens directly outwards by a pair of
branchial clefts. These instructive Copelata, comparable to permanent
Ascidia-larvæ, come next to the extinct Prochordonia, those ancient
worms which we must regard as the common ancestors of the Tunicates and
Vertebrates. The chorda of the Appendicaria is a long, cylindrical
string (Fig. 225 _ c_), and serves as an attachment for the muscles
that work the flat oar-tail.

Among the various modifications which the Ascidia-larva undergoes after
its establishment at the sea-floor, the most interesting (after the
loss of the axial rod) is the atrophy of one of its chief organs, the
medullary tube. In the Amphioxus the spinal marrow continues to
develop, but in the Ascidia the tube soon shrinks into a small and
insignificant nervous ganglion that lies above the mouth and the
gill-crate, and is in accord with the extremely slight mental power of
the animal. This insignificant relic of the medullary tube seems to be
quite beyond comparison with the nervous centre of the vertebrate, yet
it started from the same structure as the spinal cord of the Amphioxus.
The sense-organs that had been developed in the fore part of the neural
tube are also lost; no trace of which can be found in the adult
Ascidia. On the other hand, the alimentary canal becomes a most
extensive organ. It divides presently into two sections—a wide fore or
branchial gut that serves for respiration, and a narrower hind or
hepatic gut that accomplishes digestion. The branchial or head-gut of
the Ascidia is small at first, and opens directly outwards only by a
couple of lateral ducts or gill-clefts—a permanent arrangement in the
Copelata. The gill-clefts are developed in the same way as in the
Amphioxus. As their number greatly increases we get a large gill-crate,
pierced like lattice work. In the middle line of its ventral side we
find the hypobranchial groove. The mantle or cloaca-cavity (the atrium)
that surrounds the gill-crate is also formed in the same way in the
Ascidia as in the Amphioxus. The ejection-opening of this peribranchial
cavity corresponds to the branchial pore of the Amphioxus. In the adult
Ascidia the branchial gut and the heart on its ventral side are almost
the only organs that recall the original affinity with the vertebrates.

The further development of the Ascidia in detail has no particular
interest for us, and we will not go into it. The chief result that we
obtain from its embryology is the complete agreement with that of the
Amphioxus in the earliest and most important embryonic stages. They do
not begin to diverge until after the medullary tube and alimentary
canal, and the axial rod with the muscles between the two, have been
formed. The Amphioxus continues to advance, and resembles the embryonic
forms of the higher vertebrates; the Ascidia degenerates more and more,
and at last, in its adult condition, has the appearance of a very
imperfect invertebrate.

If we now look back on all the remarkable features we have encountered
in the structure and the embryonic development of the Amphioxus and the
Ascidia, and compare them with the features of man’s embryonic
development which we have previously studied, it will be clear that I
have not exaggerated the importance of these very interesting animals.
It is evident that the Amphioxus from the vertebrate side and the
Ascidia from the invertebrate form the bridge by which we can span the
deep gulf that separates the two great divisions of the animal kingdom.
The radical agreement of the lancelet and the sea-squirt in the first
and most important stages of development shows something more than
their close anatomic affinity and their proximity in classification; it
shows also their real blood-relationship and their common origin from
one and the same stem-form. In this way, it throws considerable light
on the oldest roots of man’s genealogical tree.



Chapter XVIII.
DURATION OF THE HISTORY OF OUR STEM


Our comparative investigation of the anatomy and ontogeny of the
Amphioxus and Ascidia has given us invaluable assistance. We have, in
the first place, bridged the wide gulf that has existed up to the
present between the Vertebrates and Invertebrates; and, in the second
place, we have discovered in the embryology of the Amphioxus a number
of ancient evolutionary stages that have long since disappeared from
human embryology, and have been lost, in virtue of the law of curtailed
heredity. The chief of these stages are the spherical blastula (in its
simplest primary form), and the succeeding archigastrula, the pure,
original form of the _gastrula_ which the Amphioxus has preserved to
this day, and which we find in the same form in a number of
Invertebrates of various classes. Not less important are the later
embryonic forms of the cœlomula, the chordula, etc.

Thus the embryology of the Amphioxus and the Ascidia has so much
increased our knowledge of man’s stem-history that, although our
empirical information is still very incomplete, there is now no defect
of any great consequence in it. We may now, therefore, approach our
proper task, and reconstruct the phylogeny of man in its chief lines
with the aid of this evidence of comparative anatomy and ontogeny. In
this the reader will soon see the immense importance of the direct
application of the biogenetic law. But before we enter upon the work it
will be useful to make a few general observations that are necessary to
understand the processes aright.

We must say a few words with regard to the period in which the human
race was evolved from the animal kingdom. The first thought that occurs
to one in this connection is the vast difference between the duration
of man’s ontogeny and phylogeny. The individual man needs only nine
months for his complete development, from the fecundation of the ovum
to the moment when he leaves the maternal womb. The human embryo runs
its whole course in the brief space of forty weeks (as a rule, 280
days). In many other mammals the time of the embryonic development is
much the same as in man—for instance, in the cow. In the horse and ass
it takes a little longer, forty-three to forty-five weeks; in the
camel, thirteen months. In the largest mammals, the embryo needs a much
longer period for its development in the womb—a year and a half in the
rhinoceros, and ninety weeks in the elephant. In these cases pregnancy
lasts twice as long as in the case of man, or one and three-quarter
years. In the smaller mammals the embryonic period is much shorter. The
smallest mammals, the dwarf-mice, develop in three weeks; hares in four
weeks, rats and marmots in five weeks, the dog in nine, the pig in
seventeen, the sheep in twenty-one and the goat in thirty-six. Birds
develop still more quickly. The chick only needs, in normal
circumstances, three weeks for its full development. The duck needs
twenty-five days, the turkey twenty-seven, the peacock thirty-one, the
swan forty-two, and the cassowary sixty-five. The smallest bird, the
humming-bird, leaves the egg after twelve days. Hence the duration of
individual development within the fœtal membranes is, in the mammals
and birds, clearly related to the absolute size of the body of the
animal in question. But this is not the only determining feature. There
are a number of other circumstances that have an influence on the
period of embryonic development. In the Amphioxus the earliest and most
important embryonic processes take place so rapidly that the blastula
is formed in four hours, the gastrula in six, and the typical
vertebrate form in twenty-four.

In every case the duration of ontogeny shrinks into insignificance when
we compare it with the enormous period that has been necessary for
phylogeny, or the gradual development of the ancestral series. This
period is not measured by years or centuries, but by thousands and
millions of years. Many millions of years had to pass before the most
advanced
vertebrate, man, was evolved, step by step, from his ancient
unicellular ancestors. The opponents of evolution, who declare that
this gradual development of the human form from lower animal forms, and
ultimately from a unicellular organism, is an incredible miracle,
forget that the same miracle takes place within the space of mine
months in the embryonic development of every human being. Each of us
has, in the forty weeks—properly speaking, in the first four weeks—of
his development in the womb, passed through the same series of
transformations that our animal ancestors underwent in the course of
millions of years.

It is impossible to determine even approximately, in hundreds or even
thousands of years, the real and absolute duration of the phylogenetic
period. But for some time now we have, through the research of
geologists, been in a position to assign the relative length of the
various sections of the organic history of the earth. The immediate
data for determining this relative length of the geological periods are
found in the thickness of the sedimentary strata—the strata that have
been formed at the bottom of the sea or in fresh water from the mud or
slime deposited there. These successive layers of limestone, sandstone,
slate, marl, etc., which make up the greater part of the rocks, and are
often several thousand feet thick, give us a standard for computing the
relative length of the various periods.

To make the point quite clear, I must say a word about the evolution of
the earth in general, and point out briefly the chief features of the
story. In the first place, we encounter the principle that on our
planet organic life began to exist at a definite period. That statement
is no longer disputed by any competent geologist or biologist. The
organic history of the earth could not commence until it was possible
for water to settle on our planet in fluid condition. Every organism,
without exception, needs fluid water as a condition of existence, and
contains a considerable quantity of it. Our own body, when fully
formed, contains sixty to seventy per cent of water in its tissues, and
only thirty to forty per cent of solid matter. There is even more water
in the body of the child, and still more in the embryo. In the earlier
stages of development the human fœtus contains more than ninety per
cent of water, and not ten per cent of solids. In the lower marine
animals, especially certain medusæ, the body consists to the extent of
more than ninety-nine per cent of sea-water, and has not one per cent
of solid matter. No organism can exist or discharge its functions
without water. No water, no life!

But fluid water, on which the existence of life primarily depends,
could not exist on our planet until the temperature of the surface of
the incandescent sphere had sunk to a certain point. Up to that time it
remained in the form of steam. But as soon as the first fluid water
could be condensed from the envelope of steam, it began its geological
action, and has continued down to the present day to modify the solid
crust of the earth. The final outcome of this incessant action of the
water—wearing down and dissolving the rocks in the form of rain, hail,
snow, and ice, as running stream or boiling surge—is the formation of
mud. As Huxley says in his admirable _Lectures on the Causes of
Phenomena in Organic Nature,_ the chief document as to the past history
of our earth is mud; the question of the history of past ages resolves
itself into a question about the formation of mud.

As I have said, it is possible to form an approximate idea of the
relative age of the various strata by comparing them at different parts
of the earth’s surface. Geologists have long been agreed that there is
a definite historical succession of the different strata. The various
superimposed layers correspond to successive periods in the organic
history of the earth, in which they were deposited in the form of mud
at the bottom of the sea. The mud was gradually converted into stone.
This was lifted out of the water owing to variations in the earth’s
surface, and formed the mountains. As a rule, four or five great
divisions are distinguished in the organic history of the earth,
corresponding to the larger and smaller groups of the sedimentary
strata. The larger periods are then sub-divided into a series of
smaller ones, which usually number from twelve to fifteen. The
comparative thickness of the groups of strata enables us to make an
approximate calculation of the relative length of these various periods
of time. We cannot say, it is true, “In a century a stratum of a
certain thickness (about two feet) is formed on the average; therefore,
a layer 1000 feet thick must be 500,000 years old.” Different strata of
the same thickness may need very different periods for their formation.
But from
the thickness or size of the stratum we can draw some conclusion as to
the _relative_ length of the period.

The first and oldest of the four or five chief divisions of the organic
history of the earth is called the primordial, archaic, or archeozoic
period. If we compute the total average thickness of the sedimentary
strata at about 130,000 feet, this first period comprises 70,000 feet,
or the greater part of the whole. For this and other reasons we may at
once conclude that the corresponding primordial or archeolithic period
must have been in itself much longer than the whole of the remaining
periods together, from its close to the present day. It was probably
much longer than the figures I have quoted (7:6) indicate—possibly 9:6.
Of late years the thickness of the archaic rocks has been put at 90,000
feet.

 SYNOPSIS OF THE PALEONTOLOGICAL FORMATIONS,
OR THE FOSSILIFEROUS STRATA OF THE CRUST

 Groups       Systems Formations        Synonyms of
 Formations

V. Anthropolithic groups, or anthropozoic (quaternary)
groups of strata.       XIV. Recent (alluvium).       38. Present 37.
Recent       Upper alluvial Lower alluvial XIII. Pleistocene (diluvium)
36. Post-glacial 35. Glacial       Upper diluvial
Lower diluvial

IV. Cenolithic groups, or cenozoic (tertiary) groups of strata.      
XII. Pliocene (neo-tertiary)       34. Arverne 33. Subapennine      
Upper pliocene Lower pliocene XI. Miocene (middle tertiary) 32. Falun
31. Limbourg       Upper miocene Lower miocene Xb. Oligocene (old
tertiary) 30. Aquitaine 29. Ligurium       Upper oligocene
Lower oligocene Xa. Eocene (primitive tertiary)       28. Gypsum 27.
Coarse chalk 26. London clay       Upper eocene Middle eocene Lower
eocene

III. Mesolithic groups, or mesozoic (secondary) groups of strata.      
IX. Chalk
(cretaceous)       25. White chalk 24. Green sand
23. Neoconian 22. Wealden       Upper cretaceous
Middle cretaceous Lower cretaceous Weald formation VIII. Jurassic     
21. Portland 20. Oxford 19. Bath 18. Lias Upper oolithic Middle
oolithic Lower oolithic Liassic VII. Triassic      17. Keuper 16.
Muschelkalk 15. Bunter       Upper triassic Middle triassic Lower
triassic

II. Paleolithic groups, or paleozoic (primary) groups of strata.
     VIb. Permian      14. Zechstein 13. Neurot sand       Upper permian
Lower permian VIa. Carboniferous coal-measures) 12. Carboniferous
      sandstone 11. Carboniferous
      limestone       Upper carboniferous
 Lower carboniferous V. Devonian      10. Pilton
  9. Ilfracombe 8. Linton Upper devonian Middle devonian Lower devonian
  IV. Silurian 7. Ludlow 6. Wenlock 5. Llandeilo       Upper silurian
  Middle silurian
Lower silurian

I. Archeolithic groups, or archeozoic (primordial)
groups of strata.      III. Cambrian        4. Potsdam 3. Longmynd      
Upper cambrian Lower cambrian II. Huronian I. Laurentian         2.
Labrador 1. Ottawa       Upper laurentian Lower laurentian

The primordial period falls into three subordinate sections—the
Laurentian, Huronian, and Cambrian, corresponding to the three chief
groups of rocks that comprise the archaic formation. The immense period
during which these rocks were forming in the primitive ocean probably
comprises more than 50,000,000 years. At the commencement of it the
oldest and simplest organisms were formed by spontaneous generation—the
Monera, with which the history of life on our planet opened. From these
were first developed unicellular organisms of the simplest character,
the Protophyta
and Protozoa (paulotomea, amœbæ, rhizopods, infusoria, and other
Protists). During this period the whole of the invertebrate ancestors
of the human race were evolved from the unicellular organisms. We can
deduce this from the fact that we already find remains of fossilised
fishes (Selachii and Ganoids) towards the close of the following
Silurian period. These are much more advanced and much younger than the
lowest vertebrate, the Amphioxus, and the numerous skull-less
vertebrates, related to the Amphioxus, that must have lived at that
time. The whole of the invertebrate ancestors of the human race must
have preceded these.

The primordial age is followed by a much shorter division, the
_paleozoic_ or Primary age. It is divided into four long periods, the
Silurian, Devonian, Carboniferous, and Permian. The Silurian strata are
particularly interesting because they contain the first fossil traces
of vertebrates—teeth and scales of Selachii ( _Palæodus_) in the lower,
and Ganoids ( _Pteraspis_) in the upper Silurian. During the Devonian
period the “old red sandstone” was formed; during the Carboniferous
period were deposited the vast coal-measures that yield us our chief
combustive material; in the Permian (or the Dyas), in fine, the new red
sandstone, the Zechstein (magnesian limestone), and the Kupferschiefer
(marl-slate) were formed. The collective depth of these strata is put
at 40,000 to 45,000 feet. In any case, the paleozoic age, taken as a
whole, was much shorter than the preceding and much longer than the
subsequent periods. The strata that were deposited during this primary
epoch contain a large number of fossils; besides the invertebrate
species there are a good many vertebrates, and the fishes preponderate.
There were so many fishes, especially primitive fishes (of the shark
type) and plated fishes, during the Devonian, and also during the
Carboniferous and Permian periods, that we may describe the whole
paleozoic period as “the age of fishes.” Among the paleozoic plated
fishes or Ganoids the Crossopterygii and the Ctenodipterina (dipneusts)
are of great importance.

During this period some of the fishes began to adapt themselves to
living on land, and so gave rise to the class of the amphibia. We find
in the Carboniferous period fossilised remains of five-toed amphibia,
the oldest terrestrial, air-breathing vertebrates. These amphibia
increase in variety in the Permian epoch. Towards the close of it we
find the first Amniotes, the ancestors of the three higher classes of
Vertebrates. These are lizard-like animals; the first to be discovered
was the _Proterosaurus,_ from the marl at Eisenach. The rise of the
earliest Amniotes, among which must have been the common ancestor of
the reptiles, birds, and mammals, is put back towards the close of the
paleozoic age by the discovery of these reptile remains. The ancestors
of our race during this period were at first represented by true
fishes, then by dipneusts and amphibia, and finally by the earliest
Amniotes, or the Protamniotes.

The third chief section of the organic history of the earth is the
_Mesozoic_ or Secondary period. This again is subdivided into three
divisions Triassic, Jurassic, and Cretaceous. The thickness of the
strata that were deposited in this period, from the beginning of the
Triassic to the end of the Cretaceous period, is altogether about
15,000 feet, or not half as much as the paleozoic deposits. During this
period there was a very brisk and manifold development in all branches
of the animal kingdom. There were especially a number of new and
interesting forms evolved in the vertebrate stem. Bony fishes (
_Teleostei_) make their first appearance. Reptiles are found in
extraordinary variety and number; the extinct giant-serpents
(dinosauria), the sea-serpents (halisauria), and the flying lizards
(pterosauria) are the most remarkable and best known of these. On
account of this predominance of the reptile-class, the period is called
“the age of reptiles.” But the bird-class was also evolved during this
period; they certainly originated from some division of the lizard-like
reptiles. This is proved by the embryological identity of the birds and
reptiles and their comparative anatomy, and, among other features, from
the circumstance that in this period there were birds with teeth in
their jaws and with tails like lizards (Archeopteryx, Odontornis).

Finally, the most advanced and (for us) the most important class of the
vertebrates, the mammals, made their appearance during the mesozoic
period. The earliest fossil remains of them were found in the latest
Triassic strata—lower jaws of small ungulates and marsupials. More
numerous remains are found a little later
in the Jurassic, and some in the Cretaceous. All the mammal remains
that we have from this section belong to the lower promammals and
marsupials; among these were most certainly the ancestors of the human
race. On the other hand, we have not found a single indisputable fossil
of any higher mammal (a placental) in the whole of this period. This
division of the mammals, which includes man, was not developed until
later, towards the close of this or in the following period.

The fourth section of the organic history of the earth, the Tertiary or
_Cenozoic_ age, was much shorter than the preceding. The strata that
were deposited during this period have a collective thickness of only
about 3,000 feet. It is subdivided into four sections—the Eocene,
Oligocene, Miocene, and Pliocene. During these periods there was a very
varied development of higher plant and animal forms; the fauna and
flora of our planet approached nearer and nearer to the character that
they bear to-day. In particular, the most advanced class, the mammals,
began to preponderate. Hence the Tertiary period may be called “the age
of mammals.” The highest section of this class, the placentals, now
made their appearance; to this group the human race belongs. The first
appearance of man, or, to be more precise, the development of man from
some closely-related group of apes, probably falls in either the
miocene or the pliocene period, the middle or the last section of the
Tertiary period. Others believe that man properly so-called—man endowed
with speech—was not evolved from the non-speaking ape-man (
_Pithecanthropus_) until the following, the anthropozoic, age.

In this fifth and last section of the organic history of the earth we
have the full development and dispersion of the various races of men,
and so it is called the _Anthropozoic_ as well as the _Quaternary_
period. In the imperfect condition of paleontological and
ethnographical science we cannot as yet give a confident answer to the
question whether the evolution of the human race from some extinct ape
or lemur took place at the beginning of this or towards the middle or
the end of the Tertiary period. However, this much is certain: the
development of civilisation falls in the anthropozoic age, and this is
merely an insignificant fraction of the vast period of the whole
history of life. When we remember this, it seems ridiculous to restrict
the word “history” to the civilised period. If we divide into a hundred
equal parts the whole period of the history of life, from the
spontaneous generation of the first Monera to the present day, and if
we then represent the relative duration of the five chief sections or
ages, as calculated from the average thickness of the strata they
contain, as percentages of this, we get something like the following
relation:—

 I.
II. III.
IV. V.           Archeolithic or archeozoic (primordial) age
Paleolithic or paleozoic (primary) age Mesolithic or mesozoic
(secondary) age Cenolithic or cenozoic (tertiary) age Anthropolithic or
anthropozoic (quaternary) age       53.6 32.1
11.5 2.3 0.5 ——— 100.0

In any case, the “historical period” is an insignificant quantity
compared with the vast length of the preceding ages, in which there was
no question of human existence on our planet. Even the important
Cenozoic or Tertiary period, in which the first placentals or higher
mammals appear, probably amounts to little over two per cent of the
whole organic age.

Before we approach our proper task, and, with the aid of our
ontogenetic acquirements and the biogenetic law, follow step by step
the paleontological development of our animal ancestors, let us glance
for a moment at another, and apparently quite remote, branch of
science, a general consideration of which will help us in the solving
of a difficult problem. I mean the science of comparative philology.
Since Darwin gave new life to biology by his theory of selection, and
raised the question of evolution on all sides, it has often been
pointed out that there is a remarkable analogy between the development
of languages and the evolution of species. The comparison is perfectly
just and very instructive. We could hardly find a better analogy when
we are dealing with some of the difficult and obscure features of the
evolution of species. In both cases we find the action of the same
natural laws.

All philologists of any competence in their science now agree that all
human languages have been gradually evolved from very rudimentary
beginnings. The
idea that speech is a gift of the gods—an idea held by distinguished
authorities only fifty years ago—is now generally abandoned, and only
supported by theologians and others who admit no natural development
whatever. Speech has been developed simultaneously with its organs, the
larynx and tongue, and with the functions of the brain. Hence it will
be quite natural to find in the evolution and classification of
languages the same features as in the evolution and classification of
organic species. The various groups of languages that are distinguished
in philology as primitive, fundamental, parent, and daughter languages,
dialects, etc., correspond entirely in their development to the
different categories which we classify in zoology and botany as stems,
classes, orders, families, genera, species, and varieties. The relation
of these groups, partly co-ordinate and partly subordinate, in the
general scheme is just the same in both cases; and the evolution
follows the same lines in both.

When, with the assistance of this tree, we follow the formation of the
various languages that have been developed from the common root of the
ancient Indo-Germanic tongue, we get a very clear idea of their
phylogeny. We shall see at the same time how analogous this is to the
development of the various groups of vertebrates that have arisen from
the common stem-form of the primitive vertebrate. The ancient
Indo-Germanic root-language divided first into two principal stems—the
Slavo-Germanic and the Aryo-Romanic. The Slavo-Germanic stem then
branches into the ancient Germanic and the ancient Slavo-Lettic
tongues; the Aryo-Romanic into the ancient Aryan and the ancient
Greco-Roman. If we still follow the genealogical tree of these four
Indo-Germanic tongues, we find that the ancient Germanic divides into
three branches—the Scandinavian, the Gothic, and the German. From the
ancient German came the High German and Low German; to the latter
belong the Frisian, Saxon, and modern Low-German dialects. The ancient
Slavo-Lettic divided first into a Baltic and a Slav language. The
Baltic gave rise to the Lett, Lithuanian, and old-Prussian varieties;
the Slav to the Russian and South-Slav in the south-east, and to the
Polish and Czech in the west.

We find an equally prolific branching of its two chief stems when we
turn to the other division of the Indo-Germanic languages. The
Greco-Roman divided into the Thracian (Albano-Greek) and the
Italo-Celtic. From the latter came the divergent branches of the Italic
(Roman and Latin) in the south, and the Celtic in the north: from the
latter have been developed all the British (ancient British, ancient
Scotch, and Irish) and Gallic varieties. The ancient Aryan gave rise to
the numerous Iranian and Indian languages.

This “comparative anatomy” and evolution of languages admirably
illustrates the phylogeny of species. It is clear that in structure and
development the primitive languages, mother and daughter languages, and
varieties, correspond exactly to the classes, orders, genera, and
species of the animal world. In both cases the “natural” system is
phylogenetic. As we have been convinced from comparative anatomy and
ontogeny, and from paleontology, that all past and living vertebrates
descend from a common ancestor, so the comparative study of dead and
living Indo-Germanic tongues proves beyond question that they are all
modifications of one primitive language. This view of their origin is
now accepted by all the chief philologists who have worked in this
branch and are unprejudiced.

But the point to which I desire particularly to draw the reader’s
attention in this comparison of the Indo-Germanic languages with the
branches of the vertebrate stem is, that one must never confuse direct
descendants with collateral branches, nor extinct forms with living.
This confusion is very common, and our opponents often make use of the
erroneous ideas it gives rise to for the purpose of attacking evolution
generally. When, for instance, we say that man descends from the ape,
this from the lemur, and the lemur from the marsupial, many people
imagine that we are speaking of the living species of these orders of
mammals that they find stuffed in our museums. Our opponents then foist
this idea on us, and say, with more astuteness than intelligence, that
it is quite impossible; or they ask us, by way of physiological
experiment, to turn a kangaroo into a lemur, a lemur into a gorilla,
and a gorilla into a man! The demand is childish, and the idea it rests
on erroneous. All these living forms have diverged more or less from
the ancestral form; none of them could engender the
same posterity that the stem-form really produced thousands of years
ago.

It is certain that man has descended from some extinct mammal; and we
should just as certainly class this in the order of apes if we had it
before us. It is equally certain that this primitive ape descended in
turn from an unknown lemur, and this from an extinct marsupial. But it
is just as clear that all these extinct ancestral forms can only be
claimed as belonging to the living order of mammals in virtue of their
essential internal structure and their resemblance in the decisive
anatomic characteristics of each _order._ In external appearance, in
the characteristics of the _genus_ or _species,_ they would differ more
or less, perhaps very considerably, from all living representatives of
those orders. It is a universal and natural procedure in phylogenetic
development that the stem-forms themselves, with their specific
peculiarities, have been extinct for some time. The forms that approach
nearest to them among the living species are more or less—perhaps very
substantially—different from them. Hence in our phylogenetic inquiry
and in the comparative study of the living, divergent descendants,
there can only be a question of determining the greater or less
remoteness of the latter from the ancestral form. Not a single one of
the older stem-forms has continued unchanged down to our time.

We find just the same thing in comparing the various dead and living
languages that have developed from a common primitive tongue. If we
examine our genealogical tree of the Indo-Germanic languages in this
light, we see at once that all the older or parent tongues, of which we
regard the living varieties of the stem as divergent daughter or
grand-daughter languages, have been extinct for some time. The
Aryo-Romanic and the Slavo-Germanic tongues have completely
disappeared; so also the Aryan, the Greco-Roman, the Slavo-Lettic, and
the ancient Germanic. Even their daughters and grand-daughters have
been lost; all the living Indo-Germanic languages are only related in
the sense that they are divergent descendants of common stem-forms.
Some forms have diverged more, and some less, from the original
stem-form.

This easily demonstrable fact illustrates very well the analogous case
of the origin of the vertebrate species. Phylogenetic comparative
philology here yields a strong support to phylogenetic comparative
zoology. But the one can adduce more direct evidence than the other, as
the paleontological material of philology—the old monuments of the
extinct tongue—have been preserved much better than the paleontological
material of zoology, the fossilised bones and imprints of vertebrates.

We may, however, trace man’s genealogical tree not only as far as the
lower mammals, but much further—to the amphibia, to the shark-like
primitive fishes, and, in fine, to the skull-less vertebrates that
closely resembled the Amphioxus. But this must not be understood in the
sense that the existing Amphioxus, or the sharks or amphibia of to-day,
can give us any idea of the external appearance of these remote
stem-forms. Still less must it be thought that the Amphioxus or any
actual shark, or any living species of amphibia, is a real ancestral
form of the higher vertebrates and man. The statement can only
rationally mean that the living forms I have referred to are
_collateral lines_ that are much more closely related to the extinct
stem-forms, and have retained the resemblance much better, than any
other animals we know. They are still so like them in regard to their
distinctive internal structure that we should put them in the same
class with the extinct forms if we had these before us. But no direct
descendants of these earlier forms have remained unchanged. Hence we
must entirely abandon the idea of finding direct ancestors of the human
race in their characteristic _external form_ among the living species
of animals. The essential and distinctive features that still connect
living forms more or less closely with the extinct common stem-forms
lie in the internal structure, not the external appearance. The latter
has been much modified by adaptation. The former has been more or less
preserved by heredity.

Comparative anatomy and ontogeny prove beyond question that man is a
true vertebrate, and, therefore, man’s special genealogical tree must
be connected with that of the other Vertebrates, which spring from a
common root with him. But we have also many important grounds in
comparative anatomy and ontogeny for assuming a common origin for all
the Vertebrates. If the general theory of
evolution is correct, all the Vertebrates, including man, come from a
single common ancestor, a long-extinct “Primitive Vertebrate.” Hence
the genealogical tree of the Vertebrates is at the same time that of
the human race.

Our task, therefore, of constructing man’s genealogy becomes the larger
aim of discovering the genealogy of the entire vertebrate stem. As we
now know from the comparative anatomy and ontogeny of the Amphioxus and
the Ascidia, this is in turn connected with the genealogical tree of
the Invertebrates (directly with that of the Vermalia), but has no
direct connection with the independent stems of the Articulates,
Molluscs, and Echinoderms. If we do thus follow our ancestral tree
through various stages down to the lowest worms, we come inevitably to
the _Gastræa,_ that most instructive form that gives the clearest
possible picture of an animal with two germinal layers. The Gastræa
itself has originated from the simple multicellular vesicle, the
_Blastæa,_ and this in turn must have been evolved from the lowest
circle of unicellular animals, to which we give the name of Protozoa.
We have already considered the most important primitive type of these,
the unicellular _Amœba,_ which is extremely instructive when compared
with the human ovum. With this we reach the lowest of the solid data to
which we are to apply our biogenetic law, and by which we may deduce
the extinct ancestor from the embryonic form. The amœboid nature of the
young ovum and the unicellular condition in which (as stem-cell or
cytula) every human being begins its existence justify us in affirming
that the earliest ancestors of the human race were simple amœboid
coils.

But the further question now arises: “Whence came these first amœbæ
with which the history of life began at the commencement of the
Laurentian epoch?” There is only one answer to this. The earliest
unicellular organisms can only have been evolved from the simplest
organisms we know, the _Monera._ These are the simplest living things
that we can conceive. Their whole body is nothing but a particle of
plasm, a granule of living albuminous matter, discharging of itself all
the essential vital functions that form the material basis of life.
Thus we come to the last, or, if you prefer, the first, question in
connection with evolution—the question of the origin of the Monera.
This is the real question of the origin of life, or of spontaneous
generation.

We have neither space nor occasion to go further in this Chapter into
the question of spontaneous generation. For this I must refer the
reader to the fifteenth chapter of the _History of Creation,_ and
especially to the second book of the _General Morphology,_ or to the
essay on “The Monera and Spontaneous Generation” in my _Studies of the
Monera and other Protists._[29] I have given there fully my own view of
this important question. The famous botanist Nägeli afterwards (1884)
developed the same ideas. I will only say a few words here about this
obscure question of the origin of life, in so far as our main subject,
organic evolution in general, is affected by it. Spontaneous
generation, in the definite and restricted sense in which I maintain
it, and claim that it is a necessary hypothesis in explaining the
origin of life, refers solely to the evolution of the Monera from
inorganic carbon-compounds. When living things made their first
appearance on our planet, the very complex nitrogenous compound of
carbon that we call _plasson,_ which is the earliest material
embodiment of vital action, must have been formed in a purely chemical
way from inorganic carbon-compounds. The first Monera were formed in
the sea by spontaneous generation, as crystals are formed in the
mother-water. Our demand for a knowledge of causes compels us to assume
this. If we believe that the whole inorganic history of the earth has
proceeded on mechanical principles without any intervention of a
Creator, and that the history of life also has been determined by the
same mechanical laws; if we see that there is no need to admit creative
action to explain the origin of the various groups of organisms; it is
utterly irrational to assume such creative action in dealing with the
first appearance of organic life on the earth.

 [29] The English reader will find a luminous and up-to-date chapter on
 the subject in Haeckel’s recently written and translated _Wonders of
 Life._—Translator.


This much-disputed question of “spontaneous generation” seems so
obscure, because people have associated with the term a mass of very
different, and often very absurd, ideas, and have attempted to solve
the difficulty by the crudest experiments. The real doctrine of the
spontaneous generation of life cannot possibly be refuted by
experiments.
Every experiment that has a negative result only proves that no
organism has been formed out of inorganic matter in the
conditions—highly artificial conditions—we have established. On the
other hand, it would be exceedingly difficult to prove the theory by
way of experiment; and even if Monera were still formed daily by
spontaneous generation (which is quite possible), it would be very
difficult, if not impossible, to find a solid proof of it. Those who
will not admit the spontaneous generation of the first living things in
our sense must have recourse to a supernatural miracle; and this is, as
a matter of fact, the desperate resource to which our “exact”
scientists are driven, to the complete abdication of reason.

A famous English physicist, Lord Kelvin (then Sir W. Thomson),
attempted to dispense with the hypothesis of spontaneous generation by
assuming that the organic inhabitants of the earth were developed from
germs that came from the inhabitants of other planets, and that chanced
to fall on our planet on fragments of their original home, or
meteorites. This hypothesis found many supporters, among others the
distinguished German physicist, Helmholtz. However, it was refuted in
1872 by the able physicist, Friedrich Zöllner, of Leipzig, in his work,
_On the Nature of Comets._ He showed clearly how unscientific this
hypothesis is; firstly in point of logic, and secondly in point of
scientific content. At the same time he pointed out that our hypothesis
of spontaneous generation is “a necessary condition for understanding
nature according to the law of causality.”

I repeat that we must call in the aid of the hypothesis only as regards
the Monera, the structureless “organisms without organs.” Every complex
organism must have been evolved from some lower organism. We must not
assume the spontaneous generation of even the simplest cell, for this
itself consists of at least two parts—the internal, firm nuclear
substance, and the external, softer cellular substance or the
protoplasm of the cell-body. These two parts must have been formed by
differentiation from the indifferent plasson of a moneron, or a cytode.
For this reason the natural history of the Monera is of great interest;
here alone can we find the means to overcome the chief difficulties of
the problem of spontaneous generation. The actual living Monera are
specimens of such organless or structureless organisms, as they must
have boon formed by spontaneous generation at the commencement of the
history of life.



Chapter XIX.
OUR PROTIST ANCESTORS


Under the guidance of the biogenetic law, and on the basis of the
evidence we have obtained, we now turn to the interesting task of
determining the series of man’s animal ancestors. Phylogeny us a whole
is an inductive science. From the totality of the biological processes
in the life of plants, animals, and man we have gathered a confident
inductive idea that the whole organic population of our planet has been
moulded on a harmonious law of evolution. All the interesting phenomena
that we meet in ontogeny and paleontology, comparative anatomy and
dysteleology, the distribution and habits of organisms—all the
important general laws that we abstract from the phenomena of these
sciences, and combine in harmonious unity—are the broad bases of our
great biological induction.

But when we come to the application of this law, and seek to determine
with its aid the origin of the various species of organisms, we are
compelled to frame
hypotheses that have essentially a _deductive_ character, and are
inferences from the general law to particular cases. But these special
deductions are just as much justified and necessitated by the rigorous
laws of logic as the inductive conclusions on which the whole theory of
evolution is built. The doctrine of the animal ancestry of the human
race is a special deduction of this kind, and follows with logical
necessity from the general inductive law of evolution.

I must point out at once, however, that the certainty of these
evolutionary hypotheses, which rest on clear special deductions, is not
always equally strong. Some of these inferences are now beyond
question; in the case of others it depends on the knowledge and the
competence of the inquirer what degree of certainty he attributes to
them. In any case, we must distinguish between the _ absolute_
certainty of the general (inductive) theory of descent and the
_relative_ certainty of special (deductive) evolutionary hypotheses. We
can never determine the whole ancestral series of an organism with the
same confidence with which we hold the general theory of evolution as
the sole scientific explanation of organic modifications. The special
indication of stem-forms in detail will always be more or less
incomplete and hypothetical. This is quite natural. The evidence on
which we build is imperfect, and always will be imperfect; just as in
comparative philology.

The first of our documents, paleontology, is exceedingly incomplete. We
know that all the fossils yet discovered are only an insignificant
fraction of the plants and animals that have lived on our planet. For
every single species that has been preserved for us in the rocks there
are probably hundreds, perhaps thousands, of extinct species that have
left no trace behind them. This extreme and very unfortunate
incompleteness of the paleontological evidence, which cannot be pointed
out too often, is easily explained. It is absolutely inevitable in the
circumstances of the fossilisation of organisms. It is also due in part
to the incompleteness of our knowledge in this branch. It must be borne
in mind that the great majority of the stratified rocks that compose
the crust of the earth have not yet been opened. We have only a few
specimens of the innumerable fossils that are buried in the vast
mountain ranges of Asia and Africa. Only a part of Europe and North
America has been investigated carefully. The whole of the fossils known
to us certainly do not amount to a hundredth part of the remains that
are really buried in the crust of the earth. We may, therefore, look
forward to a rich harvest in the future as regards this science.
However, our paleontological evidence will (for reasons that I have
fully explained in the sixteenth chapter of the _History of Creation_)
always be defective.

The second chief source of evidence, ontogeny, is not less incomplete.
It is the most important source of all for special phylogeny; but it
has great defects, and often fails us. We must, above all, clearly
distinguish between palingenetic and cenogenetic phenomena. We must
never forget that the laws of curtailed and disturbed heredity often
make the original course of development almost unrecognisable. The
recapitulation of phylogeny by ontogeny is only fairly complete in a
few cases, and is never wholly complete. As a rule, it is precisely the
earliest and most important embryonic stages that suffer most from
alteration and condensation. The earlier embryonic forms have had to
adapt themselves to new circumstances, and so have been modified. The
struggle for existence has had just as profound an influence on the
freely moving and still immature young forms as on the adult forms.
Hence in the embryology of the higher animals, especially, palingenesis
is much restricted by cenogenesis; it is to-day, as a rule, only a
faded and much altered picture of the original evolution of the
animal’s ancestors. We can only draw conclusions from the embryonic
forms to the stem-history with the greatest caution and discrimination.
Moreover, the embryonic development itself has only been fully studied
in a few species.

Finally, the third and most valuable source of evidence, comparative
anatomy, is also, unfortunately, very imperfect; for the simple reason
that the whole of the living species of animals are a mere fraction of
the vast population that has dwelt on our planet since the beginning of
life. We may confidently put the total number of these at more than a
million species. The number of animals whose organisation has been
studied up to the present in comparative anatomy is proportionately
very small. Here, again, future research will yield incalculable
treasures.
But, for the present, in view of this patent incompleteness of our
chief sources of evidence, we must naturally be careful not to lay too
much stress in human phylogeny on the particular animals we have
studied, or regard all the various stages of development with equal
confidence as stem-forms.

In my first efforts to construct the series of man’s ancestors I drew
up a list of, at first ten, afterwards twenty to thirty, forms that may
be regarded more or less certainly as animal ancestors of the human
race, or as stages that in a sense mark off the chief sections in the
long story of evolution from the unicellular organism to man. Of these
twenty to thirty stages, ten to twelve belong to the older group of the
Invertebrates and eighteen to twenty to the younger division of the
Vertebrates.

In approaching, now, the difficult task of establishing the
evolutionary succession of these thirty ancestors of humanity since the
beginning of life, and in venturing to lift the veil that covers the
earliest secrets of the earth’s history, we must undoubtedly look for
the first living things among the wonderful organisms that we call the
Monera; they are the simplest organisms known to us—in fact, the
simplest we can conceive. Their whole body consists merely of a simple
particle or globule of structureless plasm or plasson. The discoveries
of the last four decades have led us to believe with increasing
certainty that wherever a natural body exhibits the vital processes of
nutrition, reproduction, voluntary movement, and sensation, we have the
action of a nitrogenous carbon-compound of the chemical group of the
albuminoids; this plasm (or protoplasm) is the material basis of all
vital functions. Whether we regarded the function, in the monistic
sense, as the direct action of the material substratum, or whether we
take matter and force to be distinct things in the dualistic sense, it
is certain that we have not as yet found any living organism in which
the exercise of the vital functions is not inseparably bound up with
plasm.

The soft slimy plasson of the body of the moneron is generally called
“protoplasm,” and identified with the cellular matter of the ordinary
plant and animal cells. But we must, to be accurate, distinguish
between the plasson of the cytodes and the protoplasm of the cells.
This distinction is of the utmost importance for the purposes of
evolution. As I have often said, we must recognise two different stages
of development in these “elementary organisms,” or plastids
(“builders”), that represent the ultimate units of organic
individuality. The earlier and lower stage are the unnucleated cytodes,
the body of which consists of only one kind of albuminous matter—the
homogeneous plasson or “formative matter.” The later and higher stage
are the nucleated cells, in which we find a differentiation of the
original plasson into two different formative substances—the caryoplasm
of the nucleus and the cytoplasm of the body of the cell (cf. pp. 37
and 42).


Fig.226. Chroococcus minor. Fig. 226—Chroococcus minor (_Nägeli_),
magnified. A phytomoneron, the globular plastids of which secrete a
gelatinous structureless membrane. The unnucleated globule of plasm
(bluish-green in colour) increases by simple cleavage (_a–d_).


The Monera are permanent cytodes. Their whole body consists of soft,
structureless plasson. However carefully we examine it with our finest
chemical reagents and most powerful microscopes, we can find no
definite parts or no anatomic structure in it. Hence, the Monera are
literally organisms without organs; in fact, from the philosophic point
of view they are not organisms at all, since they have no organs. They
can only be called organisms in the sense that they are capable of the
vital functions of nutrition, reproduction, sensation, and movement. If
we were to try to imagine the simplest possible organism, we should
frame something like the moneron.

The Monera that we find to-day in various forms fall into two groups
according to the nature of their nutrition—the _ Phytomonera_ and the
_Zoomonera_; from the physiological point of view, the former are the
simplest specimens of the plant (_phyton_) kingdom, and the latter of
the animal (_zoon_) world. The Phytomonera, especially in their
simplest form, the Chromacea (_Phycochromacea_ or _Cyanophycea_), are
the most primitive and the
oldest of living organisms. The typical genus _ Chroococcus_ (Fig. 226)
is represented by several fresh-water species, and often forms a very
delicate bluish-green deposit on stones and wood in ponds and ditches.
It consists of round, light green particles, from 1/7000 to 1/2500 of
an inch in diameter.


Fig.227. Aphanocapsa primordialis. Fig. 227—Aphanocapsa primordialis
(_Nägeli_), magnified. A phytomoneron, the round plastids of which
(bluish-green in colour) secrete a shapeless gelatinous mass; in this
the unnucleated cytodes increase continually by simple cleavage.

The whole life of these homogeneous globules of plasm consists of
simple growth and reproduction by cleavage. When the tiny particle has
reached a certain size by the continuous assimilation of inorganic
matter, it divides into two equal halves, by a constriction in the
middle. The two daughter-monera that are thus formed immediately begin
a similar vital process. It is the same with the brown _Procytella
primordialis_ (formerly called the _Protococcus marinus_); it forms
large masses of floating matter in the arctic seas. The tiny
plasma-globules of this species are of a greenish-brown colour, and
have a diameter of 1/10,000 to 1/5000 of an inch. There is no membrane
discoverable in the simplest _Chroococcacea,_ but we find one in other
members of the same family; in _Aphanocapsa_ (Fig. 227) the enveloping
membranes of the social plastids combine; in _Glœcapsa_ they are
retained through several generations, so that the little
plasma-globules are enfolded in many layers of membrane.

Next to the Chromacea come the Bacteria, which have been evolved from
them by the remarkable change in nutrition which gives us the simple
explanation of the differentiation of plant and animal in the protist
kingdom. The Chromacea build up their plasm directly from inorganic
matter; the Bacteria feed on organic matter. Hence, if we logically
divide the protist kingdom into plasma-forming Protophyta and
plasma-consuming Protozoa, we must class the Bacteria with the latter;
it is quite illogical to describe them—as is still often done—as
_Schizomycetes,_ and class them with the true fungi. The Bacteria, like
the Chromacea, have no nucleus. As is well-known, they play an
important part in modern biology as the causes of fermentation and
putrefaction, and of tuberculosis, typhus, cholera, and other
infectious diseases, and as parasites, etc. But we cannot linger now to
deal with these very interesting features; the Bacteria have no
relation to man’s genealogical tree.

We may now turn to consider the remarkable Protamœba, or unnucleated
Amœba. I have, in the first volume, pointed out the great importance of
the ordinary Amœba in connection with several weighty questions of
general biology. The tiny Protamœbæ, which are found both in fresh and
salt water, have the same unshapely form and irregular movements of
their simple naked body as the real Amœbæ; but they differ from them
very materially in having no nucleus in their cell-body. The short,
blunt, finger-like processes that are thrust out at the surface of the
creeping Protamœba serve for getting food as well as for locomotion.
They multiply by simple cleavage (Fig. 228).

The next stage to the simple cytode-forms of the Monera in the
genealogy of mankind (and all other animals) is the simple cell, or the
most rudimentary form of the cell which we find living independently
to-day as the Amœba. The earliest process of inorganic differentiation
in the structureless body of the Monera led to its division into two
different substances—the caryoplasm and the cytoplasm. The caryoplasm
is the inner and firmer part of the cell, the substance of the nucleus.
The cytoplasm is the outer and softer part, the substance of the body
of the cell. By this important differentiation of the plasson into
nucleus and cell-body, the
organised cell was evolved from the structureless cytode, the nucleated
from the unnucleated plastid. That the first cells to appear on the
earth were formed from the Monera by such a differentiation seems to us
the only possible view in the present condition of science. We have a
direct instance of this earliest process of differentiation to-day in
the ontogeny of many of the lower Protists (such as the Gregarinæ).


Fig.228. A moneron (Protamoeba) in the act of reproduction. Fig. 228—A
moneron (Protamœba) in the act of reproduction. _A_ The whole moneron,
moving like an ordinary amœba by thrusting out changeable processes.
_B_ It divides into two halves by a constriction in the middle. _C_ The
two halves separate, and each becomes an independent individual.
(Highly magnified.)


The unicellular form that we have in the ovum has already been
described as the reproduction of a corresponding unicellular stem-form,
and to this we have ascribed the organisation of an Amœba (cf. Chapter
VI). The irregular-shaped Amœba, which we find living independently
to-day in our fresh and salt water, is the least definite and the most
primitive of all the unicellular Protozoa (Fig. 16). As the unripe ova
(the _protova_ that we find in the ovaries of animals) cannot be
distinguished from the common Amœbæ, we must regard the Amœba as the
primitive form that is reproduced in the embryonic stage of the amœboid
ovum to-day, in accordance with the biogenetic law. I have already
pointed out, in proof of the striking resemblance of the two cells,
that the ova of many of the sponges were formerly regarded as parasitic
Amœbæ (Figure 1.18). Large unicellular organisms like the Amœbæ were
found creeping about inside the body of the sponge, and were thought to
be parasites. It was afterwards discovered that they were really the
ova of the sponge from which the embryos were developed. As a matter of
fact, these sponge-ova are so much like many of the Amœbæ in size,
shape, the character of their nucleus, and movement of the pseudopodia,
that it is impossible to distinguish them without knowing their
subsequent development.

Our phylogenetic interpretation of the ovum, and the reduction of it to
some ancient amœboid ancestral form, supply the answer to the old
problem: “Which was first, the egg or the chick?” We can now give a
very plain answer to this riddle, with which our opponents have often
tried to drive us into a corner. The egg came a long time before the
chick. We do not mean, of course, that the egg existed from the first
as a bird’s egg, but as an indifferent amœboid cell of the simplest
character. The egg lived for thousands of years as an independent
unicellular organism, the Amœba. The egg, in the modern physiological
sense of the word, did not make its appearance until the descendants of
the unicellular Protozoon had developed into multicellular animals, and
these had undergone sexual differentiation. Even then the egg was first
a gastræa-egg, then a platode-egg, then a vermalia-egg, and
chordonia-egg; later still acrania-egg, then fish-egg, amphibia-egg,
reptile-egg, and finally bird’s egg. The bird’s egg we have experience
of daily is a highly complicated historical product, the result of
countless hereditary processes that have taken place in the course of
millions of years.

The earliest ancestors of our race were simple Protophyta, and from
these our protozoic ancestors were developed afterwards. From the
morphological point of view both the vegetal and the animal Protists
were simple organisms, individualities of the first order, or plastids.
All our later ancestors are complex organisms, or individualities of a
higher order—social aggregations of a plurality of cells. The earliest
of these, the _ Moræada,_ which represent the third stage in our
genealogy, are very simple associations of homogeneous, indifferent
cells—undifferentiated colonies of social Amœbæ or Infusoria. To
understand the nature and origin of these protozoa-colonies we need
only follow step by step the first embryonic products of the stem-cell.
In all the Metazoa the first embryonic process is the repeated cleavage
of the stem-cell, or first segmentation-cell (Fig. 229). We have
already fully considered this process, and found that all the different
forms of it may be reduced to one type, the original equal or
primordial segmentation (cf. Chapter VIII). In the genealogical tree
of the Vertebrates this palingenetic form of segmentation has been
preserved in the Amphioxus alone, all the other Vertebrates having
cenogenetically modified forms of cleavage. In any case, the latter
were developed from the former, and so the segmentation of the ovum in
the Amphioxus has a great interest for us (cf. Fig. 38). The outcome of
this repeated cleavage is the formation of a round cluster of cells,
composed of homogeneous, indifferent cells of the simplest character
(Fig. 230). This is called the _morula_ (= mulberry-embryo) on account
of its resemblance to a mulberry or blackberry.


Fig.229. Original or primordial ovum-cleavage. Fig. 229—Original or
primordial ovum-cleavage. The stem-cell or cytula, formed by
fecundation of the ovum, divides by repeated regular cleavage first
into two (_A_), then four (_B_), then eight (_C_), and finally a large
number of segmentation-cells (_D_).


It is clear that this morula reproduces for us to-day the simple
structure of the multicellular animal that succeeded the unicellular
amœboid form in the early Laurentian period. In accordance with the
biogenetic law, the morula recalls the ancestral form of the Moræa, or
simple colony of Protozoa. The first cell-communities to be formed,
which laid the early foundation of the higher multicellular body, must
have consisted of homogeneous and simple amœboid cells. The oldest
Amœbæ lived isolated lives, and even the amœboid cells that were formed
by the segmentation of these unicellular organisms must have continued
to live independently for a long time. But gradually small communities
of Amœbæ arose by the side of these eremitical Protozoa, the
sister-cells produced by cleavage remaining joined together. The
advantages in the struggle for life which these communities had over
the isolated cells favoured their formation and their further
development. We find plenty of these cell-colonies or communities
to-day in both fresh and salt water. They belong to various groups both
of the Protophyta and Protozoa.


Fig.230. Morula, or mulberry-shaped embryo. Fig. 230—Morula, or 
mulberry-shaped embryo.


To have some idea of those ancestors of our race that succeeded
phylogenetically to the Moræada, we have only to follow the further
embryonic development of the morula. We then see that the social cells
of the round cluster secrete a sort of jelly or a watery fluid inside
their globular body, and they themselves rise to the surface of it
(Fig. 29 _F, G_). In this way the solid mulberry-embryo becomes a
hollow sphere, the wall of which is composed of a single layer of
cells. We call this layer the _blastoderm,_ and the sphere itself the
_blastula,_ or embryonic vesicle.

This interesting blastula is very important. The conversion of the
morula into a hollow ball proceeds on the same lines originally in the
most diverse stems—as, for instance, in many of the zoophytes and
worms, the ascidia, many of the echinoderms and molluscs, and in the
amphioxus. Moreover, in the animals in which we do not find a real
palingenetic blastula the defect is clearly due to cenogenetic causes,
such as the formation of food-yelk and other embryonic adaptations. We
may, therefore, conclude that the ontogenetic blastula is the
reproduction of a very early phylogenetic ancestral form, and that all
the Metazoa are descended from a common stem-form, which was in the
main constructed like the blastula. In many of the lower animals the
blastula is not developed
within the fœtal membranes, but in the open water. In those cases each
blastodermic cell begins at an early stage to thrust out one or more
mobile hair-like processes; the body swims about by the vibratory
movement of these lashes or whips (Fig. 29 _F_).

We still find, both in the sea and in fresh water, various kinds of
primitive multicellular organisms that substantially resemble the
blastula in structure, and may be regarded in a sense as permanent
blastula-forms—hollow vesicles or gelatinous balls, with a wall
composed of a single layer of ciliated homogeneous cells. There are
“blastæads” of this kind even among the Protophyta—the familiar
Volvocina, formerly classed with the infusoria. The common _Volvox
globator_ is found in the ponds in the spring—a small, green,
gelatinous globule, swimming about by means of the stroke of its
lashes, which rise in pairs from the cells on its surface. In the
similar _ Halosphæra viridis_ also, which we find in the marine
plancton (floating matter), a number of green cells form a simple layer
at the surface of the gelatinous ball; but in this case there are no
cilia.

Some of the infusoria of the flagellata-class (_Signura, Magosphæra,_
etc.) are similar in structure to these vegetal clusters, but differ in
their animal nutrition; they form the special group of the
_Catallacta._ In September, 1869, I studied the development of one of
these graceful animals on the island of Gis-Oe, off the coast of Norway
(_Magosphæra planula_), Figures 2.231 and 2.232). The fully-formed body
is a gelatinous ball, with its wall composed of thirty-two to
sixty-four ciliated cells; it swims about freely in the sea. After
reaching maturity the community is dissolved. Each cell then lives
independently for some time, grows, and changes into a creeping amœba.
This afterwards contracts, and clothes itself with a structureless
membrane. The cell then looks just like an ordinary animal ovum. When
it has been in this condition for some time the cell divides into two,
four, eight, sixteen, thirty-two, and sixty-four cells. These arrange
themselves in a round vesicle, thrust out vibratory lashes, burst the
capsule, and swim about in the same magosphæra-form with which we
started. This completes the life-circle of the remarkable and
instructive animal.

If we compare these permanent blastulæ with the free-swimming ciliated
larvæ or blastulæ, with similar construction, of many of the lower
animals, we can confidently deduce from them that there was a very
early and long-extinct common stem-form of substantially the same
structure as the blastula. We may call it the _Blastæa._ Its body
consisted, when fully formed, of a simple hollow ball, filled with
fluid or structureless jelly, with a wall composed of a single stratum
of ciliated cells. There were probably many genera and species of these
blastæads in the Laurentian period, forming a special class of marine
protists.

It is an interesting fact that in the plant kingdom also the simple
hollow sphere is found to be an elementary form of the multicellular
organism. At the surface and below the surface (down to a depth of 2000
yards) of the sea there are green globules swimming about, with a wall
composed of a single layer of chlorophyll-bearing cells. The botanist
Schmitz gave them the name of _Halosphæra viridis_ in 1879.

The next stage to the _Blastæa,_ and the sixth in our genealogical
tree, is the Gastræa that is developed from it. As we have already
seen, this ancestral form is particularly important. That it once
existed is proved with certainty by the gastrula, which we find
temporarily in the ontogenesis of all the Metazoa (Fig. 29 _J, K_). As
we saw, the original, palingenetic form of the gastrula is a round or
oval uni-axial body, the simple cavity of which (the primitive gut) has
an aperture at one pole of its axis (the primitive mouth). The wall of
the gut consists of two strata of cells, and these are the primary
germinal layers, the animal skin-layer (ectoderm) and vegetal gut-layer
(entoderm).

The actual ontogenetic development of the gastrula from the blastula
furnishes sound evidence as to the phylogenetic origin of the _Gastræa_
from the _Blastæa._ A pit-shaped depression appears at one side of the
spherical blastula (Fig. 29 _H_). In the end this invagination goes so
far that the outer or invaginated part of the blastoderm lies close on
the inner or non-invaginated part (Fig. 29 _J_). In explaining the
phylogenetic origin of the gastræa in the light of this ontogenetic
process, we may assume that the one-layered cell-community of the
blastæa began to take in food more largely at one particular part of
its surface. Natural selection would gradually lead to
the formation of a depression or pit at this alimentary spot on the
surface of the ball. The depression would grow deeper and deeper. In
time the vegetal function of taking in and digesting food would be
confined to the cells that lined this hole; the other cells would see
to the animal functions of locomotion, sensation, and protection. This
was the first division of labour among the originally homogeneous cells
of the blastæa.


Fig.231. The Norwegian Magosphaera planula, swimming about by means of
the lashes or cilia at its surface. Fig. 232. Section of same, showing
how the pear-shaped cells in the centre of the gelatinous ball are
connected by a fibrous process. Fig. 231—The Norwegian Magosphæra
planula, swimming about by means of the lashes or cilia at its surface.
Fig. 232—Section of same, showing how the pear-shaped cells in the
centre of the gelatinous ball are connected by a fibrous process. Each
cell has a contractile vacuole as well as a nucleus.


The effect, then, of this earliest histological differentiation was to
produce two different kinds of cells—nutritive cells in the depression
and locomotive cells on the surface outside. But this involved the
severance of the two primary germinal layers—a most important process.
When we remember that even man’s body, with all its various parts, and
the body of all the other higher animals, are built up originally out
of these two simple layers, we cannot lay too much stress on the
phylogenetic significance of this gastrulation. In the simple primitive
gut or gastric cavity of the gastrula and its rudimentary mouth we have
the first real organ of the animal frame in the morphological sense;
all the other organs were developed afterwards from these. In reality,
the whole body of the gastrula is merely a “primitive gut.” I have
shown already (Chapters VIII and XIX) that the two-layered embryos of
all the Metazoa can be reduced to this typical gastrula. This important
fact justifies us in concluding, in accordance with the biogenetic law,
that their ancestors also were phylogenetically developed from a
similar stem-form. This ancient stem-form is the gastræa.

The gastræa probably lived in the sea during the Laurentian period,
swimming about in the water by means of its ciliary coat much as free
ciliated gastrulæ do to-day. Probably it differed from the existing
gastrula only in one essential point, though extinct millions of years
ago. We have reason, from comparative anatomy and ontogeny, to believe
that it multiplied by sexual generation, not merely asexually (by
cleavage, gemmation, and spores), as was no doubt the case with the
earlier ancestors. Some of the cells of the primary germ-layers
probably became ova and others fertilising sperm. We base these
hypotheses on the fact that we do to-day find the simplest form of
sexual reproduction in some of the living gastræads and other lower
animals, especially the sponges.

The fact that there are still in existence various kinds of gastræads,
or lower Metazoa with an organisation little higher than that of the
hypothetical gastræa, is a strong point in favour of our theory. There
are not very many species of these living gastræads; but their
morphological and phylogenetic interest is so great, and their
intermediate position between the Protozoa and Metazoa so instructive,
that I proposed long ago (1876) to make a special class of them. I
distinguished three orders in this class—the Gastremaria, Physemaria,
and Cyemaria (or Dicyemida).
But we might also regard these three orders as so many independent
classes in a primitive gastræad stem.

The Gastremaria and Cyemaria, the chief of these living gastræads, are
small Metazoa that live parasitically inside other Metazoa, and are, as
a rule, 1/50 to 1/25 of an inch long, often much less (Fig. 233, 1–15).
Their soft body, devoid of skeleton, consists of two simple strata of
cells, the primary germinal layers; the outer of these is thickly
clothed with long hair-like lashes, by which the parasites swim about
in the various cavities of their host. The inner germinal layer
furnishes the sexual products. The pure type of the original gastrula
(or _ archigastrula,_ Fig. 29 _ I_) is seen in the _Pemmatodiscus
gastrulaceus,_ which Monticelli discovered in the umbrella of a large
medusa (_Pilema pulmo_) in 1895; the convex surface of this gelatinous
umbrella was covered with numbers of clear vesicles, of 1/25 to 1/8
inch in diameter, in the fluid contents of which the little parasites
were swimming. The cup-shaped body of the _Pemmatodiscus_ (Fig. 233,
_1_) is sometimes rather flat, and shaped like a hat or cone, at other
times almost curved into a semi-circle. The simple hollow of the cup,
the primitive gut (_g_), has a narrow opening (_o_). The skin layer
(_e_) consists of long slender cylindrical cells, which bear long
vibratory hairs; it is separated by a thin structureless, gelatinous
plate (_f_) from the visceral or gut layer (_i_), the prismatic cells
of which are much smaller and have no cilia. Pemmatodiscus propagates
asexually, by simple longitudinal cleavage; on this account it has
recently been regarded as the representative of a special order of
gastræads (_Mesogastria_).

Probably a near relative of the _Pemmatodiscus_ is the _ Kunstleria
Gruveli_ (Fig. 233, _2_). It lives in the body-cavity of Vermalia
(Sipunculida), and differs from the former in having no lashes either
on the large ectodermic cells (_e_) or the small entodermic (_i_); the
germinal layers are separated by a thick, cup-shaped, gelatinous mass,
which has been called the “clear vesicle” (_f_). The primitive mouth is
surrounded by a dark ring that bears very strong and long vibratory
lashes, and effects the swimming movements.

_Pemmatodiscus_ and _Kunstleria_ may be included in the family of the
Gastremaria. To these gastræads with open gut are closely related the
Orthonectida (_Rhopalura,_ Fig. 233, _3–5_). They live parasitically in
the body-cavity of echinoderms (Ophiura) and vermalia; they are
distinguished by the fact that their primitive gut-cavity is not empty,
but filled with entodermic cells, from which the sexual cells are
developed. These gastræads are of both sexes, the male (Fig. 3) being
smaller and of a somewhat different shape from the oval female (Fig.
4).

The somewhat similar _Dicyemida_ (Fig. 6) are distinguished from the
preceding by the fact that their primitive gut-cavity is occupied by a
single large entodermic cell instead of a crowded group of sexual
cells. This cell does not yield sexual products, but afterwards divides
into a number of cells (spores), each of which, without being
impregnated, grows into a small embryo. The Dicyemida live
parasitically in the body-cavity, especially the renal cavities, of the
cuttle-fishes. They fall in several genera, some of which are
characterised by the possession of special polar cells; the body is
sometimes roundish, oval, or club-shaped, at other times long and
cylindrical. The genus _Conocyema_ (Figs. 7–15) differs from the
ordinary _Dicyema_ in having four polar pimples in the form of a cross,
which may be incipient tentacles.

The classification of the Cyemaria is much disputed; sometimes they are
held to be parasitic infusoria (like the _Opalina_), sometimes platodes
or vermalia, related to the suctorial worms or rotifers, but having
degenerated through parasitism. I adhere to the phylogenetically
important theory that I advanced in 1876, that we have here real
gastræads, primitive survivors of the common stem-group of all the
Metazoa. In the struggle for life they have found shelter in the
body-cavity of other animals.

The small Cœlenteria attached to the floor of the sea that I have
called the Physemaria (_Haliphysema_ and _ Gastrophysema_) probably
form a third order (or class) of the living gastræads. The genus
_Haliphysema_ (Figs. 234, 235) is externally very similar to a large
rhizopod (described by the same name in 1862) of the family of the
_Rhabdamminida,_ which was at first taken for a sponge. In order to
avoid confusion with these, I afterwards gave them the name of
Prophysema. The whole mature body of the _Prophysema_ is a simple
cylindrical or oval tube, with a two-layered wall. The hollow of the
tube is the gastric cavity, and the upper opening of it the mouth (Fig.
235 _m_).


Fig.233. Modern gastræads. Fig. 1. Pemmatodiscus gastrulaceus
(Monticelli), in longitudinal section. Fig. 2. Kunstleria gruveli
(Delage), in longitudinal section. (From Kunstler and Gruvel.) Figs.
3-5. Rhopalura Giardi (Julin): Fig. 3 male, Fig. 4 female, Fig. 5
planula. Fig. 6. Dicyema macrocephala (Van Beneden). Fig. 7-15.
Conocyema polymorpha (Van Beneden): Fig. 7 the mature gastræad, Fig.
8-15 its gastrulation. Fig. 233—Modern gastræads. Fig. 1. Pemmatodiscus
gastrulaceus (_Monticelli_), in longitudinal section. Fig. 2.
Kunstleria gruveli (_Delage_), in longitudinal section. (From _
Kunstler_ and _Gruvel._) Figs. 3–5. Rhopalura Giardi (_Julin_): Fig. 3
male, Fig. 4 female, Fig. 5 planula. Fig. 6. Dicyema macrocephala (_Van
Beneden_). Figs. 7–15. Conocyema polymorpha (_Van Beneden_): Fig. 7 the
mature gastræad, Figs. 8–15 its gastrulation. _d_ primitive gut, _o_
primitive mouth, _ e_ ectoderm, _i_ entoderm, _f_ gelatinous plate
between _e_ and _i_ (supporting plate, blastocœl).


The two strata of cells that form the wall of the tube are the primary
germinal layers. These rudimentary zoophytes differ from the swimming
gastræads chiefly in being attached at one end (the end opposite to the
mouth) to the floor of the sea.


Figs. 234 and 235. Prophysema primordiale, a living gastraead. Figs.
234 and 235—Prophysema primordiale, a living gastræad. Fig. 234. The
whole of the spindle-shaped animal (attached below to the floor of the
sea. Fig. 235. The same in longitudinal section. The primitive gut
(_d_) opens above at the primitive mouth (_m_). Between the ciliated
cells (_g_) are the amœboid ova (_e_). The skin-layer (_h_) is
encrusted with grains of sand below and sponge-spicules above.


In _Prophysema_ the primitive gut is a simple oval cavity, but in the
closely related _Gastrophysema_ it is divided into two chambers by a
transverse constriction; the hind and smaller chamber above furnishes
the sexual products, the anterior one being for digestion.


Figs. 236-237. Ascula of gastrophysema, attached to the floor of the
sea. Figs. 236–237—Ascula of gastrophysema, attached to the floor of
the sea. Fig. 236 external view, 237 longitudinal section. _g_
primitive gut, _o_ primitive mouth, _i_ visceral layer, _e_ cutaneous
layer. (Diagram.)

The simplest sponges (_Olynthus,_ Fig. 238) have the same organisation
as the Physemaria. The only material difference between them is that in
the sponge the thin two-layered body-wall is pierced by numbers of
pores. When these are closed they resemble the Physemaria. Possibly the
gastræads that we call Physemaria are only olynthi with the pores
closed. The _ Ammoconida,_ or the simple tubular sand-sponges of the
deep-sea (_Ammolynthus,_ etc.), do not differ from the gastræads in any
important point when the pores are closed. In my _ Monograph on the
Sponges_ (with sixty plates) I endeavoured to prove analytically that
all the species of this class can be traced phylogenetically to a
common stem-form (_Calcolynthus_).

The lowest form of the Cnidaria is also not far removed from the
gastræads. In the interesting common fresh-water polyp (_Hydra_) the
whole body is simply an oval tube with a double wall; only in this case
the mouth has a crown of tentacles. Before these develop the hydra
resembles an ascula (Figs. 236, 237). Afterwards there are slight
histological differentiations in its ectoderm, though the entoderm
remains
a single stratum of cells. We find the first differentiation of
epithelial and stinging cells, or of muscular and neural cells, in the
thick ectoderm of the hydra.


Fig.238. Olynthus, a very rudimentary sponge. Fig. 238—Olynthus, a very
rudimentary sponge. A piece cut away in front.

In all these rudimentary living cœlenteria the sexual cells of both
kinds—ova and sperm cells—are formed by the same individual; it is
possible that the oldest gastræads were hermaphroditic. It is clear
from comparative anatomy that hermaphrodism—the combination of both
kinds of sexual cells in one individual—is the earliest form of sexual
differentiation; the separation of the sexes (gonochorism) was a much
later phenomenon. The sexual cells originally proceeded from the edge
of the primitive mouth of the gastræad.



Chapter XX.
OUR WORM-LIKE ANCESTORS


The gastræa theory has now convinced us that all the Metazoa or
multicellular animals can be traced to a common stem-form, the Gastræa.
In accordance with the biogenetic law, we find solid proof of this in
the fact that the two-layered embryos of all the Metazoa can be reduced
to a primitive common type, the gastrula. Just as the countless species
of the Metazoa do actually develop in the individual from the simple
embryonic form of the gastrula, so they have all descended in past time
from the common stem-form of the Gastræa. In this fact, and the fact we
have already established that the Gastræa has been evolved from the
hollow vesicle of the one-layered Blastæa, and this again from the
original unicellular stem-form, we have obtained a solid basis for our
study of evolution. The clear path from the stem-cell to the gastrula
represents the first section of our human stem-history (Chapters VIII,
IX, and XIX).

The second section, that leads from the Gastræa to the Prochordonia, is
much more difficult and obscure. By the Prochordonia we mean the
ancient and long-extinct animals which the important embryonic form of
the chordula proves to have once existed (cf. Figs. 83–86). The nearest
of living animals to this embryonic structure are the lowest Tunicates,
the Copelata ( _Appendicaria_) and the larvæ of the Ascidia. As both
the Tunicates and the Vertebrates develop from the same chordula, we
may infer that there was a corresponding common ancestor of both stems.
We may call this the _Chordæa,_ and the corresponding stem-group the
_Prochordonia_ or _Prochordata._

From this important stem-group of the unarticulated Prochordonia (or
“primitive chorda-animals”) the stems of the Tunicates and Vertebrates
have been divergently evolved. We shall see presently how this
conclusion is justified in the present condition of morphological
science.

We have first to answer the difficult and much-discussed question of
the development of the Chordæa from the Gastræa; in other words, “How
and by what transformations were the characteristic animals, resembling
the embryonic chordula, which we regard as the common stem-forms of all
the Chordonia, both
Tunicates and Vertebrates, evolved from the simplest two-layered
Metazoa?”

The descent of the Vertebrates from the Articulates has been maintained
by a number of zoologists during the last thirty years with more zeal
than discernment; and, as a vast amount has been written on the
subject, we must deal with it to some extent. All three classes of
Articulates in succession have been awarded the honour of being
considered the “real ancestors” of the Vertebrates: first, the Annelids
(earth-worms, leeches, and the like), then the Crustacea (crabs, etc.),
and, finally, the Tracheata (spiders, insects, etc.). The most popular
of these hypotheses was the annelid theory, which derived the
Vertebrates from the Worms. It was almost simultaneously (1875)
formulated by Carl Semper, of Würtzburg, and Anton Dohrn, of Naples.
The latter advanced this theory originally in favour of the failing
degeneration theory, with which I dealt in my work, _Aims and Methods
of Modern Embryology._

This interesting degeneration theory—much discussed at that time, but
almost forgotten now—was formed in 1875 with the aim of harmonising the
results of evolution and ever-advancing Darwinism with religious
belief. The spirited struggle that Darwin had occasioned by the
reformation of the theory of descent in 1859, and that lasted for a
decade with varying fortunes in every branch of biology, was drawing to
a close in 1870–1872, and soon ended in the complete victory of
transformism. To most of the disputants the chief point was not the
general question of evolution, but the particular one of “man’s place
in nature”—“the question of questions,” as Huxley rightly called it. It
was soon evident to every clear-headed thinker that this question could
only be answered in the sense of our anthropogeny, by admitting that
man had descended from a long series of Vertebrates by gradual
modification and improvement.

In this way the real affinity of man and the Vertebrates came to be
admitted on all hands. Comparative anatomy and ontogeny spoke too
clearly for their testimony to be ignored any longer. But in order
still to save man’s unique position, and especially the dogma of
personal immortality, a number of natural philosophers and theologians
discovered an admirable way of escape in the “theory of degeneration.”
Granting the affinity, they turned the whole evolutionary theory upside
down, and boldly contended that “man is not the most highly developed
animal, but the animals are degenerate men.” It is true that man is
closely related to the ape, and belongs to the vertebrate stem; but the
chain of his ancestry goes upward instead of downward. In the beginning
“God created man in his own image,” as the prototype of the perfect
vertebrate; but, in consequence of original sin, the human race sank so
low that the apes branched off from it, and afterwards the lower
Vertebrates. When this theory of degeneration was consistently
developed, its supporters were bound to hold that the entire animal
kingdom was descended from the debased children of men.

This theory was most strenuously defended by the Catholic priest and
natural philosopher, Michelis, in his _Hæckelogony: An Academic Protest
against Hæckel’s Anthropogeny_ (1875). In still more “academic” and
somewhat mystic form the theory was advanced by a natural philosopher
of the older Jena school—the mathematician and physicist, Carl Snell.
But it received its chief support on the zoological side from Anton
Dohrn, who maintained the anthropocentric ideas of Snell with
particular ability. The Amphioxus, which modern science now almost
unanimously regards as the real Primitive Vertebrate, the ancient model
of the original vertebrate structure, is, according to Dohrn, a late,
degenerate descendant of the stem, the “prodigal son” of the vertebrate
family. It has descended from the Cyclostoma by a profound
degeneration, and these in turn from the fishes; even the Ascidia and
the whole of the Tunicates are merely degenerate fishes! Following out
this curious theory, Dohrn came to contest the general belief that the
Cœlenterata and Worms are “lower animals”; he even declared that the
unicellular Protozoa were degenerate Cœlenterata. In his opinion
“degeneration is the great principle that explains the existence of all
the lower forms.”

If this Michelis-Dohrn theory were true, and all animals were really
degenerate descendants of an originally perfect humanity, man would
assuredly be the true centre and goal of all terrestrial life; his
anthropocentric position and his immortality would be saved.
Unfortunately, this trustful theory is in such
flagrant contradiction to all the known facts of paleontology and
embryology that it is no longer worth serious scientific consideration.

But the case is no better for the much-discussed descent of the
Vertebrates from the Annelids, which Dohrn afterwards maintained with
great zeal. Of late years this hypothesis, which raised so much dust
and controversy, has been entirely abandoned by most competent
zoologists, even those who once supported it. Its chief supporter,
Dohrn, admitted in 1890 that it is “dead and buried,” and made a
blushing retraction at the end of his _Studies of the Early History of
the Vertebrate._

Now that the annelid-hypothesis is “dead and buried,” and other
attempts to derive the Vertebrates from Medusæ, Echinoderms, or
Molluscs, have been equally unsuccessful, there is only one hypothesis
left to answer the question of the origin of the Vertebrates—the
hypothesis that I advanced thirty-six years ago and called the
“chordonia-hypothesis.” In view of its sound establishment and its
profound significance, it may very well claim to be a _theory,_ and so
should be described as the chordonia or chordæa theory.

I first advanced this theory in a series of university lectures in
1867, from which the _History of Creation_ was composed. In the first
edition of this work (1868) I endeavoured to prove, on the strength of
Kowalevsky’s epoch-making discoveries, that “of all the animals known
to us the Tunicates are undoubtedly the nearest blood-relatives of the
Vertebrates; they are the most closely related to the Vermalia, from
which the Vertebrates have been evolved. Naturally, I do not mean that
the Vertebrates have descended from the Tunicates, but that the two
groups have sprung from a common root. It is clear that the real
Vertebrates (primarily the Acrania) were evolved in very early times
from a group of Worms, from which the degenerate Tunicates also
descended in another and retrogressive direction.” This common extinct
stem-group are the Prochordonia; we still have a silhouette of them in
the chordula-embryo of the Vertebrates and Tunicates; and they still
exist independently, in very modified form, in the class of the
Copelata ( _Appendicaria,_ Fig. 225).

The chordæa-theory received the most valuable and competent support
from Carl Gegenbaur. This able comparative morphologist defended it in
1870, in the second edition of his _Elements of Comparative Anatomy_ ;
at the same time he drew attention to the important relations of the
Tunicates to a curious worm, _Balanoglossus_ : he rightly regards this
as the representative of a special class of worms, which he called
“gut-breathers” ( _Enteropneusta_). Gegenbaur referred on many other
occasions to the close blood-relationship of the Tunicates and
Vertebrates, and luminously explained the reasons that justify us in
framing the hypothesis of the descent of the two stems from a common
ancestor, an unsegmented worm-like animal with an axial chorda between
the dorsal nerve-tube and the ventral gut-tube.

The theory afterwards received a good deal of support from the research
made by a number of distinguished zoologists and anatomists, especially
C. Kupffer, B. Hatschek, F. Balfour, E. Van Beneden, and Julin. Since
Hatschek’s _Studies of the Development of the Amphioxus_ gave us full
information as to the embryology of this lowest vertebrate, it has
become so important for our purpose that we must consider it a document
of the first rank for answering the question we are dealing with.

The ontogenetic facts that we gather from this sole survivor of the
Acrania are the more valuable for phylogenetic purposes, as
paleontology, unfortunately, throws no light whatever on the origin of
the Vertebrates. Their invertebrate ancestors were soft organisms
without skeleton, and thus incapable of fossilisation, as is still the
case with the lowest vertebrates—the Acrania and Cyclostoma. The same
applies to the greater part of the Vermalia or worm-like animals, the
various classes and orders of which differ so much in structure. The
isolated groups of this rich stem are living branches of a huge tree,
the greater part of which has long been dead, and we have no fossil
evidence as to its earlier form. Nevertheless, some of the surviving
groups are very instructive, and give us clear indications of the way
in which the Chordonia were developed from the Vermalia, and these from
the Cœlenteria.

While we seek the most important of these palingenetic forms among the
groups of Cœlenteria and Vermalia, it is understood that not a single
one of them
must be regarded as an unchanged, or even little changed, copy of the
extinct stem-form. One group has retained one feature, another a
different feature, of the original organisation, and other organs have
been further developed and characteristically modified. Hence here,
more than in any other part of our genealogical tree, we have to keep
before our mind the _full picture_ of development, and separate the
unessential secondary phenomena from the essential and primary. It will
be useful first to point out the chief advances in organisation by
which the simple Gastræa gradually became the more developed Chordæa.

We find our first solid datum in the gastrula of the Amphioxus (Figure
1.38). Its bilateral and tri-axial type indicates that the
Gastræads—the common ancestors of all the Metazoa—divided at an early
stage into two divergent groups. The uni-axial Gastræa became sessile,
and gave rise to two stems, the Sponges and the Cnidaria (the latter
all reducible to simple polyps like the hydra). But the tri-axial
Gastræa assumed a certain pose or direction of the body on account of
its swimming or creeping movement, and in order to sustain this it was
a great advantage to share the burden equally between the two halves of
the body (right and left). Thus arose the typical bilateral form, which
has three axes. The same bilateral type is found in all our artificial
means of locomotion—carts, ships, etc.; it is by far the best for the
movement of the body in a certain direction and steady position. Hence
natural selection early developed this bilateral type in a section of
the Gastræads, and thus produced the stem-forms of all the bilateral
animals.

The _Gastræa bilateralis,_ of which we may conceive the bilateral
gastrula of the amphioxus to be a palingenetic reproduction,
represented the two-sided organism of the earliest Metazoa in its
simplest form. The vegetal entoderm that lined their simple gut-cavity
served for nutrition; the ciliated ectoderm that formed the external
skin attended to locomotion and sensation; finally, the two primitive
mesodermic cells, that lay to the right and left at the ventral border
of the primitive mouth, were sexual cells, and effected reproduction.
In order to understand the further development of the gastræa, we must
pay particular attention to: (1) the careful study of the embryonic
stages of the amphioxus that lie between the gastrula and the chordula;
(2) the morphological study of the simplest Platodes ( _Platodaria_ and
_Turbellaria_) and several groups of unarticulated Vermalia (
_Gastrotricha, Nemertina, Enteropneusta_).

We have to consider the Platodes first, because they are on the border
between the two principal groups of the Metazoa, the Cœlenteria and the
Cœlomaria. With the former they share the lack of body-cavity, anus,
and vascular system; with the latter they have in common the bilateral
type, the possession of a pair of nephridia or renal canals, and the
formation of a vertical brain or cerebral ganglion. It is now usual to
distinguish four classes of Platodes: the two free-living classes of
the primitive worms ( _Platodaria_) and the coiled-worms (
_Turbellaria_), and the two parasitic classes of the suctorial worms (
_Trematoda_) and the tape-worms ( _Cestoda_). We have only to consider
the first two of these classes; the other two are parasites, and have
descended from the former by adaptation to parasitic habits and
consequent degeneration.

The primitive worms ( _Platodaria_) are very small flat worms of simple
construction, but of great morphological and phylogenetic interest.
They have been hitherto, as a rule, regarded as a special order of the
_Turbellaria,_ and associated with the _Rhabdocœla_ ; but they differ
considerably from these and all the other Platodes (flat worms) in the
absence of renal canals and a special central nervous system; the
structure of their tissue is also simpler than in the other Platodes.
Most of the Platodes of this group ( _Aphanostomum, Amphichœrus,
Convoluta, Schizoprora,_ etc.) are very soft and delicate animals,
swimming about in the sea by means of a ciliary coat, and very small
(1/10 to 1/20 inch long). Their oval body, without appendages, is
sometimes spindle-shaped or cylindrical, sometimes flat and
leaf-shaped. Their skin is merely a layer of ciliated ectodermic cells.
Under this is a soft medullary substance, which consists of entodermic
cells with vacuoles. The food passes through the mouth directly into
this digestive medullary substance, in which we do not generally see
any permanent gut-cavity (it may have entirely collapsed); hence these
primitive Platodes have been called _Acœla_ (without gut-cavity or
cœlom), or, more correctly, _Cryptocœla,_ or _Pseudocœla._ The sexual
organs of these hermaphroditic
Platodaria are very simple—two pairs of strings of cells, the inner of
which (the ovaries, Fig. 239 _o_) produce ova, and the outer (the
spermaria, _s_) sperm-cells. These gonads are not yet independent
sexual glands, but sexually differentiated cell-groups in the medullary
substance, or, in other words, parts of the gut-wall. Their products,
the sex-cells, are conveyed out behind by two pairs of short canals;
the male opening ( _m_) lies just behind the female ( _f_). Most of the
Platodaria have not the muscular pharynx, which is very advanced in the
_Turbellaria_ and _Trematoda._ On the other hand, they have, as a rule,
before or behind the mouth, a bulbous sense-organ (auditory vesicle or
organ of equilibrium, _g_), and many of them have also a couple of
simple optic spots. The cell-pit of the ectoderm that lies underneath
is rather thick, and represents the first rudiment of a neural ganglion
(vertical brain or acroganglion).


Fig.239. Aphanostomum Langii (Haekel), a primitive worm of the
platodaria class, of the order of Cryptocoela or Acoela. Fig.
239—Aphanostomum Langii ( _Haeckel_), a primitive worm of the
platodaria class, of the order of _Cryptocoela_ or _Acoela._ This new
species of the genus Aphanostomum, named after Professor Arnold Lang of
Zurich, was found in September, 1899, at Ajaccio in Corsica (creeping
between fucoidea). It is one-twelfth of an inch long, one-twenty-fifth
of an inch broad, and violet in colour. _a_ mouth, _g_ auditory
vesicle, _e_ ectoderm, _i_ entoderm, _o_ ovaries, _a_ spermaries, _f_
female aperture, _m_ male aperture.


The _Turbellaria,_ with which the similar _Platodaria_ were formerly
classed, differ materially from them in the more advanced structure of
their organs, and especially in having a central nervous system
(vertical brain) and excretory renal canals (nephridia); both originate
from the ectoderm. But between the two germinal layers a mesoderm is
developed, a soft mass of connective tissue, in which the organs are
embedded. The _Turbellaria_ are still represented by a number of
different forms, in both fresh and sea-water. The oldest of these are
the very rudimentary and tiny forms that are known as _Rhabdocœla_ on
account of the simple construction of their gut; they are, as a rule,
less than a quarter of an inch long and of a simple oval or lancet
shape (Fig. 240). The surface is covered with ciliated epithelium, a
stratum of ectodermic cells. The digestive gut is still the simple
primitive gut of the gastræa ( _d_), with a single aperture that is
both mouth and anus ( _m_). There is, however, an invagination of the
ectoderm at the mouth, which has given rise to a muscular pharynx (
_sd_). It is noteworthy that the mouth of the Turbellaria (like the
primitive mouth of the Gastræa) may, in this class, change its position
considerably in the middle line of the ventral surface; sometimes it
lies behind ( _Opisthostomum_), sometimes in the middle (
_Mesostomum_), sometimes in front ( _Prosostomum_). This displacement
of the mouth from front to rear is very interesting, because it
corresponds to a phylogenetic displacement of the mouth. This probably
occurred in the Platode ancestors of most (or all?) of the Cœlomaria;
in these the permanent mouth ( _metastoma_) lies at the fore end (oral
pole), whereas the primitive mouth ( _prostoma_) lay at the hind end of
the bilateral body.

In most of the Turbellaria there is a narrow cavity, containing a
number of secondary organs, between the two primary germinal layers,
the outer or animal layer of which forms the epidermis and the inner
vegetal layer the visceral epithelium. The earliest of these organs are
the sexual organs; they are very variously constructed in the
Platode-class; in the simplest case there are merely two pairs of
gonads or sexual glands—a pair of testicles (Fig. 241 _h_)
and a pair of ovaries ( _e_). They open externally, sometimes by a
common aperture ( _Monogonopora_), sometimes by separate ones, the
female behind the male ( _Digonopora,_ Fig. 241). The sexual glands
develop originally from the two promesoblasts or primitive mesodermic
cells (Fig. 83 _p_). As these earliest mesodermic structures extended,
and became spacious sexual pouches in the later descendants of the
Platodes, probably the two cœlom-pouches were formed from them, the
first trace of the real body-cavity of the higher Metazoa (
_Enterocœla_).

The gonads are among the oldest organs, the few other organs that we
find in the Platodes between the gut-wall and body-wall being later
evolutionary products. One of the oldest and most important of these
are the kidneys or _nephridia,_ which remove unusable matter from the
body (Fig. 240 _nc_). These urinary or excretory organs were originally
enlarged skin-glands—a couple of canals that run the length of the
body, and have a separate or common external aperture ( _nm_). They
often have a number of branches. These special excretory organs are not
found in the other Cœlenteria (Gastræads, Sponges, Cnidaria) or the
Cryptocœla. They are first met in the _Turbellaria,_ and have been
transmitted direct from these to the Vermalia, and from these to the
higher stems.


Fig.240. A simple turbellarian (Rhabdocoelum). Fig. 241. The same,
showing the other organs. Fig. 240—A simple turbellarian (
_Rhabdocœlum_). _m_ mouth, _sd_ gullet epithelium, _sm_ gullet muscles,
_d_ gastric gut, _nc_ renal canals, _nm_ renal aperture, _au_ eye, _na_
olfactory pit. (Diagram.)

Fig. 241—The same, showing the other organs. _g_ brain, _au_ eye, _na_
olfactory pit, _n_ nerves, _h_ testicles, _ma_ male aperture, _fa_
female aperture, _e_ ovary, _f_ ciliated epiderm. (Diagram.)


Finally, there is a very important new organ in the Turbellaria, which
we do not find in the _Cryptocœla_ (Fig. 239) and their gastræad
ancestors—the rudimentary nervous system. It consists of a couple of
simple cerebral ganglia (Fig. 241 _g_) and fine nervous fibres that
radiate from them; these are partly voluntary nerves (or motor fibres)
that go to the thin muscular layer developing under the skin; and
partly sensory nerves that proceed to the sense-cells of the ciliated
epiderm ( _f_). Many of the Turbellaria have also special sense-organs;
a couple of ciliated smell pits ( _na_), rudimentary eyes ( _au_), and,
less frequently, auditory vesicles.

On these principles I assume that the oldest and simplest Turbellaria
arose from Platodaria, and these directly from bilateral Gastræads. The
chief advances were the formation of gonads and nephridia, and of the
rudimentary brain. On this hypothesis, which I advanced in 1872 in the
first sketch of the gastræa-theory ( _Monograph on the Sponges_), there
is no direct affinity between the Platodes and the Cnidaria.

Next to the ancient stem-group of the Turbellaria come a number of more
recent chordonia ancestors, which we class with the _Vermalia_ or
_Helminthes,_ the unarticulated worms. These true worms ( _Vermes,_
lately also called _Scolecida_) are the difficulty or the lumber-room
of the zoological classifier, because the various classes have very
complicated relations to the lower Platodes on the one hand and the
more advanced animals on the other. But if we exclude the Platodes and
the Annelids from this stem, we find a fairly satisfactory unity of
organisation
in the remaining classes. Among these worms we find some important
forms that show considerable advance in organisation from the platode
to the chordonia stage. Three of these phenomena are particularly
instructive: (1) The formation of a true (secondary) body-cavity
(cœloma); (2) the formation of a second aperture of the gut, the anus;
and (3) the formation of a vascular system. The great majority of the
Vermalia have these three features, and they are all wanting in the
Platodes; in the rest of the worms at least one or two of them are
developed.


Figs. 242 and 243. Chaetonotus, a rudimentary vermalian, of the group
of Gastrotricha. Figs. 242 and 243—Chætonotus, a rudimentary vermalian,
of the group of Gastrotricha. _m_ mouth, _s_ gullet, _d_ gut, _a_ anus,
_g_ brain, _n_ nerves, _ss_ sensory hairs, _au_ eye, _ms_ muscular
cells, _h_ skin, _f_ ciliated bands of the ventral surface, _nc_
nephridia, _nm_ their aperture, _e_ ovaries.


Next and very close to the Platodes we have the Ichthydina (
_Gastrotricha_), little marine and fresh-water worms, about 1/250 to
1/1000 inch long. Zoologists differ as to their position in
classification. In my opinion, they approach very close to the
Rhabdocœla (Figs. 240, 241), and differ from them chiefly in the
possession of an anus at the posterior end (Fig. 242 _a_). Further, the
cilia that cover the whole surface of the Turbellaria are confined in
the Gastrotricha to two ciliated bands ( _f_) on the ventral surface of
the oval body, the dorsal surface having bristles. Otherwise the
organisation of the two classes is the same. In both the gut consists
of a muscular gullet ( _s_) and a glandular primitive gut ( _d_). Over
the gullet is a double brain (acroganglion, _g_). At the side of the
gut are two serpentine prorenal canals (water-vessels or pronephridia,
_nc_), which open on the ventral side ( _nm_). Behind are a pair of
simple sexual glands or gonads (Fig. 243 _e_).

While the Ichthydina are thus closely related to the Platodes, we have
to go farther away for the two classes of Vermalia which we unite in
the group of the “snout-worms” ( _Frontonia_). These are the
_Nemertina_ and the _Enteropneusta._
Both classes have a complete ciliary coat on the epidermis, a heritage
from the Turbellaria and the Gastræads; also, both have two openings of
the gut, the mouth and anus, like the Gastrotricha. But we find also an
important organ that is wanting in the preceding forms—the vascular
system. In their more advanced mesoderm we find a few contractile
longitudinal canals which force the blood through the body by their
contractions; these are the first blood-vessels.


Fig.244. A simple Nemertine. Fig. 244—A simple Nemertine. _m_ mouth,
_d_ gut, _a_ anus, _g_ brain, _n_ nerves, _h_ ciliary coat, _ss_
sensory pits (head-clefts), _au_ eyes, _r_ dorsal vessel, _l_ lateral
vessels. (Diagram.)


Fig. 245. A young Enteropneust. Fig. 245—A young Enteropneust (
_Balanaglossus_). (From _Alexander Agassiz._) _r_ acorn-shaped snout,
_h_ neck, _k_ gill-clefts and gill-arches of the fore-gut, in long rows
on each side, _d_ digestive hind-gut, filling the greater part of the
body-cavity, _v_ intestinal vein or ventral vessel, lying between the
parallel folds of the skin, _a_ anus.


The Nemertina were formerly classed with the much less advanced
Turbellaria. But they differ essentially from them in having an anus
and blood-vessels, and several other marks of higher organisation. They
have generally long and narrow bodies, like a more or less flattened
cord; there are, besides several small species, giant-forms with a
width of 1/5 to 2/5 inch and a length of several yards (even ten to
fifteen). Most of them live in the sea, but some in fresh water and
moist earth. In their internal structure they approach the Turbellaria
on the one hand and the higher Vermalia (especially the Enteropneusta)
on the other. They have a good deal of interest as the lowest and
oldest of all animals with blood. In them we find blood-vessels for the
first time, distributing real blood through the body. The blood is red,
and the red colouring-matter is hæmoglobin, connected with elliptic
discoid blood-cells, as in the Vertebrates. Most of them have two or
three parallel blood-canals, which run the whole length of the body,
and are connected in front and behind by loops, and often by a number
of ring-shaped pieces. The chief of these primitive blood-vessels is
the one that lies above the gut in the middle line of the back (Fig.
244 _r_); it may be compared to either the dorsal vessel of the
Articulates or the aorta of the Vertebrates. To the right and left are
the two serpentine lateral vessels (Fig. 244 _l_).


Fig.246. Transverse section of the branchial gut. Fig. 246—Transverse
section of the branchial gut. _A_ of Balanoglossus, _B_ of Ascidia. _r_
branchial gut, _n_ pharyngeal groove, * ventral folds between the two.
Diagrammatic illustration from _Gegenbaur,_ to show the relation of the
dorsal branchial-gut cavity ( _r_) to the pharyngeal or hypobranchial
groove ( _n_).


After the Nemertina, I take (as distant relatives) the _Enteropneusta_
; they may be classed together with them as _Frontonia_ or _Rhyncocœla_
(snout-worms). There is now only one genus of this class, with several
species ( _Balanoglossus_); but it is very remarkable, and may be
regarded as the last survivor of an ancient and long-extinct class of
Vermalia. They are related, on the one hand, to the Nemertina and their
immediate ancestors, the Platodes, and to the lowest and oldest forms
of the Chordonia on the other.

The Enteropneusta (Fig. 245) live in the sea sand, and are long worms
of very simple shape, like the Nemertina. From the latter they have
inherited: (1) The bilateral type, with incomplete segmentation; (2)
the ciliary coat of the soft epidermis; (3) the double rows of gastric
pouches, alternating with a single or double row of gonads; (4)
separation of the sexes (the Platode ancestors were hermaphroditic);
(5) the ventral mouth, underneath a protruding snout; (6) the anus
terminating the simple gut-tube; and (7) several parallel blood-canals,
running the length of the body, a dorsal and a ventral principal stem.

On the other hand, the Enteropneusta differ from their Nemertine
ancestors in several features, some of which are important, that we may
attribute to adaptation. The chief of these is the branchial gut (Fig.
245 _k_). The anterior section of the gut is converted into a
respiratory organ, and pierced by two rows of gill-clefts; between
these there is a branchial (gill) skeleton, formed of rods and plates
of chitine. The water that enters at the mouth makes its exit by these
clefts. They lie in the dorsal half of the fore-gut, and this is
completely separated from the ventral half by two longitudinal folds
(Fig. 246 _A*_). This ventral half, the glandular walls of which are
clothed with ciliary epithelium and secrete mucus, corresponds to the
pharyngeal or hypo-branchial groove of the Chordonia ( _Bn_), the
important organ from which the later thyroid gland is developed in the
Craniota (cf. p. 184). The agreement in the structure of the branchial
gut of the Enteropneusts, Tunicates, and Vertebrates was first
recognised by Gegenbaur (1878); it is the more significant as at first
we find only a couple of gill-clefts in the young animals of all three
groups; the number gradually increases. We can infer from this the
common descent of the three groups with all the more
confidence when we find the _Balanoglossus_ approaching the Chordonia
in other respects. Thus, for instance, the chief part of the central
nervous system is a long dorsal neural string that runs above the gut
and corresponds to the medullary tube of the Chordonia. Bateson
believes he has detected a rudimentary chorda between the two.

Of all extant invertebrate animals the Enteropneusts come nearest to
the Chordonia in virtue of these peculiar characters; hence we may
regard them as the survivors of the ancient gut-breathing Vermalia from
which the Chordonia also have descended. Again, of all the
chorda-animals the Copelata (Fig. 225) and the tailed larvæ of the
ascidia approach nearest to the young _Balanoglossus._ Both are, on the
other hand, very closely related to the _Amphioxus,_ the Primitive
Vertebrate of which we have considered the importance (Chapters XVI and
XVII). As we saw there, the unarticulated Tunicates and the articulated
Vertebrates must be regarded as two independent stems, that have
developed in divergent directions. But the common root of the two
stems, the extinct group of the Prochordonia, must be sought in the
vermalia stem; and of all the living Vermalia those we have considered
give us the safest clue to their origin. It is true that the actual
representatives of the important groups of the Copelata, Balanoglossi,
Nemertina, Icthydina, etc., have more or less departed from the
primitive model owing to adaptation to special environment. But we may
just as confidently affirm that the main features of their organisation
have been preserved by heredity.

We must grant, however, that in the whole stem-history of the
Vertebrates the long stretch from the Gastræads and Platodes up to the
oldest Chordonia remains by far the most obscure section. We might
frame another hypothesis to raise the difficulty—namely, that there was
a long series of very different and totally extinct forms between the
Gastræa and the Chordæa. Even in this modified chordæa-theory the six
fundamental organs of the chordula would retain their great value. The
medullary tube would be originally a chemical sensory organ, a dorsal
olfactory tube, taking in respiratory-water and food by the neuroporus
in front and conveying them by the neurenteric canal into the primitive
gut. This olfactory tube would afterwards become the nervous centre,
while the expanding gonads (lying to right and left of the primitive
mouth) would form the cœloma. The chorda may have been originally a
digestive glandular groove in the dorsal middle line of the primitive
gut. The two secondary gut-openings, mouth and anus, may have arisen in
various ways by change of functions. In any case, we should ascribe the
same high value to the chordula as we did before to the gastrula.

In order to explain more fully the chief stages in the advance of our
race, I add the hypothetical sketch of man’s ancestry that I published
in my _Last Link_ [a translation by Dr. Gadow of the paper read at the
International Zoological Congress at Cambridge in 1898]:—


A.—Man’s Genealogical Tree, First Half:
EARLIER SERIES OF ANCESTORS, WITHOUT FOSSIL EVIDENCE.


 Chief Stages       Ancestral Stem-groups        Living Relatives of
 Ancestors Stages 1–5:
Protist ancestors Unicellular organisms.
1–2:

Prototypes 3–5: Protozoa       1. Monera

Without nucleus
 2. Algaria Unicellular algæ
         1. Chromacea _ (Chroococcus) Phycochromacea _ 2. Paulotomea _
         Palmellacea Eremosphæra _ 3. Lobosa Unicellular (amœbina)
rhizopods 4. Infusoria Unicellular
 5. Blastæades Multicellular hollow spheres 3. Amœbina _Amœba
 Leucocyta_


4. Flagellata _ Euflagellata Zoomonades _ 5. Catallacta _ Magosphæra,
Volvocina, Blastula _ Stages 6–11:
Invertebrate metazoa ancestors 6–8:
Cœlenteria without anus and body-cavity 9–11:
Vermalia, with anus and body-cavity       6. Gastræades With two
germ-layers
 7 Platodes I _Platodaria_ (without nephridia) 8. Platodes II
 _Platodinia_ (with nephridia)       6. Gastrula _ Hydra, Olynthus,
 Gastremaria _ 7. Cryptocœla _Convoluta, Porporus_
 8. Rhabdocœla _Vortex, Monolus_ 9. Provermalia (Primitive worms)
_Rotatoria_ 10. Frontonia _(Rhynchelminthes)_
 Snout-worms 11. Prochordonia Chorda-worms
 9. Gastrotricha _Trochozoa, Trochophora_
 10. Enteropneusta _ Balanglossus Cephalodiscus _ 11. Copelata
_Appendicaria_ Chordula-larvæ Stages 12–15: Monorhina
 ancestors Oldest vertebrates without jaws or pairs of limbs, single
 nose       12. Acrania I
 (Prospondylia) 13. Acrania II More recent 14. Cyclostoma I
 (Archicrania) 15. Cyclostoma II
More recent       12. Amphioxus larvæ
 13. Leptocardia Amphioxus 14. Petromyzonta larvæ

 15. Marsipobranchia Petromyzonta



B.—Man’s Genealogical Tree, Second Half:
LATER ANCESTORS, WITH FOSSIL EVIDENCE.


  Geological Periods        Ancestral Stem-groups        Living Relatives
  of Ancestors Silurian 16. Selachii Primitive fishes _Proselachii_
        16. Natidanides Chlamydoselachius Heptanchus Silurian 17.
  Ganoids Plated-fishes _Proganoids_       17. Accipenserides
  (Sturgeons) Polypterus Devonian       18. Dipneusta _Paladipneusta_
        18. Neodipneusta Ceratodus Proptopterus Carboniferous       19.
  Amphibia _Stegocephala_ 19. Phanerobranchia Salamandrina
(Proteus, triton) Permian       20. Reptilia _Proreptilia_       20.
Rhynchocephalia Primitive lizards
Hatteria Triassic       21. Monotrema _Promammalia_       21.
Ornithodelphia _ Echidna
Ornithorhyncus _ Jurassic       22. Marsupialia _Prodidelphia_       22.
Didelphia _ Didelphys Perameles _ Cretaceous       23. Mallotheria
_Prochoriata_       23. Insectivora Erinaceida (Ictopsia +) Older Eocene
      24. Lemuravida Older lemurs Dentition 3. 1. 4. 3.       24.
Pachylemures _ (Hyopsodus +)
 (Adapis +) _ Neo-Eocene 25. Lemurogona Later lemurs Dentition 2. 1. 4.
 3.       25. Autolemures _ Eulemur Stenops _ Oligocene 26. Dysmopitheca
 Western apes Dentition 2. 1. 3. 3.       26. Platyrrhinæ _ (Anthropops
 +)
(Homunculus +) _ Older Miocene       27. Cynopitheca Dog-faced apes
(tailed)       27. Papiomorpha _Cynocephalus_ Neo-Miocene       28.
Anthropoides Man-like apes (tail-less)       28. Hylobatida Hylobates
Satyrus Pliocene       29. Pithecanthropi Ape-men (alali, speechless)
29. Anthropitheca Chimpanzee
Gorilla Pleistocene       30. Homines Men with speech       30. Weddahs
Australian negroes



Chapter XXI.
OUR FISH-LIKE ANCESTORS


Our task of detecting the extinct ancestors of our race among the vast
numbers of animals known to us encounters very different difficulties
in the various sections of man’s stem-history. These were very great in
the series of our invertebrate ancestors; they are much slighter in the
subsequent series of our vertebrate ancestors. Within the vertebrate
stem there is, as we have already seen, so complete an agreement in
structure and embryology that it is impossible to doubt their
phylogenetic unity. In this case the evidence is much clearer and more
abundant.

The characteristics that distinguish the Vertebrates as a whole from
the Invertebrates have already been discussed in our description of the
hypothetical Primitive Vertebrate (Chapter XI, Figs. 98–102). The chief
of these are: (1) The evolution of the primitive brain into a dorsal
medullary tube; (2) the formation of the chorda between the medullary
tube and the gut; (3) the division of the gut into branchial (gill) and
hepatic (liver) gut; and (4) the internal articulation or metamerism.
The first three features are shared by the Vertebrates with the
ascidia-larvæ and the Prochordonia; the fourth is peculiar to them.
Thus the chief advantage in organisation by which the earliest
Vertebrates took precedence of the unsegmented Chordonia consisted in
the development of internal segmentation.

The whole vertebrate stem divides first into the two chief sections of
Acrania and Craniota. The Amphioxus is the only surviving
representative of the older and lower section, the Acrania
(“skull-less”). All the other vertebrates belong to the second
division, the Craniota (“skull-animals”). The Craniota descend directly
from the Acrania, and these from the primitive Chordonia. The
exhaustive study that we made of the comparative anatomy and ontogeny
of the Ascidia and the Amphioxus has proved these relations for us.
(See Chapters XVI and XVII.) The Amphioxus, the lowest Vertebrate, and
the Ascidia, the nearest related Invertebrate, descend from a common
extinct stem-form, the Chordæa; and this must have had, substantially,
the organisation of the chordula.

However, the Amphioxus is important not merely because it fills the
deep gulf between the Invertebrates and Vertebrates, but also because
it shows us to-day the typical vertebrate in all its simplicity. We owe
to it the most important data that we proceed on in reconstructing the
gradual historical development of the whole stem. All the Craniota
descend from a common stem-form, and this was substantially identical
in structure with the Amphioxus. This stem-form, the Primitive
Vertebrate (_Prospondylus,_ Figs. 98–102), had the characteristics of
the vertebrate as such, but not the important features that distinguish
the Craniota from the Acrania. Though the Amphioxus has many
peculiarities of structure and has much degenerated, and though it
cannot be regarded as an unchanged descendant of the Primitive
Vertebrate, it must have inherited from it the specific characters we
enumerated above. We may not say that “Amphioxus is the ancestor of the
Vertebrates”; but we can say: “Amphioxus is the nearest relation to the
ancestor of all the animals we know.” Both belong to the same small
family, or lowest class of the Vertebrates, that we call the Acrania.
In our genealogical tree this group forms the twelfth stage, or the
first stage among the vertebrate ancestors (p. 228). From this group of
Acrania both the Amphioxus and the Craniota were evolved.

The vast division of the Craniota embraces all the Vertebrates known to
us, with the exception of the Amphioxus. All of them have a head
clearly differentiated from the trunk, and a skull enclosing a brain.
The head has also three pairs of higher sense-organs (nose, eyes, and
ears). The brain is very rudimentary at first, a mere bulbous
enlargement of the
fore end of the medullary tube. But it is soon divided by a number of
transverse constrictions into, first three, then five successive
cerebral vesicles. In this formation of the head, skull, and brain,
with further development of the higher sense-organs, we have the
advance that the Craniota made beyond their skull-less ancestors. Other
organs also attained a higher development; they acquired a compact
centralised heart with valves and a more advanced liver and kidneys,
and made progress in other important respects.


Fig.247. The large marine lamprey (Petromyzon marinus). Fig. 247—The
large marine lamprey _(Petromyzon marinus),_ much reduced. Behind the
eye there is a row of seven gill-clefts visible on the left, in front
the round suctorial mouth.


We may divide the Craniota generally into _Cyclostoma_
(“round-mouthed”) and _Gnathostoma_ (“jaw-mouthed”). There are only a
few groups of the former in existence now, but they are very
interesting, because in their whole structure they stand midway between
the Acrania and the Gnathostoma. They are much more advanced than the
Acrania, much less so than the fishes, and thus form a very welcome
connecting-link between the two groups. We may therefore consider them
a special intermediate group, the fourteenth and fifteenth stages in
the series of our ancestors.

The few surviving species of the Cyclostoma are divided into two
orders—the _Myxinoides_ and the _Petromyzontes._ The former, the
hag-fishes, have a long, cylindrical, worm-like body. They were classed
by Linné with the worms, and by later zoologists, with the fishes, or
the amphibia, or the molluscs. They live in the sea, usually as
parasites of fishes, into the skin of which they bore with their round
suctorial mouths and their tongues, armed with horny teeth. They are
sometimes found alive in the body cavity of fishes (such as the torsk
or sturgeon); in these cases they have passed through the skin into the
interior. The second order consists of the Petromyzontes or lampreys;
the small river lamprey (_Petromyzon fluviatilis_) and the large marine
lamprey (_Petromyzon marinus,_ Fig. 247). They also have a round
suctorial mouth, with horny teeth inside it; by means of this they
attach themselves by sucking to fishes, stones, and other objects
(hence the name _Petromyzon_ = stone-sucker). It seems that this habit
was very widespread among the earlier Vertebrates; the larvæ of many of
the Ganoids and frogs have suctorial disks near the mouth.

The class that is formed of the Myxinoides and Petromyzontes is called
the Cyclostoma (round-mouthed), because their mouth has a circular or
semi-circular aperture. The jaws (upper and lower) that we find in all
the higher Vertebrates are completely wanting in the Cyclostoma, as in
the Amphioxus. Hence the other Vertebrates are collectively opposed to
them as Gnathostoma (jaw-mouthed). The Cyclostoma might also be called
_Monorhina_ (single-nosed), because they have only a single nasal
passage, while all the Gnathostoma have two nostrils (_Amphirhina_ =
double-nosed). But apart from these peculiarities the Cyclostoma differ
more widely from the fishes in other special features of their
structure than the fishes do from man. Hence they are obviously the
last survivors of a very ancient class of Vertebrates, that was far
from attaining the advanced organisation of the true fish. To mention
only the chief points, the Cyclostoma show no trace of pairs of limbs.
Their mucous skin is quite naked and smooth and devoid of scales. There
is no bony skeleton. A very rudimentary skull is developed at the
foremost end of their chorda. At this point a soft membranous (partly
turning into cartilage), small skull-capsule is formed, and encloses
the brain.


Fig.248. Fossil Permian primitive fish (Pleuracanthus Dechenii), from
the red sandstone of Saarbrücken. Fig. 248—Fossil Permian primitive
fish _(Pleuracanthus Dechenii),_ from the red sandstone of Saarbrücken.
(From _Döderlein._) _I_ Skull and branchial skeleton: _o_ eye-region,
_pq_ palatoquadratum, _nd_ lower jaw, _hm_ hyomandibular, _hy_
tongue-bone, _k_ gill-radii, _kb_ gill-arches, _z_ jaw-teeth, _sz_
gullet-teeth, _st_ neck-spine. _II_ Vertebral column: _ob_ upper
arches, _ub_ lower arches, _hc_ intercentra, _r_ ribs. _III_ Single
fins: _d_ dorsal fin, _c_ tail-fin (tail-end wanting), _an_ anus-fin,
_ft_ supporter of fin-rays. _IV_ Breast-fin: _sg_ shoulder-zone, _ax_
fin-axis, _ss_ double lines of fin-rays, _bs_ additional rays, _sch_
plates. _V_ Ventral fin: _p_ pelvis, _ax_ fin-axis, _ss_ single row of
fin-rays, _bs_ additional rays, _sch_ scales, _cop_ penis.


The brain of the Cyclostoma is merely a very small and comparatively
insignificant swelling of the spinal marrow, a simple vesicle at first.
It afterwards divides into five successive cerebral vesicles, like the
brain of the Gnathostoma. These five primitive cerebral vesicles, that
are found in the embryos of all the higher vertebrates from the fishes
to man, and grow into very complex structures, remain at a very
rudimentary stage in the Cyclostoma. The histological structure of the
nerves is also less advanced than in the rest of the vertebrates. In
these the auscultory organ always contains three circular canals, but
in the lampreys there are only two, and in the hag-fishes only one. In
most other respects the organisation of the Cyclostoma is much
simpler—for instance, in the structure of the heart, circulation, and
kidneys. We must especially note the absence of a very important organ
that we find in the fishes, the floating-bladder, from which the lungs
of the higher Vertebrates have been developed.

When we consider all these peculiarities in the structure of the
Cyclostoma, we may formulate the following thesis: Two divergent lines
proceeded from the earliest Craniota, or the primitive Craniota
(_Archicrania_). One of these lines is preserved in a greatly modified
condition: these are the Cyclostoma, a very backward and partly
degenerate side-line. The other, the chief line of the Vertebrate stem,
advanced straight to the fishes, and by fresh adaptations acquired a
number of important improvements.

The Cyclostoma are almost always classified by zoologists among the
fishes; but the incorrectness of this may be judged from the fact that
in all the chief and distinctive features of organisation they are
further removed from the fishes than the fishes are from the Mammals,
and even man. With the fishes we enter upon the vast division of the
jaw-mouthed
or double-nosed Vertebrates (_Gnathostoma_ or _Amphirhina_). We have to
consider the fishes carefully as the class which, on the evidence of
palæontology, comparative anatomy, and ontogeny, may be regarded with
absolute certainty as the stem-class of all the higher Vertebrates or
Gnathostomes. Naturally, none of the actual fishes can be considered
the direct ancestor of the higher Vertebrates. But it is certain that
all the Vertebrates or Gnathostomes, from the fishes to man, descend
from a common, extinct, fish-like ancestor. If we had this ancient
stem-form before us, we would undoubtedly class it as a true fish.
Fortunately the comparative anatomy and classification of the fishes
are now so far advanced that we can get a very clear idea of these
interesting and instructive features.


Fig.249. Embryo of a shark (Scymnus lichia), seen from the ventral
side. Fig. 249—Embryo of a shark (_Scymnus lichia_), seen from the
ventral side. _v_ breast-fins (in front five pairs of gill-clefts), _h_
belly-fins, _a_ anus, _s_ tail-fin, _k_ external gill-tuft, _d_
yelk-sac (removed for most part), _g_ eye, _n_ nose, _m_ mouth-cleft.


In order to understand properly the genealogical tree of our race
within the vertebrate stem, it is important to bear in mind the
characteristics that separate the whole of the Gnathostomes from the
Cyclostomes and Craniota. In these respects the fishes agree entirely
with all the other Gnathostomes up to man, and it is on this that we
base our claim of relationship to the fishes. The following
characteristics of the Gnathostomes are anatomic features of this kind:
(1) The internal gill-arch apparatus with the jaw arches; (2) the pair
of nostrils; (3) the floating bladder or lungs; and (4) the two pairs
of limbs.

The peculiar formation of the frame work of the branchial (gill) arches
and the connected maxillary (jaw) apparatus is of importance in the
whole group of the Gnathostomes. It is inherited in rudimentary form by
all of them, from the earliest fishes to man. It is true that the
primitive transformation (which we find even in the Ascidia) of the
fore gut into the branchial gut can be traced in all the Vertebrates to
the same simple type; in this respect the gill-clefts, which pierce the
walls of the branchial gut in all the Vertebrates and in the Ascidia,
are very characteristic. But the _external,_ superficial branchial
skeleton that supports the gill-crate in the Cyclostoma is replaced in
the Gnathostomes by an _internal_ branchial skeleton. It consists of a
number of successive cartilaginous arches, which lie in the wall of the
gullet between the gill-clefts, and run round the gullet from both
sides. The foremost pair of gill-arches become the maxillary arches,
from which we get our upper and lower jaws.

The olfactory organs are at first found in the same form in all the
Gnathostomes, as a pair of depressions in the fore part of the skin of
the head, above the mouth; hence, they are also called the Amphirhina
(“double-nosed”). The Cyclostoma are “one-nosed” (_Monorhina_); their
nose is a single passage in the middle of the frontal surface. But as
the olfactory nerve is double in both cases, it is possible that the
peculiar form of the nose in the actual Cyclostomes is a secondary
acquisition (by adaptation to suctorial habits).


Fig.250. Fully-developed man-eating shark (Carcharias melanopterus),
left view. Fig. 250—Fully developed man-eating shark (_Carcharias
melanopterus_), left view. _r1_ first, _r2_ second dorsal fin, _s_
tail-fin, _a_ anus-fin, _v_ breast-fins, _h_ belly-fins.)


A third essential character of the Gnathostomes, that distinguishes
them very conspicuously from the lower vertebrates we have dealt with,
is the formation of a blind sac by invagination from the fore part of
the gut, which becomes in the fishes the air-filled floating-bladder.
This organ acts as a hydrostatic apparatus, increasing or reducing the
specific gravity of the fish by compressing or altering the quantity of
air in it. The fish can rise or sink in the water by means of it. This
is the organ from which the lungs of the higher vertebrates are
developed.

Finally, the fourth character of the Gnathostomes in their simple
embryonic form is the two pairs of extremities or limbs—a pair of fore
legs (breast-fins in the fish, Fig. 250 _v_) and a pair of hind legs
(ventral fins in the fish, Fig. 250 _h_). The comparative anatomy of
these fins is very interesting, because they contain the rudiments of
all the skeletal parts that form the framework of the fore and hind
legs in all the higher vertebrates right up to man. There is no trace
of these pairs of limbs in the Acrania and Cyclostomes.

Turning, now, to a closer inspection of the fish class, we may first
divide it into three groups or sub-classes, the genealogy of which is
well known to us. The first and oldest group is the sub-class of the
_Selachii_ or primitive fishes; the best-known representatives of which
to-day are the orders of the sharks and rays (Figs. 248–252). Next to
this is the more advanced sub-class of the plated fishes or _Ganoids_
(Figs. 253–5). It has been long extinct for the most part, and has very
few living representatives, such as the sturgeon and the bony pike; but
we can form some idea of the earlier extent of this interesting group
from the large numbers of fossils. From these plated fishes the
sub-class of the bony fishes
or _Teleostei_ was developed, to which the great majority of living
fishes belong (especially nearly all our river fishes). Comparative
anatomy and ontogeny show clearly that the Ganoids descended from the
Selachii, and the Teleostei from the Ganoids. On the other hand, a
collateral line, or rather the advancing chief line of the vertebrate
stem, was developed from the earlier Ganoids, and this leads us through
the group of the Dipneusta to the important division of the Amphibia.


Fig.251. Fossil angel-shark (Squatina alifera) from the upper Jurassic
at Eichstätt. Fig. 251—Fossil angel-shark (_Squatina alifera_), from
the upper Jurassic at Eichstätt. (From _Zittel._) The cartilaginous
skull is clearly seen in the broad head, and the gill-arches behind.
The wide breast-fin and the narrower belly-fin have a number of radii;
between these and the vertebral column are a number of ribs.


The earliest fossil remains of Vertebrates that we know were found in
the Upper Silurian (p. 201), and belong to two groups—the Selachii and
the Ganoids. The most primitive of all known representatives of the
earliest fishes are probably the remarkable _Pleuracanthida,_ the
genera _Pleuracanthus, Xenacanthus, Orthocanthus,_ etc. (Fig. 248).
These ancient cartilaginous fishes agree in most points of structure
with the real sharks (Figs. 249, 250); but in other respects they seem
to be so much simpler in organisation that many palæontologists
separate them altogether, and regard them as _Proselachii_; they are
probably closely related to the extinct ancestors of the Gnathostomes.
We find well-preserved remains of them in the Permian period.
Well-preserved impressions of other sharks are found in the Jurassic
schist, such as of the angel-fish (_Squatina,_ Fig. 251). Among the
extinct earlier sharks of the Tertiary period there were some twice as
large as the biggest living fishes; _Carcharodon_ was more than 100
feet long. The sole surviving species of this genus (_C. Rondeleti_) is
eleven yards long, and has teeth two inches long; but among the fossil
species we find teeth six inches long (Fig. 252).

From the primitive fishes or Selachii, the earliest Gnathostomes, was
developed the legion of the Ganoids. There are very few genera now of
this interesting and varied group—the ancient sturgeons (_Accipenser_),
the eggs of which are eaten as caviare, and the stratified pikes
(_Polypterus,_ Fig. 255) in African rivers, and bony pikes
(_Lepidosteus_) in the rivers of North America. On the other hand, we
have a great variety of specimens of this group in the fossil state,
from the Upper Silurian onward. Some of these fossil Ganoids approach
closely to the Selachii; others are nearer to the Dipneusts; others
again represent a transition to the Teleostei. For our genealogical
purposes the most interesting are the intermediate forms between the
Selachii and the Dipneusts. Huxley, to whom we owe particularly
important works on the fossil Ganoids, classed them in the order of the
_Crossopterygii._ Many genera and species of this order are found in
the Devonian and Carboniferous strata (Fig. 253); a single, greatly
modified survivor of the group is still found in the large rivers of
Africa (_Polypterus,_ Fig. 255, and the closely related
_Calamichthys_). In many impressions of the Crossopterygii the floating
bladder seems to be ossified,
and therefore well preserved—for instance, in the _Undina_ (Fig. 254,
immediately behind the head).

Part of these Crossopterygii approach very closely in their chief
anatomic features to the Dipneusts, and thus represent phylogenetically
the transition from the Devonian Ganoids to the earliest air-breathing
vertebrates. This important advance was made in the Devonian period.
The numerous fossils that we have from the first two geological
sections, the Laurentian and Cambrian periods, belong exclusively to
aquatic plants and animals. From this paleontological fact, in
conjunction with important geological and biological indications, we
may infer with some confidence that there were no terrestrial animals
at that time. During the whole of the vast archeozoic period—many
millions of years—the living population of our planet consisted almost
exclusively of aquatic organisms; this is a very remarkable fact, when
we remember that this period embraces the larger half of the whole
history of life. The lower animal-stems are wholly (or with very few
exceptions) aquatic. But the higher stems also remained in the water
during the primordial epoch. It was only towards its close that some of
them came to live on land. We find isolated fossil remains of
terrestrial animals first in the Upper Silurian, and in larger numbers
in the Devonian strata, which were deposited at the beginning of the
second chief section of geology (the paleozoic age). The number
increases considerably in the Carboniferous and Permian deposits. We
find many species both of the articulate and the vertebrate stem that
lived on land and breathed the atmosphere; their aquatic ancestors of
the Silurian period only breathed water. This important change in
respiration is the chief modification that the animal organism
underwent in passing from the water to the solid land. The first
consequence was the formation of lungs for breathing air; up to that
time the gills alone had served for respiration. But there was at the
same time a great change in the circulation and its organs; these are
always very closely correlated to the respiratory organs. Moreover, the
limbs and other organs were also more or less modified, either in
consequence of remote correlation to the preceding or owing to new
adaptations.


Fig.252. Tooth of a gigantic shark (Carcharodon megalodon), from the
Pliocene at Malta. Fig. 252—Tooth of a gigantic shark (_Carcharodon
megalodon_), from the Pliocene at Malta. (From _Zittel._)


In the vertebrate stem it was unquestionably a branch of the fishes—in
fact, of the Ganoids—that made the first fortunate experiment during
the Devonian period of adapting themselves to terrestrial life and
breathing the atmosphere. This led to a modification of the heart and
the nose. The true fishes have merely a pair of blind olfactory pits on
the surface of the head; but a connection of these with the cavity of
the mouth was now formed. A canal made its appearance on each side, and
led directly from the nasal depression into the mouth-cavity, thus
conveying atmospheric air to the lungs even when the mouth was closed.
Further, in all true fishes the heart has only two sections—an atrium
that receives the venous blood from the veins, and a ventricle that
propels it through a conical artery to the gills; the atrium was now
divided into two halves, or right and left auricles, by an incomplete
partition. The right auricle alone now received the venous blood from
the body, while the left auricle received the venous blood that flowed
from the lungs and gills to the heart. Thus the double circulation of
the higher vertebrates was evolved from the simple
circulation of the true fishes, and, in accordance with the laws of
correlation, this advance led to others in the structure of other
organs.


Fig.253. A Devonian Crossopterygius (Holoptychius nobilissimus), from
the Scotch old red sandstone. Fig. 254. A Jurassic Crossopterygius
(Undina penicillata), from the upper Jurassic at Eichstätt. Fig. 255. A
living Crossopterygius, from the Upper Nile. Fig. 253—A Devonian
Crossopterygius (_Holoptychius nobilissimus_), from the Scotch old red
sandstone. (From _Huxley._) Fig. 254.—A Jurassic Crossopterygius
(_Undina penicillata_), from the upper Jurassic at Eichstätt. (From
_Zittel._) _j_ jugular plates, _b_ three ribbed scales.
Fig. 255—A living Crossopterygius, from the Upper Nile ((Polypterus
bichir).


The vertebrate class, that thus adapted itself to breathing the
atmosphere, and was developed from a branch of the Ganoids, takes the
name of the _Dipneusts_ or _Dipnoa_ (“double-breathers”), because they
retained the earlier gill-respiration along with the new pulmonary
(lung) respiration, like the lowest amphibia. This class was
represented during the paleozoic age (or the Devonian, Carboniferous,
and Permian periods) by a number of different genera. There are only
three genera of the class living to-day: _Protopterus annectens_ in the
rivers
of tropical Africa (the White Nile, the Niger, Quelliman, etc.),
_Lepidosiren paradoxa_ in tropical South America (in the tributaries of
the Amazon), and _Ceratodus Forsteri_ in the rivers of East Australia.
This wide distribution of the three isolated survivors proves that they
represent a group that was formerly very large. In their whole
structure they form a transition from the fishes to the amphibia. The
transitional formation between the two classes is so pronounced in the
whole organisation of these remarkable animals that zoologists had a
lively controversy over the question whether they were really fishes or
amphibia. Several distinguished zoologists classed them with the
amphibia, though most now associate them with the fishes. As a matter
of fact, the characters of the two classes are so far united in the
Dipneusts that the answer to the question depends entirely on the
definition we give of “fish” and “amphibian.” In habits they are true
amphibia. During the tropical winter, in the rainy season, they swim in
the water like the fishes, and breathe water by gills. During the dry
season they bury themselves in the dry mud, and breathe the atmosphere
through lungs, like the amphibia and the higher vertebrates. In this
double respiration they resemble the lower amphibia, and have the same
characteristic formation of the heart; in this they are much superior
to the fishes. But in most other features
they approach nearer to the fishes, and are inferior to the amphibia.
Externally they are entirely fish-like.


Fig.256. Fossil Dipneust (Dipterus Valenciennesi), from the old red
sandstone (Devon). Fig. 257. The Australian Dipneust (Ceratodus
Forsteri). Fig. 256—Fossil Dipneust (_Dipterus Valenciennesi_), from
the old red sandstone (Devon). (From _Pander._) Fig. 257—The Australian
Dipneust (_Ceratodus Forsteri_). _B_ view from the right, _A_ lower
side of the skull, _C_ lower jaw. (From _Gunther._) _Qu_ quadrate bone,
_Psph_ parasphenoid, _Pt P_ pterygopalatinum, _Vo_ vomer, _d_ teeth,
_na_ nostrils, _Br_ branchial cavity, _C_ first rib. _D_ lower-jaw
teeth of the fossil _Ceratodus Kaupi_ (from the Triassic).


In the Dipneusts the head is not marked off from the trunk. The skin is
covered with large scales. The skeleton is soft, cartilaginous, and at
a low stage of development, as in the lower Selachii and the earliest
Ganoids. The chorda is completely retained, and surrounded by an
unsegmented sheath. The two pairs of limbs are very simple fins of a
primitive type, like those of the lowest Selachii. The formation of the
brain, the gut, and the sexual organs is also the same as in the
Selachii. Thus the Dipneusts have preserved by heredity many of the
less advanced features of our primitive fish-like ancestors, and at the
same time have made a great step forward in adaptation to air-breathing
by means of lungs and the correlative improvement of the heart.


Fig.258. Young ceratodus, shortly after issuing from the egg. Fig. 259.
Young ceratodus six weeks after issuing from the egg. Fig. 258—Young
ceratodus, shortly after issuing from the egg, magnified. _k_
gill-cover, _l_ liver. (From _Richard Semon._) Fig. 259—Young ceratodus
six weeks after issuing from the egg. _s_ spiral fold of gut, _b_
rudimentary belly-fin. (From _Richard Semon._)


Ceratodus is particularly interesting on account of the primitive build
of its skeleton; the cartilaginous skeleton of its two pairs of fins,
for instance, has still the original form of a bi-serial or feathered
leaf, and was on that account described by Gegenbaur as a “primitive
fin-skeleton.” On the other hand, the skeleton of the pairs of fins is
greatly reduced in the African dipneust (_Protopterus_) and the
American (_Lepidosiren_). Further, the lungs are double in these modern
dipneusts, as in all the other air-breathing vertebrates; they have on
that account been called “double-lunged” (_Dipneumones_) in contrast to
the Ceratodus; the latter has only a single lung (_Monopneumones_). At
the same time the gills also are developed as water-breathing organs in
all these lung-fishes. Protopterus has external as well as internal
gills.

The paleozoic Dipneusts that are in the direct line of our ancestry,
and form the connecting-bridge between the Ganoids and the Amphibia,
differ in many respects
from their living descendants, but agree with them in the above
essential features. This is confirmed by a number of interesting facts
that have lately come to our knowledge in connection with the embryonic
development of the Ceratodus and Lepidosiren; they give us important
information as to the stem-history of the lower Vertebrates, and
therefore of our early ancestors of the paleozoic age.



Chapter XXII.
OUR FIVE-TOED ANCESTORS


With the phylogenetic study of the four higher classes of Vertebrates,
which must now engage our attention, we reach much firmer ground and
more light in the construction of our genealogy than we have, perhaps,
enjoyed up to the present. In the first place, we owe a number of very
valuable data to the very interesting class of Vertebrates that come
next to the Dipneusts and have been developed from them—the Amphibia.
To this group belong the salamander, the frog, and the toad. In earlier
days all the reptiles were, on the example of Linne, classed with the
Amphibia (lizards, serpents, crocodiles, and tortoises). But the
reptiles are much more advanced than the Amphibia, and are nearer to
the birds in the chief points of their structure. The true Amphibia are
nearer to the Dipneusta and the fishes; they are also much older than
the reptiles. There were plenty of highly-developed (and sometimes
large) Amphibia during the Carboniferous period; but the earliest
reptiles are only found in the Permian period. It is probable that the
Amphibia were evolved even earlier—during the Devonian period—from the
Dipneusta. The extinct Amphibia of which we have fossil remains from
that remote period (very numerous especially in the Triassic strata)
were distinguished for a graceful scaly coat or a powerful bony armour
on the skin (like the crocodile), whereas the living amphibia have
usually a smooth and slippery skin.

The earliest of these armoured Amphibia (_Phractamphibia_) form the
order of _Stegocephala_ (“roof-headed”) (Fig. 260). It is among these,
and not among the actual Amphibia, that we must look for the forms that
are directly related to the genealogy of our race, and are the
ancestors of the three higher classes of Vertebrates. But even the
existing Amphibia have such important relations to us in their anatomic
structure, and especially their embryonic development, that we may say:
Between the Dipneusts and the Amniotes there was a series of extinct
intermediate forms which we should certainly class with the Amphibia if
we had them before us. In their whole organisation even the actual
Amphibia seem to be an instructive transitional group. In the important
respects of respiration and circulation they approach very closely to
the Dipneusta, though in other respects they are far superior to them.

This is particularly true of the development of their limbs or
extremities. In them we find these for the first time as five-toed
feet. The thorough investigations of Gegenbaur have shown that the
fish’s fins, of which very erroneous opinions were formerly held, are
many-toed feet. The various cartilaginous or bony radii that are found
in large numbers in each fin correspond to the fingers or toes of the
higher Vertebrates. The several joints of each fin-radius correspond to
the various parts of the toe. Even in the Dipneusta the fin is of the
same construction as in the fishes; it was afterwards gradually evolved
into the five-toed form, which we first encounter in the Amphibia. This
reduction of the number of the toes to six, and then to five, probably
took place in the second half of the Devonian period—at the latest, in
the subsequent Carboniferous period—in those Dipneusta which we regard
as the ancestors of the Amphibia. We have several fossil remains of
five-toed Amphibia from this period. There are numbers of fossil
impressions of them in the Triassic of Thuringia (_Chirotherium_).


The fact that the toes number five is of great importance, because they
have clearly been transmitted from the Amphibia to all the higher
Vertebrates. Man entirely resembles his amphibian ancestors in this
respect, and indeed in the whole structure of the bony skeleton of his
five-toed extremities. A careful comparison of the skeleton of the frog
with our own is enough to show this. It is well known that this
hereditary number of the toes has assumed a very great practical
importance from remote times; on it our whole system of enumeration
(the decimal system applied to measurement of time, mass, weight, etc.)
is based. There is absolutely no reason why there should be five toes
in the fore and hind feet in the lowest Amphibia, the reptiles, and the
higher Vertebrates, unless we
ascribe it to inheritance from a common stem-form. Heredity alone can
explain it. It is true that we find less than five toes in many of the
Amphibia and of the higher Vertebrates. But in all these cases we can
prove that some of the toes atrophied, and were in time lost
altogether.


Fig.260. Fossil amphibian from the Permian, found in the Plauen terrain
near Dresden (Branchiosaurus amblystomus). Fig. 260—Fossil amphibian
from the Permian, found in the Plauen terrain near Dresden
(_Branchiosaurus amblystomus_). (From _Credner._) _A_ skeleton of a
young larva. _B_ larva, restored, with gills. _C_ the adult form.)


The causes of this evolution of the five-toed foot from the many-toed
fin in the amphibian ancestor must be sought in adaptation to the
entire change of function that the limbs experienced in passing from an
exclusively aquatic to a partly terrestrial life. The many-toed fin had
been used almost solely for motion in the water; it had now also to
support the body in creeping on the solid ground. This led to a
modification both of the skeleton and the muscles of the limbs. The
number of the fin-radii was gradually reduced, and sank finally to
five. But these five remaining radii became much stronger. The soft
cartilaginous radii became bony rods. The rest of the skeleton was
similarly strengthened. Thus from the one-armed lever of the many-toed
fish-fin arose the improved many-armed lever system of the five-toed
amphibian limbs. The movements of the body gained in variety as well as
in strength. The various parts of the skeletal system and correlated
muscular system began to differentiate more and more. In view of the
close correlation of the muscular and nervous systems, this also made
great advance in structure and function. Hence we find, as a matter of
fact, that the brain is much more developed in the higher Amphibia than
in the fishes, the Dipneusta, and the lower Amphibia.


Fig.261. Larva of the Spotted Salamander (Salamandra maculata), seen
from the ventral side. Fig. 261—Larva of the Spotted Salamander
(_Salamandra maculata_), seen from the ventral side. In the centre a
yelk-sac still hangs from the gut. The external gills are gracefully
ramified. The two pairs of legs are still very small.


The first advance in organisation that was occasioned by the adoption
of life on land was naturally the construction of an organ for
breathing air—a lung. This was formed directly from the
floating-bladder inherited from the fishes. At first its function was
insignificant beside that of the gills, the older organ for
water-respiration. Hence we find in the lowest Amphibia, the gilled
Amphibia, that, like the Dipneusta, they pass the greater part of their
life in the water, and breathe water through gills. They only come to
the surface at brief intervals, or creep on to the land, and then
breathe air by their lungs. But some of the tailed Amphibia—the
salamanders—remain entirely in the water when they are young, and
afterwards spend most of their time on land. In the adult state they
only breathe air through lungs. The same applies to the most advanced
of the Amphibia, the Batrachia (frogs and toads); some of them have
entirely lost the gill-bearing larva form.[30] This is also the case
with certain small, serpentine Amphibia, the Cæcilia (which live in the
ground like earth-worms).

 [30] The tree-frog of Martinique (_Hylades martinicensis_) loses the
 gills on the seventh, and the tail and yelk-sac on the eighth, day of
 fœtal life. On the ninth or tenth day after fecundation the frog
 emerges from the egg.


The great interest of the natural history of the Amphibia consists
especially in their intermediate position between the lower and higher
Vertebrates. The lower Amphibia approach very closely to the Dipneusta
in their whole organisation, live mainly in the water, and breathe by
gills; but the higher Amphibia are just as close to the Amniotes, live
mainly on land, and breathe by lungs. But in their younger state the
latter resemble the former, and only reach the higher stage by a
complete metamorphosis. The embryonic development of most of the
higher Amphibia still faithfully reproduces the stem-history of the
whole class, and the various stages of the advance that was made by the
lower Vertebrates in passing from aquatic to terrestrial life during
the Devonian or the Carboniferous period are repeated in the spring by
every frog that develops from an egg in our ponds.


Fig.262. Larva of the common grass-frog (Rana temporaria), or
“tadpole.” Fig. 262—Larva of the common grass-frog (_Rana temporaria_),
or “tadpole.” _m_ mouth, _n_ a pair of suckers for fastening on to
stones, _d_ skin-fold from which the gill-cover develops; behind it the
gill-clefts, from which the branching gills (_k_) protrude, _s_
tail-muscles, _f_ cutaneous fin-fringe of the tail.


The common frog leaves the egg in the shape of a larva, like the tailed
salamander (Fig. 261), and this is altogether different from the mature
frog (Fig. 262). The short trunk ends in a long tail, with the form and
structure of a fish’s tail (_s_). There are no limbs at first. The
respiration is exclusively branchial, first through external (_k_) and
then internal gills. In harmony with this the heart has the same
structure as in the fish, and consists of two sections—an atrium that
receives the venous blood from the body, and a ventricle that forces it
through the arteries into the gills.

We find the larvæ of the frog (or tadpoles, _Gyrini_) in great numbers
in our ponds every spring in this fish-form, using their muscular tails
in swimming, just like the fishes and young Ascidia. When they have
reached a certain size, the remarkable metamorphosis from the fish-form
to the frog begins. A blind sac grows out of the gullet, and expands
into a couple of spacious sacs: these are the lungs. The simple chamber
of the heart is divided into two sections by the development of a
partition, and there are at the same time considerable changes in the
structure of the chief arteries. Previously all the blood went from the
auricle through the aortic arches into the gills, but now only part of
it goes to the gills, the other part passing to the lungs through the
new-formed pulmonary artery. From this point arterial blood returns to
the left auricle of the heart, while the venous blood gathers in the
right auricle. As both auricles open into a single ventricle, this
contains mixed blood. The dipneust form has now succeeded to the
fish-form. In the further course of the metamorphosis the gills and the
branchial vessels entirely disappear, and the respiration becomes
exclusively pulmonary. Later, the long swimming tail is lost, and the
frog now hops to the land with the legs that have grown meantime.

This remarkable metamorphosis of the Amphibia is very instructive in
connection with our human genealogy, and is particularly interesting
from the fact that the various groups of actual Amphibia have remained
at different stages of their stem-history, in harmony with the
biogenetic law. We have first of all a very low order of Amphibia—the
_Sozobranchia_ (“gilled-amphibia”), which retain their gills throughout
life, like the fishes. In a second order of the salamanders the gills
are lost in the metamorphosis, and when fully grown they have only
pulmonary respiration. Some of the tailed Amphibia still retain the
gill-clefts in the side of the neck, though they have lost the gills
themselves (_Menopoma_). If we force the larvæ of our salamanders (Fig.
261) and tritons to remain in the water, and prevent them from reaching
the land, we can in favourable circumstances make them retain their
gills. In this fish-like condition they reach sexual maturity, and
remain throughout life at the lower stage of the gilled Amphibia.


fish-like axolotl (_Siredon pisciformis_). It was formerly regarded as
a permanent gilled amphibian persisting throughout life at the
fish-stage. But some of the hundreds of these animals that are kept in
the Botanical Garden at Paris got on to the land for some reason or
other, lost their gills, and changed into a form closely resembling the
salamander (_Amblystoma_). Other species of the genus became sexually
mature for the first time in this condition. This has been regarded as
an astounding phenomenon, although every common frog and salamander
repeats the metamorphosis in the spring. The whole change from the
aquatic and gill-breathing animal to the terrestrial lung-breathing
form may be followed step by step in this case. But what we see here in
the development of the individual has happened to the whole class in
the course of its stem-history.


Fig.263. Fossil mailed amphibian, from the Bohemian Carboniferous
(Seeleya). Fig. 263—Fossil mailed amphibian, from the Bohemian
Carboniferous (_Seeleya_). (From _Fritsch._) The scaly coat is retained
on the left.


The metamorphosis goes farther in a third order of Amphibia, the
_Batrachia_ or _Anura,_ than in the salamander. To this belong the
various kinds of toads, ringed snakes, water-frogs, tree-frogs, etc.
These lose, not only the gills, but also (sooner or later) the tail,
during metamorphosis.

The ontogenetic loss of the gills and the tail in the frog and toad can
only be explained on the assumption that they are descended from
long-tailed Amphibia of the salamander type. This is also clear from
the comparative anatomy of the two groups. This remarkable
metamorphosis is, however, also interesting because it throws a certain
light on the phylogeny of the tail-less apes and man. Their ancestors
also had long tails and gills like the gilled Amphibia, as the tail and
the gill-arches of the human embryo clearly show.

For comparative anatomical and ontogenetic reasons, we must not seek
these amphibian ancestors of ours—as one would be inclined to do,
perhaps—among the tail-less Batrachia, but among the tailed lower
Amphibia.

The vertebrate form that comes next to the Amphibia in the series of
our ancestors is a lizard-like animal, the earlier existence of which
can be confidently deduced from the facts of comparative anatomy and
ontogeny. The living _Hatteria_ of New Zealand (Fig. 264) and the
extinct _Rhyncocephala_ of the Permian period (Fig. 265) are closely
related to this important stem-form; we may call them the
_Protamniotes,_ or Primitive Amniotes. All the Vertebrates above the
Amphibia—or the three classes of reptiles, birds, and mammals—differ so
much in their whole organisation from all the lower Vertebrates we have
yet considered, and have so great a resemblance to each other, that we
put them all together in a single group with the title of _Amniotes._
In these three classes alone we find the remarkable embryonic membrane,
already mentioned, which we called the _amnion_; a cenogenetic
adaptation that we may regard as a result of the sinking of the growing
embryo into the yelk-sac.

All the Amniotes known to us—all reptiles, birds, and mammals
(including man)—agree in so many important points of internal structure
and development that their descent from a common ancestor can be
affirmed with tolerable certainty. If the evidence of comparative
anatomy and ontogeny is ever entirely beyond suspicion, it is certainly
the case here. All the peculiarities that accompany and follow the
formation of the amnion, and that we have learned in our consideration
of human embryology; all the peculiarities in the development of the
organs which we will presently follow in detail; finally, all the
principal special features of the internal structure of the full-grown
Amniotes—prove so clearly the common origin of all the Amniotes from
single extinct stem-form that it is difficult to entertain the idea of
their evolution from several independent stems. This unknown common
stem-form is our primitive Amniote (_Protamnion_). In outward
appearance it was probably something between the salamander and the
lizard.

It is very probable that some part of the Permian period was the age of
the origin of the Protamniotes. This follows from the fact that the
Amphibia are not fully developed until the Carboniferous period, and
that the first fossil reptiles (_Palæhatteria, Homœosaurus,
Proterosaurus_) are found towards the close of the Permian period.
Among the important changes of the vertebrate organisation that marked
the rise of the first Amniotes from salamandrine Amphibia during this
period the following three are especially noteworthy: the entire
disappearance of the water-breathing gills and the conversion of the
gill-arches into other organs, the formation of the allantois or
primitive urinary sac, and the development of the amnion.

One of the most salient characteristics of the Amniotes is the complete
loss of the gills. All Amniotes, even if living in water (such as
sea-serpents and whales), breathe air through lungs, never water
through gills. All the Amphibia (with very rare exceptions) retain
their gills for some time when young, and have for a time (if not
permanently) branchial respiration; but after these there is no
question of branchial respiration. The Protamniote itself must have
entirely abandoned water-breathing. Nevertheless, the gill-arches are
preserved by heredity, and develop into totally different (in part
rudimentary) organs—various parts of the bone of the tongue, the frame
of the jaws, the organ of hearing, etc. But we do not find in the
embryos of the Amniotes any trace of gill-leaves, or of real
respiratory organs on the gill-arches.

With this complete abandonment of the gills is probably connected the
formation of another organ, to which we have already referred in
embryology—namely, the allantois or primitive urinary sac (cf. p. 166).
It is very probable that the urinary bladder of the Dipneusts is the
first structure of the allantois. We find in these a urinary bladder
that proceeds from the lower wall of the hind end of the gut, and
serves as receptacle for the renal secretions. This organ has been
transmitted to the Amphibia, as we can see in the frog.

The formation of the amnion and the allantois and the complete
disappearance of the gills are the chief characteristics that
distinguish the Amniotes from the lower Vertebrates we have hitherto
considered. To these we may add several subordinate features that are
transmitted to all the Amniotes, and are found in these only. One
striking embryonic character of the Amniotes is the great curve of the
head and neck in the embryo. We also find an advance in the structure
of several of the internal organs of the Amniotes which raises them
above the highest of the anamnia. In particular, a partition is formed
in the simple ventricle of the heart, dividing into right and left
chambers. In connection with the complete metamorphosis of the
gill-arches we find a further development of the auscultory organs.
Also, there is a great advance in the structure of the brain, skeleton,
muscular system, and other parts. Finally, one of the most important
changes is the reconstruction of the kidneys. In all the earlier
Vertebrates we have found the primitive kidneys as excretory organs,
and these appear at an early stage in the embryos of all the higher
Vertebrates up to man. But in the Amniotes these primitive kidneys
cease to act at an early stage of embryonic life, and their function is
taken up by the permanent or secondary kidneys, which develop from the
terminal section of the prorenal ducts.

Taking all these peculiarities of the Amniotes together, it is
impossible to doubt that all the animals of this group—all reptiles,
birds, and mammals—have a common origin, and form a single
blood-related stem. Our own race belongs to this stem. Man is, in every
feature of his organisation and embryonic development, a true Amniote,
and has descended from the Protamniote with all the other
Amniotes. Though they appeared at the end (possibly even in the middle)
of the Paleozoic age, the Amniotes only reached their full development
during the Mesozoic age. The birds and mammals made their first
appearance during this period. Even the reptiles show their greatest
growth at this time, so that it is called “the reptile age.” The
extinct Protamniote, the ancestor of the whole group, belongs in its
whole organisation to the reptile class.


Fig.264. The lizard (Hatteria punctata = Sphenodon punctatus) of New
Zealand. Fig. 264—The lizard (_Hatteria punctata = Sphenodon
punctatus_) of New Zealand. The sole surviving proreptile. (From
_Brehm._)


The genealogical tree of the amniote group is clearly indicated in its
chief lines by their paleontology, comparative anatomy, and ontogeny.
The group succeeding the Protamniote divided into two branches. The
branch that will claim our whole interest is the class of the Mammals.
The other branch, which developed in a totally different direction, and
only comes in contact with the Mammals at its root, is the combined
group of the reptiles and birds; these two classes may, with Huxley, be
conveniently grouped together as the _Sauropsida._ Their common
stem-form is an extinct lizard-like reptile of the order of the
Rhyncocephalia. From this have been developed in various directions the
serpents, crocodiles, tortoises, etc.—in a word, all the members of the
reptile class. But the remarkable class of the birds has also been
evolved directly from a branch of the reptile group, as is now
established beyond question. The embryos of the reptiles and birds are
identical until a very late stage, and have an astonishing resemblance
even later. Their whole structure agrees so much that no anatomist now
questions the descent of the birds from the reptiles. On the other
hand, the mammal line has descended from the group of the
Sauromammalia, a different branch of the Proreptilia. It is connected
at its deepest roots with the reptile line, but it then diverges
completely from it and follows a distinctive development. Man is the
highest outcome of this class, the “crown of creation.” The hypothesis
that the three higher Vertebrate classes represent a single
Amniote-stem, and that the common root of this stem is to be found in
the amphibian class, is now generally admitted.


Fig.265. Homoeosaurus pulchellus, a Jurassic proreptile from Kehlheim.
Fig. 265—Homœosaurus pulchellus, a Jurassic proreptile from Kehlheim.
(From _Zittel._)


The instructive group of the Permian Tocosauria, the common root from
which the divergent stems of the Sauropsids and mammals have issued,
merits our particular attention as the stem-group of all the Amniotes.
Fortunately a living representative of this extinct ancestral group has
been preserved to our day; this is the remarkable lizard of New
Zealand, _Hatteria punctata_ (Fig. 264). Externally it differs little
from the ordinary lizard; but in many important points of internal
structure, especially in the primitive construction of the vertebral
column, the skull, and the limbs, it occupies a much lower position,
and approaches its amphibian ancestors, the Stegocephala. Hence
Hatteria is the phylogenetically oldest of all living reptiles, an
isolated survivor from the Permian period, closely resembling the
common ancestor of the Amniotes. It must differ so little from this
extinct form, our hypothetical Protamniote, that we put it next to the
Proreptilia. The remarkable Permian _Palæhatteria,_ that Credner
discovered in the Plauen terrain at Dresden in 1888, belongs to the
same group (Fig. 266). The Jurassic genus _Homœosaurus_ (Fig. 265), of
which well-preserved skeletons are found in the Solenhofen schists, is
perhaps still more closely related to them.

Unfortunately, the numerous fossil remains of Permian and Triassic
Tocosauria that we have found in the last two decades are, for the most
part, very imperfectly preserved. Very often we can make only
precarious inferences from these skeletal fragments as to the anatomic
characters of the soft parts that went with the bony skeleton of the
extinct Tocosauria. Hence it has not yet been possible to arrange these
important fossils with any confidence in the ancestral series that
descend from the Protamniotes to the Sauropsids on the one side and the
Mammals on the other. Opinions are particularly divided as to the place
in classification and the phylogenetic significance of the remarkable
_Theromorpha._ Cope gives this name to a very interesting and extensive
group of extinct terrestrial reptiles, of which we have only fossil
remains from the Permian and Triassic strata. Forty years ago some of
these Therosauria (fresh-water animals) were described by Owen as
_Anomodontia._ But during the last twenty years the distinguished
American paleontologists, Cope and Osborn, have greatly increased our
knowledge of them, and have claimed that the stem-forms of the Mammals
must be sought in this order. As a matter of fact, the Theromorpha are
nearer to the Mammals in the chief points of structure than any other
reptiles. This is especially true of the Thereodontia, to which the
_Pureosauria_ and _Pelycosauria_ belong (Fig. 267). The whole structure
of their pelvis and hind-feet has attained the same form as in the
Monotremes, the lowest Mammals. The formation of the
scapula and the quadrate bone shows an approach to the Mammals such as
we find in no other group of reptiles. The teeth also are already
divided into incisors, canines, and molars. Nevertheless, it is very
doubtful whether the Theromorpha really are in the ancestral line of
the Sauromammals, or lead direct from the Tocosauria to the earliest
Mammals. Other experts on this group believe that it is an independent
legion of the reptiles, connected, perhaps, at its lowest root, with
the Sauromammals, but developed quite independently of the
Mammals—though parallel to them in many ways.

One of the most important of the zoological facts that we rely on in
our investigation of the genealogy of the human race is the position of
man in the Mammal class. However different the views of zoologists may
have been as to this position in detail, and as to his relations to the
apes, no scientist has ever doubted that man is a true mammal in his
whole organisation and development. Linné drew attention to this fact
in the first edition of his famous _Systema Naturæ_ (1735). As will be
seen in any museum of anatomy or any manual of comparative anatomy; the
human frame has all the characteristics that are common to the Mammals
and distinguish them conspicuously from all other animals.


Fig.266. Skull of a Permian lizard (Palaehatteria longicaudata). Fig.
266—Skull of a Permian lizard (_Palæhatteria longicaudata_). (From
_Credner._) _n_ nasal bone, _pf_ frontal bone, _l_ lachrymal bone, _po_
postorbital bone, _sq_ covering bone, _i_ cheek-bone, _vo_ vomer, _im_
inter-maxillary.


If we examine this undoubted fact from the point of view of phylogeny,
in the light of the theory of descent, it follows at once that man is
of a common stem with all the other Mammals, and comes from the same
root as they. But the various features in which the Mammals agree and
by which they are distinguished are of such a character as to make a
polyphyletic hypothesis quite inadmissible. It is impossible to
entertain the idea that all the living and extinct Mammals come from a
number of separate roots. If we accept the general theory of evolution,
we are bound to admit the monophyletic hypothesis of the descent of all
the Mammals (including man) from a single mammalian stem-form. We may
call this long-extinct root-form and its earliest descendants (a few
genera of one family) “primitive mammals” or “stem-mammals”
(_Promammalia_). As we have already seen, this root-form developed from
the primitive Proreptile stem in a totally different direction from the
birds, and soon separated from the main stem of the reptiles. The
differences between the Mammals and the reptiles and birds are so
important and characteristic that we can assume with complete
confidence this division of the vertebrate stem at the commencement of
the development of the Amniotes. The reptiles and birds, which we group
together as the _Sauropsids,_ generally agree in the characteristic
structure of the skull and brain, and this is notably different from
that of the Mammals. In most of the reptiles and birds the skull is
connected with the first cervical vertebra (the _atlas_) by a single,
and in the Mammals (and Amphibia) by a double, condyle at the back of
the head. In the former the lower jaw is composed of several pieces,
and connected with the skull so that it can move by a special maxillary
bone (the _quadratum_); in the Mammals the lower jaw consists of one
pair of bony pieces, which articulate directly with the temporal bone.
Further, in the Sauropsids the skin is clothed with scales or feathers;
in the Mammals with hair. The red blood-cells of the former have a
nucleus; those of the latter have not. In fine, two quite
characteristic features of the Mammals, which distinguish them not only
from the birds and reptiles, but from all other animals, are the
possession of a
complete diaphragm and of mammary glands that produce the milk for the
nutrition of the young. It is only in the Mammals that the diaphragm
forms a transverse partition of the body-cavity, completely separating
the pectoral from the abdominal cavity. It is only in the mammals that
the mother suckles its young, and this rightly gives the name to the
whole class (_mamma_ = breast).


Fig.267. Skull of a Triassic theromorphum (Galesaurus planiceps), from
the Karoo formation in South Africa. Fig. 267—Skull of a Triassic
theromorphum (_Galesaurus planiceps_), from the Karoo formation in
South Africa. (From _Owen._) a from the right, _b_ from below, _c_ from
above, _d_ tricuspid tooth. _N_ nostrils, _Na_ nasal bone, _Mx_ upper
jaw, _Prf_ prefrontal, _Fr_ frontal bone, _A_ eye-pits, _S_
temple-pits. _Pa_ Parietal eye, _Bo_ joint at back of head, _Pt_
pterygoid-bone, _Md_ lower jaw.


From these pregnant facts of comparative anatomy and ontogeny it
follows absolutely that the whole of the Mammals belong to a single
natural stem, which branched off at an early date from the
reptile-root. It follows further with the same absolute certainty that
the human race is also a branch of this stem. Man shares all the
characteristics I have described with all the Mammals, and differs in
them from all other animals. Finally, from these facts we deduce with
the same confidence those advances in the vertebrate organisation by
which one branch of the Sauromammals was converted into the stem-form
of the Mammals. Of these advances the chief were: (1) The
characteristic modification of the skull and the brain; (2) the
development of a hairy coat; (3) the complete formation of the
diaphragm; and (4) the construction of the mammary glands and
adaptation to suckling. Other important changes of structure proceeded
step by step with these.

The epoch at which these important advances were made, and the
foundation of the Mammal class was laid, may be put with great
probability in the first section of the Mesozoic or secondary age—the
Triassic period. The oldest fossil remains of mammals that we know were
found in strata that belong to the earliest Triassic period—the upper
Kueper. One of the earliest forms is the genus _Dromatherium,_ from the
North American Triassic (Fig. 268). Their teeth still strikingly recall
those of the Pelycosauria. Hence we may assume that this small and
probably insectivorous mammal belonged to the stem-group of the
Promammals. We do not find any positive trace of the third and most
advanced division of the Mammals—the Placentals. These (including man)
are much younger, and we do not find indisputable fossil remains of
them until the Cenozoic age, or the Tertiary period. This
paleontological fact is very important, because it fully harmonises
with the evolutionary succession of the Mammal orders that is deduced
from their comparative anatomy and ontogeny.

The latter science teaches us that the whole Mammal class divides into
three main groups or sub-classes, which correspond to three successive
phylogenetic stages. These three stages, which also represent three
important stages in our human genealogy, were first distinguished in
1816 by the eminent French zoologist, Blainville, and received the
names of _Ornithodelphia, Didelphia,_ and _Monodelphia,_ according to
the construction of the female organs (_delphys_ = uterus or womb).
Huxley afterwards gave them the names of _Prototheria, Metatheria,_ and
_Epitheria._ But the three sub-classes differ so widely from each
other, not only in the construction of the sexual organs, but in many
other respects also, that we may confidently draw up the following
important phylogenetic thesis: The Monodelphia or Placentals descend
from the Didelphia or Marsupials; and the latter, in turn, are
descended from the Monotremes or Ornithodelphia.

Thus we must regard as the twenty-first stage in our genealogical tree
the earliest and lowest chief group of the Mammals—the sub-class of the
Monotremes (“cloaca-animals,” Ornithodelphia, or Prototheria, Figs. 269
and 270). They take their name from the cloaca which they share with
all the lower Vertebrates. This cloaca is the common outlet for the
passage of the excrements, the urine, and the sexual products. The
urinary ducts and sexual canals open into the hindmost part of the gut,
while in all the other Mammals they are separated from the rectum and
anus. The latter have a special uro-genital outlet (_porus
urogenitalis_). The bladder also opens into the cloaca in the
Monotremes, and, indeed, apart from the two urinary ducts; in all the
other Mammals the latter open directly into the bladder. It was proved
by Haacke and Caldwell in 1884 that the Monotremes lay large eggs like
the reptiles, while all the other Mammals are viviparous. In 1894
Richard Semon further proved that these large eggs, rich in food-yelk,
have a partial segmentation and discoid gastrulation, as I had
hypothetically assumed in 1879; here again they resemble their
reptilian ancestors. The construction of the mammary gland is also
peculiar in the Monotremes. In them the glands have no teats for the
young animal to suck, but there is a special part of the breast pierced
with holes like a sieve, from which the milk issues, and the young
Monotreme must lick it off. Further, the brain of the Monotremes is
very little advanced. It is feebler than that of any of the other
Mammals. The fore-brain or cerebrum, in particular, is so small that it
does not cover the cerebellum. In the skeleton (Fig. 270) the formation
of the scapula among other parts is curious; it is quite different from
that of the other Mammals, and rather agrees with that of the reptiles
and Amphibia. Like these, the Monotremes have a strongly developed
_caracoideum._ From these and other less prominent characteristics it
follows absolutely that the Monotremes occupy the lowest place among
the Mammals, and represent a transitional group between the Tocosauria
and the rest of the Mammals. All these remarkable reptilian characters
must have been possessed by the stem-form of the whole mammal class,
the Promammal of the Triassic period, and have been inherited from the
Proreptiles.


Fig.268. Lower jaw of a Primitive Mammal or Promammal (Dromatherium
silvestre) from the North American Triassic. Fig. 268—Lower jaw of a
Primitive Mammal or Promammal (_Dromatherium silvestre_) from the North
American Triassic. _i_ incisors, _c_ canine, _p_ premolars, _m_ molars.
(From _Döderlein._)


During the Triassic and Jurassic periods the sub-class of the
Monotremes was represented by a number of different stem-mammals.
Numerous fossil remains of them have lately been discovered in the
Mesozoic strata of Europe, Africa, and America. To-day there are only
two surviving specimens of the group, which we place together in the
family of the duck-bills, _Ornithostoma._ They are confined to
Australia and the neighbouring island of Van Diemen’s Land (or
Tasmania); they become scarcer every year, and will soon, like their
blood-relatives, be counted among the extinct animals. One form lives
in the rivers, and builds subterraneous dwellings on the banks; this is
the _Ornithorhyncus paradoxus,_ with webbed feet, a thick soft fur, and
broad flat jaws, which look very much like the bill of a duck (Figs.
269, 270). The other form, the land duck-bill, or spiny ant-eater
(_Echidna hystrix_), is very much like the anteaters in its habits and
the peculiar construction of its thin snout and very long tongue; it is
covered with needles, and can roll itself up like a hedgehog. A cognate
form (_Parechidna Bruyni_) has lately been found in New Guinea.

These modern Ornithostoma are the scattered survivors of the vast
Mesozoic group of Monotremes; hence they have the same interest in
connection with the stem history of the Mammals as the living
stem-reptiles (_Hatteria_) for that of the reptiles, and the isolated
Acrania (_Amphioxus_) for the phylogeny of the Vertebrate stem.

The Australian duck-bills are distinguished externally by a toothless
bird-like
beak or snout. This absence of real bony teeth is a late result of
adaptation, as in the toothless Placentals (_Edentata,_ armadillos and
ant-eaters). The extinct Monotremes, to which the Promammalia belonged,
must have had developed teeth, inherited from the reptiles. Lately
small rudiments of real molars have been discovered in the young of the
Ornithorhyncus, which has horny plates in the jaws instead of real
teeth.


Fig. 269. The Ornithorhyncus or Duck-mole. (Ornithorhyncus paradoxus).
Fig. 269—The Ornithorhyncus or Duck-mole. (_Ornithorhyncus paradoxus_).


Fig. 270. Skeleton of the Ornithorhyncus. Fig. 270—Skeleton of the
Ornithorhyncus.


The living Ornithostoma and the stem-forms of the Marsupials (or
_Didelphia_) must be regarded as two widely diverging lines from the
Promammals. This second sub-class of the Mammals is very interesting as
a perfect intermediate stage between the other two. While the
Marsupials retain a great part of the characteristics of the
Monotremes, they have also acquired some of the chief features of the
Placentals. Some features
are also peculiar to the Marsupials, such as the construction of the
male and female sexual organs and the form of the lower jaw. The
Marsupials are distinguished by a peculiar hook-like bony process that
bends from the corner of the lower jaw and points inwards. As most of
the Placentals have not this process, we can, with some probability,
recognise the Marsupial from this feature alone. Most of the mammal
remains that we have from the Jurassic and Cretaceous deposits are
merely lower jaws, and most of the jaws found in the Jurassic deposits
at Stonesfield and Purbeck have the peculiar hook-like process that
characterises the lower jaw of the Marsupial. On the strength of this
paleontological fact, we may suppose that they belonged to Marsupials.
Placentals do not seem to have existed at the middle of the Mesozoic
age—not until towards its close (in the Cretaceous period). At all
events, we have no fossil remains of indubitable Placentals from that
period.

The existing Marsupials, of which the plant-eating kangaroo and the
carnivorous opossum (Fig. 272) are the best known, differ a good deal
in structure, shape, and size, and correspond in many respects to the
various orders of Placentals. Most of them live in Australia, and a
small part of the Australian and East Malayan islands. There is now not
a single living Marsupial on the mainland of Europe, Asia, or Africa.
It was very different during the Mesozoic and even during the Cenozoic
age. The sedimentary deposits of these periods contain a great number
and variety of marsupial remains, sometimes of a colossal size, in
various parts of the earth, and even in Europe. We may infer from this
that the existing Marsupials are the remnant of an extensive earlier
group that was distributed all over the earth. It had to give way in
the struggle for life to the more powerful Placentals during the
Tertiary period. The survivors of the group were able to keep alive in
Australia and South America because the one was completely separated
from the other parts of the earth during the whole of the Tertiary
period, and the other during the greater part of it.


Fig.271. Lower jaw of a Promammal (Dryolestes priscus), from the
Jurassic of the Felsen strata. Fig. 271—Lower jaw of a Promammal
(_Dryolestes priscus_), from the Jurassic of the Felsen strata. (From
_Marsh._)


From the comparative anatomy and ontogeny of the existing Marsupials we
may draw very interesting conclusions as to their intermediate position
between the earlier Monotremes and the later Placentals. The defective
development of the brain (especially the cerebrum), the possession of
marsupial bones, and the simple construction of the allantois (without
any placenta as yet) were inherited by the Marsupials, with many other
features, from the Monotremes, and preserved. On the other hand, they
have lost the independent bone (_caracoideum_) at the shoulder-blade.
But we have a more important advance in the disappearance of the
cloaca; the rectum and anus are separated by a partition from the
uro-genital opening (_sinus urogenitalis_). Moreover, all the
Marsupials have teats on the mammary glands, at which the new-born
animal sucks. The teats pass into the cavity of a pouch or pocket on
the ventral side of the mother, and this is supported by a couple of
marsupial bones. The young are born in a very imperfect condition, and
carried by the mother for some time longer in her pouch, until they are
fully developed (Fig. 272). In the giant kangaroo, which is as tall as
a man, the embryo only develops for a month in the uterus, is then born
in a very imperfect state, and finishes its growth in the mother’s
pouch (_marsupium_); it remains in this about nine months, and at first
hangs continually on to the teat of the mammary gland.

From these and other characteristics (especially the peculiar
construction of the internal and external sexual organs in male and
female) it is clear that we must conceive the whole sub-class of the
Marsupials as one stem group, which has been developed from the
Promammalia. From one branch of these Marsupials (possibly from more
than one) the stem-forms of the higher Mammals, the Placentals, were
afterwards evolved. Of the existing forms of the Marsupials,
which have undergone various modifications through adaptation to
different environments, the family of the opossums (_Didelphida_ or
_Pedimana_) seems to be the oldest and nearest to the common stem-form
of the whole class. To this family belong the crab-eating opossum of
Brazil (Fig. 272) and the opossum of Virginia, on the embryology of
which Selenka has given us a valuable work (cf. Figs. 63–67 and
131–135). These Didelphida climb trees like the apes, grasping the
branches with their hand-shaped hind feet. We may conclude from this
that the stem-forms of the Primates, which we must regard as the
earliest Lemurs, were evolved directly from the opossum. We must not
forget, however, that the conversion of the five-toed foot into a
prehensile hand is polyphyletic. By the same adaptation to climbing
trees the habit of grasping their branches with the feet has in many
different cases brought about that opposition of the thumb or great toe
to the other toes which makes the hand prehensile. We see this in the
climbing lizards (chameleon), the birds, and the tree-dwelling mammals
of various orders.


Fig.272. The crab-eating Opossum (Philander cancrivorus). The female
has three young in the pouch. Fig. 272—The crab-eating Opossum
(_Philander cancrivorus_). The female has three young in the pouch.
(From _Brehm._)


Some zoologists have lately advanced the opposite opinion, that the
Marsupials represent a completely independent
sub-class of the Mammals, with no direct relation to the Placentals,
and developing independently of them from the Monotremes. But this
opinion is untenable if we examine carefully the whole organisation of
the three sub-classes, and do not lay the chief stress on incidental
features and secondary adaptations (such as the formation of the
marsupium). It is then clear that the Marsupials—viviparous Mammals
without placenta—are a necessary transition from the oviparous
Monotremes to the higher Placentals with chorion-villi. In this sense
the Marsupial class certainly contains some of man’s ancestors.



Chapter XXIII.
OUR APE ANCESTORS


The long series of animal forms which we must regard as the ancestors
of our race has been confined within narrower and narrower circles as
our phylogenetic inquiry has progressed. The great majority of known
animals do not fall in the line of our ancestry, and even within the
vertebrate stem only a small number are found to do so. In the most
advanced class of the stem, the mammals, there are only a few families
that belong directly to our genealogical tree. The most important of
these are the apes and their predecessors, the half-apes, and the
earliest Placentals (_Prochoriata_).

The Placentals (also called _Choriata, Monodelphia, Eutheria_ or
_Epitheria_) are distinguished from the lower mammals we have just
considered, the Monotremes and Marsupials, by a number of striking
peculiarities. Man has all these distinctive features; that is a very
significant fact. We may, on the ground of the most careful
comparative-anatomical and ontogenetic research, formulate the thesis:
“Man is in every respect a true Placental.” He has all the
characteristics of structure and development that distinguish the
Placentals from the two lower divisions of the mammals, and, in fact,
from all other animals. Among these characteristics we must especially
notice the more advanced development of the brain. The fore-brain or
cerebrum especially is much more developed in them than in the lower
animals. The _corpus callosum,_ which forms a sort of wide bridge
connecting the two hemispheres of the cerebrum, is only fully formed in
the Placentals; it is very rudimentary in the Marsupials and
Monotremes. It is true that the lowest Placentals are not far removed
from the Marsupials in cerebral development; but within the placental
group we can trace an unbroken gradation of progressive development of
the brain, rising gradually from this lowest stage up to the elaborate
psychic organ of the apes and man. The human soul—a physiological
function of the brain—is in reality only a more advanced ape-soul.

The mammary glands of the Placentals are provided with teats like those
of the Marsupials; but we never find in the Placentals the pouch in
which the latter carry and suckle their young. Nor have they the
marsupial bones in the ventral wall at the anterior border of the
pelvis, which the Marsupials have in common with the Monotremes, and
which are formed by a partial ossification of the sinews of the inner
oblique abdominal muscle. There are merely a few insignificant remnants
of them in some of the Carnivora. The Placentals are also generally
without the hook-shaped process at the angle of the lower jaw which is
found in the Marsupials.

However, the feature that characterises the Placentals above all
others, and that has given its name to the whole sub-class, is the
formation of the placenta. We have already considered the formation and
significance of this remarkable embryonic organ when we traced the
development of the chorion and the allantois in the human embryo
(pp.165–9) The urinary sac or the allantois, the
curious vesicle that grows out of the hind part of the gut, has
essentially the same structure and function in the human embryo as in
that of all the other Amniotes (cf. Figs. 194–6). There is a quite
secondary difference, on which great stress has wrongly been laid, in
the fact that in man and the higher apes the original cavity of the
allantois quickly degenerates, and the rudiment of it sticks out as a
solid projection from the primitive gut. The thin wall of the allantois
consists of the same two layers or membranes as the wall of the gut—the
gut-gland layer within and the gut-fibre layer without. In the
gut-fibre layer of the allantois there are large blood-vessels, which
serve for the nutrition, and especially the respiration, of the
embryo—the umbilical vessels (p. 170).In the reptiles and birds the
allantois enlarges into a spacious sac, which encloses the embryo with
the amnion, and does not combine with the outer fœtal membrane (the
chorion). This is the case also with the lowest mammals, the oviparous
Monotremes and most of the Marsupials. It is only in some of the later
Marsupials (_Peramelida_) and all the Placentals that the allantois
develops into the distinctive and remarkable structure that we call the
_placenta._


Fig.273. Foetal membranes of the human embryo (diagrammatic). Fig.
273—Fœtal membranes of the human embryo (diagrammatic). _m_ the thick
muscular wall of the womb. _plu_ placenta [the inner layer (_plu_′) of
which penetrates into the chorion-villi (_chz_) with its processes].
_chf_ tufted, _chl_ smooth chorion. _a_ amnion, _ah_ amniotic cavity,
_as_ amniotic sheath of the umbilical cord (which passes under into the
navel of the embryo—not given here), _dg_ vitelline duct, _ds_ yelk
sac, _dv, dr_ decidua (vera and reflexa). The uterine cavity (_uh_)
opens below into the vagina and above on the right into an oviduct
(_t_). (From _Kölliker._)


The placenta is formed by the branches of the blood-vessels in the wall
of the allantois growing into the hollow ectodermic tufts (villi) of
the chorion, which run into corresponding depressions in the mucous
membrane of the womb. The latter also is richly permeated with
blood-vessels which bring the mother’s blood to the embryo. As the
partition in the villi between the maternal blood-vessels and those of
the fœtus is extremely thin, there is a direct exchange of fluid
between the two, and this is of the greatest importance in the
nutrition of the young mammal. It is true that the maternal vessels do
not entirely pass into the fœtal vessels, so that the two kinds of
blood are simply mixed. But the partition between them is so thin that
the nutritive fluid easily transudes through it. By means of this
transudation or diosmosis the exchange of fluids takes place without
difficulty. The larger the embryo is in the placentals, and the longer
it remains in the womb, the more necessary it is to have special
structures to meet its great consumption of food.

In this respect there is a very conspicuous difference between the
lower and higher mammals. In the Marsupials, in which the embryo is
only a comparatively short time in the womb and is born in a very
immature condition, the vascular arrangements in the yelk-sac and the
allantois suffice for its nutrition, as we find them in the Monotremes,
birds, and reptiles. But in the Placentals, where gestation lasts a
long time, and the embryo reaches its full development under the
protection of its enveloping membranes, there has to be a new mechanism
for the direct supply of a large quantity of food, and this is
admirably met by the formation of the placenta.

Branches of the blood-vessels penetrate into the chorion-villi from
within, starting from the gut-fibre layer of the allantois, and
bringing the blood of the fœtus through the umbilical vessels (Fig. 273
_chz_). On the other hand, a thick network of blood-vessels develops in
the mucous membrane that clothes the inner surface of the womb,
especially in the region of the depressions into which the
chorion-villi penetrate (_plu_). This network of arteries contains
maternal blood, brought by the uterine vessels. As the connective
tissue between the enlarged capillaries of
the uterus disappears, wide cavities filled with maternal blood appear,
and into these the chorion-villi of the embryo penetrate. The sum of
these vessels of both kinds, that are so intimately correlated at this
point, together with the connective and enveloping tissue, is the
_placenta._ The placenta consists, therefore, properly speaking, of two
different though intimately connected parts—the fœtal placenta (Fig.
273 _chz_) within and the maternal or uterine placenta (_plu_) without.
The latter is made up of the mucous coat of the uterus and its
blood-vessels, the former of the tufted chorion and the umbilical
vessels of the embryo (cf. Fig. 196).


Fig.274. Skull of a fossil lemur (Adapis parisiensis,), from the
Miocene at Quercy. Fig. 274—Skull of a fossil lemur (_Adapis
parisiensis_), from the Miocene at Quercy. _A_ lateral view from the
right. _B_ lower jaw, _C_ lower molar, _i_ incisors, _c_ canines, _p_
premolars, _m_ molars.


The manner in which these two kinds of vessels combine in the placenta,
and the structure, form, and size of it, differ a good deal in the
various Placentals; to some extent they give us valuable data for the
natural classification, and therefore the phylogeny, of the whole of
this sub-class. On the ground of these differences we divide it into
two principal sections; the lower Placentals or _Indecidua,_ and the
higher Placentals or _ Deciduata._

To the Indecidua belong three important groups of mammals: the Lemurs
(_Prosimiæ_), the Ungulates (tapirs, horses, pigs, ruminants, etc.),
and the Cetacea (dolphins and whales). In these Indecidua the villi are
distributed over the whole surface of the chorion (or its greater part)
either singly or in groups. They are only loosely connected with the
mucous coat of the uterus, so that the whole fœtal membrane with its
villi can be easily withdrawn from the uterine depressions like a hand
from a glove. There is no real coalescence of the two placentas at any
part of the surface of contact. Hence at birth the fœtal placenta alone
comes away; the uterine placenta is not torn away with it.

The formation of the placenta is very different in the second and
higher section of the Placentals, the _Deciduata._ Here again the whole
surface of the chorion is thickly covered with the villi in the
beginning. But they afterwards disappear from one part of the surface,
and grow proportionately thicker on the other part. We thus get a
differentiation between the smooth chorion (_chorion laeve,_ Fig. 273
_chl_) and the thickly-tufted chorion (_chorion frondosum,_ Fig. 273
_chf_). The former has only a few small villi or none at all; the
latter is thickly covered with large and well-developed villi; this
alone now constitutes the placenta. In the great majority of the
Deciduata the placenta has the same shape as in man >(Figs. 197 and
200)—namely a thick, circular disk like a cake; so we find in the
Insectivora, Chiroptera, Rodents, and Apes. This _discoplacenta_ lies
on one side of the chorion. But in the Sarcotheria (both the Carnivora
and the seals, _Pinnipedia_) and in the elephant and several other
Deciduates we find a _zonoplacenta_; in these the rich mass of villi
runs like a girdle round the middle of the ellipsoid chorion, the two
poles of it being free from them.

Still more characteristic of the Deciduates is the peculiar and very
intimate connection between the _chorion frondosum_ and the
corresponding part of the mucous coat of the womb, which we must regard
as a real coalescence of the two. The villi of the chorion push their
branches into the blood-filled tissues of the coat of the uterus, and
the vessels of each loop together so intimately that it is no longer
possible to separate the fœtal
from the maternal placenta; they form henceforth a compact and
apparently simple placenta. In consequence of this coalescence, a whole
piece of the lining of the womb comes away at birth with the fœtal
membrane that is interlaced with it. This piece is called the
“falling-away” membrane (_decidua_). It is also called the serous
(spongy) membrane, because it is pierced like a sieve or sponge. All
the higher Placentals that have this decidua are classed together as
the “Deciduates.” The tearing away of the decidua at birth naturally
causes the mother to lose a quantity of blood, which does not happen in
the Indecidua. The last part of the uterine coat has to be repaired by
a new growth after birth in the Deciduates. (Cf. Figs. 199, 200, pp.
168–70.)


Fig.275. The Slender Lori (Stenops gracilis) of Ceylon, a tail-less
lemur. Fig. 275—The Slender Lori (_Stenops gracilis_) of Ceylon, a
tail-less lemur.


In the various orders of the Deciduates, the placenta differs
considerably both in outer form and internal structure. The extensive
investigations of the last ten years have shown that there is more
variation in these respects among the higher mammals than was formerly
supposed. The physiological work of this important embryonic organ, the
nutrition of the fœtus during its long sojourn in the womb, is
accomplished in the various groups of the Placentals by very different
and sometimes very elaborate structures. They have lately been fully
described by Hans Strahl.

The phylogeny of the placenta has become more intelligible from the
fact that we have found a number of transitional forms of it. Some of
the Marsupials (_Perameles_) have the beginning of a placenta. In some
of the Lemurs (_Tarsius_) a discoid placenta with decidua is developed.

While these important results of comparative embryology have been
throwing further light on the close blood-relationship of man and the
anthropoid apes in the last few years (p. 172), the great advance of
paleontology has at the same time been affording us a deeper insight
into the stem-history of the Placental group. In the seventh chapter of
my _Systematic Phylogeny of the Vertebrates_ I advanced the hypothesis
that the Placentals form a single stem with many branches, which has
been evolved from an older group of the Marsupials (_Prodidelphia_).
The four great legions of the Placentals—Rodents, Ungulates, Carnassia,
and Primates—are sharply separated to-day by important features of
organisation. But if we consider their extinct ancestors of the
Tertiary period, the differences gradually disappear, the deeper we go
in the Cenozoic deposits; in the end we find that they vanish
altogether.
The primitive stem-forms of the Rodents (_Esthonychida_), the Ungulates
(_Chondylarthra_), the Carnassia (_Ictopsida_), and the Primates
(_Lemuravida_) are so closely related at the beginning of the Tertiary
period that we might group them together as different families of one
order, the Proplacentals (_Mallotheria_ or _Prochoriata_).

Hence the great majority of the Placentals have no direct and close
relationship to man, but only the legion of the _ Primates._ This is
now generally divided into three orders—the half-apes (_Prosimiæ_),
apes (_Simiæ_), and man (_Anthropi_). The lemurs or half-apes are the
stem-group, descending from the older _ Mallotheria_ of the Cretaceous
period. From them the apes were evolved in the Tertiary period, and man
was formed from these towards its close.

The Lemurs (_Prosimiæ_) have few living representatives. But they are
very interesting, and are the last survivors of a once extensive group.
We find many fossil remains of them in the older Tertiary deposits of
Europe and North America, in the Eocene and Miocene. We distinguish two
sub-orders, the fossil _Lemuravida_ and the modern _Lemurogona._ The
earliest and most primitive forms of the Lemuravida are the Pachylemurs
(_Hypopsodina_); they come next to the earliest Placentals
(_Prochoriata_), and have the typical full dentition, with forty-four
teeth (3.1.4.3. over 3.1.4.3.). The Necrolemurs (_Adapida,_ Fig. 274)
have only forty teeth, and have lost an incisor in each jaw (2.1.4.3.
over 2.1.4.3.). The dentition is still further reduced in the
Lemurogona (_Autolemures_), which usually have only thirty-six teeth
(2.1.3.3. over 2.1.3.3.). These living survivors are scattered far over
the southern part of the Old World. Most of the species live in
Madagascar, some in the Sunda Islands, others on the mainland of Asia
and Africa. They are gloomy and melancholic animals; they live a quiet
life, climbing trees, and eating fruit and insects. They are of
different kinds. Some are closely related to the Marsupials (especially
the opossum). Others (_Macrotarsi_) are nearer to the Insectivora,
others again (_Chiromys_) to the Rodents. Some of the lemurs
(_Brachytarsi_) approach closely to the true apes. The numerous fossil
remains of half-apes and apes that have been recently found in the
Tertiary deposits justify us in thinking that man’s ancestors were
represented by several different species during this long period. Some
of these were almost as big as men, such as the diluvial lemurogonon
_Megaladapis_ of Madagascar.


Fig.276. The white-nosed ape (Cercopithecus petaurista). Fig. 276—The
white-nosed ape (_Cercopithecus petaurista_).


Next to the lemurs come the true apes (_Simiæ_), the twenty-sixth stage
in our ancestry. It has been beyond question for some time now that the
apes approach nearest to man in every respect of all the animals. Just
as the lowest apes come close to the lemurs, so the highest come next
to man. When we carefully study the comparative anatomy of the apes and
man, we can trace a gradual and uninterrupted advance in the
organisation of the ape up to the purely human frame, and, after
impartial examination of the “ape problem” that has been discussed of
late years with such passionate interest, we come infallibly to the
important conclusion, first formulated by Huxley in 1863: “Whatever
systems of organs we take, the comparison of their modifications in the
series of apes leads to the same result: that the anatomic differences
that separate man from the gorilla and chimpanzee are not as great as
those that separate the gorilla from the lower apes.” Translated into
phylogenetic language, this “pithecometra-law,” formulated in such
masterly fashion by Huxley, is quite equivalent to the popular saying:
“Man is descended from the apes.”

In the very first exposition of his profound natural classification
(1735) Linné
placed the anthropoid mammals at the head of the animal kingdom, with
three genera: man, the ape, and the sloth. He afterwards called them
the “Primates”—the “lords” of the animal world; he then also separated
the lemur from the true ape, and rejected the sloth. Later zoologists
divided the order of Primates. First the Gottingen anatomist,
Blumenbach, founded a special order for man, which he called _ Bimana_
(“two-handed”); in a second order he united the apes and lemurs under
the name of _Quadrumana_ (“four-handed”); and a third order was formed
of the distantly-related _Chiroptera_ (bats, etc.). The separation of
the Bimana and Quadrumana was retained by Cuvier and most of the
subsequent zoologists. It seems to be extremely important, but, as a
matter of fact, it is totally wrong. This was first shown in 1863 by
Huxley, in his famous _Man’s Place in Nature._ On the strength of
careful comparative anatomical research he proved that the apes are
just as truly “two-handed” as man; or, if we prefer to reverse it, that
man is as truly four-handed as the ape. He showed convincingly that the
ideas of hand and foot had been wrongly defined, and had been
improperly based on physiological instead of morphological grounds. The
circumstance that we oppose the
thumb to the other four fingers in our hand, and so can grasp things,
seemed to be a special distinction of the hand in contrast to the foot,
in which the corresponding great toe cannot be opposed in this way to
the others. But the apes can grasp with the hind-foot as well as the
fore, and so were regarded as quadrumanous. However, the inability to
grasp that we find in the foot of civilised man is a consequence of the
habit of clothing it with tight coverings for thousands of years. Many
of the bare-footed lower races of men, especially among the negroes,
use the foot very freely in the same way as the hand. As a result of
early habit and continued practice, they can grasp with the foot (in
climbing trees, for instance) just as well as with the hand. Even
new-born infants of our own race can grasp very strongly with the great
toe, and hold a spoon with it as firmly as with the hand. Hence the
physiological distinction between hand and foot can neither be pressed
very far, nor has it a scientific basis. We must look to morphological
characters.


Fig.277. The drill-baboon (Cynocephalus leucophaeus) (From Brehm.) Fig.
277—The drill-baboon (_Cynocephalus leucophæus_). (From _Brehm._)


As a matter of fact, it is possible to draw such a sharp morphological
distinction—a distinction based on anatomic structure—between the fore
and hind extremity. In the formation both of the bony skeleton and of
the muscles that are connected with the hand and foot before and behind
there are material and constant differences; and these are found both
in man and the ape. For instance, the number and arrangement of the
smaller bones of the hand and foot are quite different. There are
similar constant differences in the muscles. The hind extremity always
has three muscles (a short flexor muscle, a short extensor muscle, and
a long calf-muscle) that are not found in the fore extremity. The
arrangement of the muscles also is different before and behind. These
characteristic differences between the fore and hind extremities are
found in man as well as the ape. There can be no doubt, therefore, that
the ape’s foot deserves that name just as much as the human foot does,
and that all true apes are just as “bimanous” as man. The common
distinction of the apes as “quadrumanous” is altogether wrong
morphologically.

But it may be asked whether, quite apart from this, we can find any
other features that distinguish man more sharply from the ape than the
various species of apes are distinguished from each other. Huxley gave
so complete and demonstrative a reply to this question that the
opposition still raised on many sides is absolutely without foundation.
On the ground of careful comparative anatomical research, Huxley proved
that in all morphological respects the differences between the highest
and lowest apes are greater than the corresponding differences between
the highest apes and man. He thus restored Linné’s order of the
Primates (excluding the bats), and divided it into three sub-orders,
the first composed of the half-apes (_Lemuridæ_), the second of the
true apes (_Simiadæ_), the third of men (_Anthropidæ_).

But, as we wish to proceed quite consistently and impartially on the
laws of systematic logic, we may, on the strength of Huxley’s own law,
go a good deal farther in this division. We are justified in going at
least one important step farther, and assigning man his natural place
within one of the sections of the order of apes. All the features that
characterise this group of apes are found in man, and not found in the
other apes. We do not seem to be justified, therefore, in founding for
man a special order distinct from the apes.

The order of the true apes (_Simiæ_ or _ Pitheca_)—excluding the
lemurs—has long been divided into two principal groups, which also
differ in their geographical distribution. One group (_Hesperopitheca,_
or western apes) live in America. The other group, to which man
belongs, are the _ Eopitheca_ or eastern apes; they are found in Asia
and Africa, and were formerly in Europe. All the eastern apes agree
with man in the features that are chiefly used in zoological
classification to distinguish between the two simian groups, especially
in the dentition. The objection might be raised that the teeth are too
subordinate an organ physiologically for us to lay stress on them in so
important a question. But there is a good reason for it; it is with
perfect justice that zoologists have for more than a century paid
particular attention to the teeth in the systematic division and
arrangement of the orders of mammals. The number, form, and arrangement
of the teeth are much more faithfully inherited in the various orders
than most other characters.

Hence the form of dentition in man is very important. In the fully
developed
condition we have thirty-two teeth; of these eight are incisors, four
canine, and twenty molars. The eight incisors, in the middle of the
jaws, have certain characteristic differences above and below. In the
upper jaw the inner incisors are larger than the outer; in the lower
jaw the inner are the smaller. Next to these, at each side of both
jaws, is a canine (or “eye tooth”), which is larger than the incisors.
Sometimes it is very prominent in man, as it is in most apes and many
of the other mammals, and forms a sort of tusk. Next to this there are
five molars above and below on each side, the first two of which (the
“pre-molars”) are small, have only one root, and are included in the
change of teeth; the three back ones are much larger, have two roots,
and only come with the second teeth. The apes of the Old World, or all
the living or fossil apes of Asia, Africa, and Europe, have the same
dentition as man.


Figs. 278 to 282. Skeletons of a man and the four anthropoid apes. Fig.
278. Gibbon (Hylobates). Fig. 279. Orang (Satyrus). Fig. 280.
Chimpanzee (Anthropithecus). Fig. 281. Gorilla (Gorilla). Fig. 282. Man
(Homo). Fig. 278–282—Skeletons of a man and the four anthropoid apes.
(Fig. 278, Gibbon; Fig. 279, Orang; Fig. 280, Chimpanzee; Fig. 281,
Gorilla; Fig. 282, Man. (From _Huxley._) Cf. Figs. 203–209.


On the other hand, all the American apes have an additional pre-molar
in each half of the jaw. They have six molars above and below on each
side, or thirty-six teeth altogether. This characteristic difference
between the eastern and western apes has been so faithfully inherited
that it is very instructive for us. It is true that there seems to be
an exception in the case of a small family of South American apes. The
small silky apes (_Arctopitheca_ or _Hapalidæ_), which include the
tamarin (_Midas_) and the brush-monkey (_Jacchus_), have only five
molars in each half of the jaw (instead of six), and so seem to be
nearer to the eastern apes. But it is found, on closer examination,
that they have three premolars, like all the western apes, and that
only the last molar has been lost. Hence the apparent exception really
confirms the above distinction.

Of the other features in which the two groups of apes differ, the
structure of the nose is particularly instructive and conspicuous. All
the eastern apes have the same type of nose as man—a comparatively
narrow partition between the two halves, so that the nostrils run
downwards. In some of them the nose protrudes as far as in man, and has
the same characteristic structure. We have already alluded to the
curious long-nosed apes, which have a long, finely-curved nose. Most of
the eastern apes have, it is true, rather flat noses, like, for
instance, the white-nosed monkey (Fig. 276); but the nasal partition is
thin and narrow in them all. The American apes have a different type of
nose. The partition is very broad and thick at the bottom, and the
wings of the nostrils are not developed, so that they point outwards
instead of downwards. This difference in the form of the nose is so
constantly inherited in both groups that the apes of the New World are
called “flat-nosed” (_Platyrrhinæ_), and those of the Old World
“narrow-nosed” (_Catarrhinæ_). The bony passage of the ear (at the
bottom of which is the tympanum) is short and wide in all the
Platyrrhines, but long and narrow in all the Catarrhines; and in man
this difference also is significant.

This division of the apes into Platyrrhines and Catarrhines, on the
ground of the above hereditary features, is now generally admitted in
zoology, and receives strong support from the geographical distribution
of the two groups in the east and west. It follows at once, as regards
the phylogeny of the apes, that two divergent lines proceeded from the
common stem-form of the ape-order in the early Tertiary period, one of
which spread over the Old, the other over the New, World. It is certain
that all the Platyrrhines come of one stock, and also all the
Catarrhines; but the former are phylogenetically older, and must be
regarded as the stem-group of the latter.

What can we deduce from this with regard to our own genealogy? Man has
just the same characters, the same form of dentition, auditory passage,
and nose, as all the Catarrhines; in this he radically differs from the
Platyrrhines. We are thus forced to assign him a position among the
eastern apes in the order of Primates, or at least place him alongside
of them. But it follows that man is a direct blood relative of the apes
of the Old World, and can be traced to a common stem-form together with
all the Catarrhines. In his whole organisation and in his origin man is
a true Catarrhine; he originated in the Old World from an unknown,
extinct group of the eastern apes. The apes of the New World, or the
Platyrrhines, form a divergent branch of our genealogical tree, and
this is only distantly related at its root to the human race. We must
assume, of course, that the earliest Eocene apes had the full dentition
of the Platyrrhines; hence we may regard this stem-group as a special
stage (the twenty-sixth) in our ancestry, and deduce from it (as the
twenty-seventh stage) the earliest Catarrhines.

We have now reduced the circle of our nearest relatives to the small
and comparatively scanty group that is represented by the sub-order of
the Catarrhines; and we are in a position to answer the question of
man’s place in this sub-order, and say whether we can deduce anything
further from this position as to our immediate ancestors. In answering
this question the comprehensive and able studies that Huxley gives of
the comparative anatomy of man and the various Catarrhines in his
_Man’s Place in Nature_ are of great assistance to us. It is quite
clear from these that the differences between man and the highest
Catarrhines (gorilla, chimpanzee, and orang) are in every respect
slighter than the corresponding differences between the highest and the
lowest Catarrhines (white-nosed monkey, macaco, baboon, etc.). In fact,
within the small group of the tail-less anthropoid apes the differences
between the various genera are not less than the differences between
them and man. This is seen by a glance at the skeletons that Huxley has
put together (Figs. 278–282). Whether we take the skull or the
vertebral column or the ribs or the fore or hind limbs, or whether we
extend the comparison to the muscles, blood-vessels, brain, placenta,
etc., we always reach the same result on impartial examination—that man
is not more different from the other Catarrhines than the extreme forms
of them (for instance, the gorilla and baboon) differ from each other.
We may now, therefore, complete the Huxleian law we have already quoted
with the following thesis: “Whatever system of organs we take, a
comparison of their modifications in the series of Catarrhines always
leads to the same conclusion; the anatomic differences that separate
man from the most advanced Catarrhines (orang, gorilla, chimpanzee) are
not as great as those that separate the latter from the lowest
Catarrhines (white-nosed monkey, macaco, baboon).”

We must, therefore, consider the descent of man from other Catarrhines
to be fully proved. Whatever further information on the comparative
anatomy and ontogeny of the living Catarrhines we may obtain in the
future, it cannot possibly disturb this conclusion. Naturally, our
Catarrhine ancestors must have passed through a long series of
different forms before the human type was produced. The chief advances
that effected this “creation of man,” or his differentiation from the
nearest related Catarrhines, were: the adoption of the erect posture
and the consequent greater differentiation of the fore and hind limbs,
the evolution of articulate speech and its organ, the larynx, and the
further development of the brain and its function, the soul; sexual
selection had a great influence in this, as Darwin showed in his famous
work.

With an eye to these advances we can distinguish at least four
important stages in our simian ancestry, which represent prominent
points in the historical process of the making of man. We may take,
after the Lemurs, the earliest and lowest Platyrrhines of South
America, with thirty-six teeth, as the twenty-sixth stage of our
genealogy; they were developed from the Lemurs by a peculiar
modification of the brain, teeth, nose, and fingers. From these Eocene
stem-apes were formed the earliest Catarrhines or eastern apes, with
the human dentition (thirty-two teeth), by modification of the nose,
lengthening of the bony channel of the ear, and the loss of four
pre-molars. These oldest stem-forms of the whole Catarrhine group were
still thickly coated with hair, and had long tails—baboons
(_Cynopitheca_) or tailed apes (_Menocerca,_ Fig. 276). They lived
during the Tertiary period, and are found fossilised in the Miocene. Of
the actual tailed apes perhaps the nearest to them are the _
Semnopitheci._

If we take these Semnopitheci as the twenty-seventh stage in our
ancestry, we may put next to them, as the twenty-eighth, the tail-less
anthropoid apes. This name is given to the most advanced and man-like
of the existing Catarrhines. They were developed from the other
Catarrhines by losing the tail and part of the hair, and by a higher
development of the brain, which found expression in the enormous growth
of the skull. Of this remarkable family there are only a few genera
to-day, and we have already dealt with them (Chapter XV)—the gibbon
(_Hylobates,_ Fig. 203) and orang (_Satyrus,_ Figs. 204, 205) in
South-Eastern Asia and the Archipelago; and the chimpanzee
(_Anthropithecus,_ Figs. 206, 207) and gorilla (_Gorilla,_ Fig. 208) in
Equatorial Africa.

The great interest that every thoughtful man takes in these nearest
relatives of ours has found expression recently in a fairly large
literature. The most distinguished of these works for impartial
treatment of the question of affinity is Robert Hartmann’s little work
on _The Anthropoid Apes._ Hartmann divides the primate order into two
families: (1) _ Primarii_ (man and the anthropoid apes); and (2) _
Simianæ_ (true apes, Catarrhines and Platyrrhines). Professor Klaatsch,
of Heidelberg, has advanced a different view in his interesting and
richly illustrated work on _The Origin and Development of the Human_
_Race._ This is a substantial supplement to my _Anthropogeny,_ in so
far as it gives the chief results of modern research on the early
history of man and civilisation. But when Klaatsch declares the descent
of man from the apes to be “irrational, narrow-minded, and false,” in
the belief that we are thinking of some living species of ape, we must
remind him that no competent scientist has ever held so narrow a view.
All of us look merely—in the sense of Lamarck and Darwin—to the
original unity (admitted by Klaatsch) of the primate stem. This common
descent of all the Primates (men, apes, and lemurs) from one primitive
stem-form, from which the most far-reaching conclusions follow for the
whole of anthropology and philosophy, is admitted by Klaatsch as well
as by myself and all other competent zoologists who accept the theory
of evolution in general. He says explicitly (p. 172): “The three
anthropoid apes—gorilla, chimpanzee, and orang—seem to be branches from
a common root, and this was not far from that of the gibbon and man.”
That is in the main the opinion that I have maintained (especially
against Virchow) in a number of works ever since 1866. The hypothetical
common ancestor of all the Primates, which must have lived in the
earliest Tertiary period (more probably in the Cretaceous), was called
by me _Archiprimus_; Klaatsch now calls it _Primatoid._ Dubois has
proposed the appropriate name of _Prothylobates_ for the common and
much younger stem-form of the anthropomorpha (man and the anthropoid
apes). The actual _ Hylobates_ is nearer to it than the other three
existing anthropoids. None of these can be said to be absolutely the
most man-like. The gorilla comes next to man in the structure of the
hand and foot, the chimpanzee in the chief features of the skull, the
orang in brain development, and the gibbon in the formation of the
chest. None of these existing anthropoid apes is among the direct
ancestors of our race; they are scattered survivors of an ancient
branch of the Catarrhines, from which the human race developed in a
particular direction.


Fig.283. Skull of the fossil ape-man of Java (Pithecanthropus erectus),
restored by Eugen Dubois. Fig. 283—Skull of the fossil ape-man of Java
(_Pithecanthropus erectus_), restored by _Eugen Dubois._


Although man is directly connected with this anthropoid family and
originates from it, we may assign an important intermediate form
between the _Prothylobates_ and him (the twenty-ninth stage in our
ancestry), the ape-men (_Pithecanthropi_). I gave this name in the
_History of Creation_ to the “speechless primitive men” (_Alali_),
which were men in the ordinary sense as far as the general structure is
concerned (especially in the differentiation of the limbs), but lacked
one of the chief human characteristics, articulate speech and the
higher intelligence that goes with it, and so had a less developed
brain. The phylogenetic hypothesis of the organisation of this
“ape-man” which I then advanced was brilliantly confirmed twenty-four
years afterwards by the famous discovery of the fossil _Pithecanthropus
erectus_ by Eugen Dubois (then military surgeon in Java, afterwards
professor at Amsterdam). In 1892 he found at Trinil, in the residency
of Madiun in Java, in Pliocene deposits, certain remains of a large and
very man-like ape (roof of the skull, femur, and teeth), which he
described as “an erect ape-man” and a survivor of a “stem-form of man”
(Fig. 283). Naturally, the Pithecanthropus excited the liveliest
interest, as the long-sought transitional form between man and the ape:
we seemed to have found “the missing link.” There were very interesting
scientific discussions of it at the last three International Congresses
of Zoology (Leyden, 1895, Cambridge, 1898, and Berlin, 1901). I took an
active part in the discussion at
Cambridge, and may refer the reader to the paper I read there on “The
Present Position of Our Knowledge of the Origin of Man” (translated by
Dr. Gadow with the title of _ The Last Link_).

An extensive and valuable literature has grown up in the last ten years
on the Pithecanthropus and the pithecoid theory connected with it. A
number of distinguished anthropologists, anatomists, paleontologists,
and phylogenists have taken part in the controversy, and made use of
the important data furnished by the new science of pre-historic
research. Hermann Klaatsch has given a good summary of them, with many
fine illustrations, in the above-mentioned work. I refer the reader to
it as a valuable supplement to the present work, especially as I cannot
go any further here into these anthropological and pre-historic
questions. I will only repeat that I think he is wrong in the attitude
of hostility that he affects to take up with regard to my own views on
the descent of man from the apes.

The most powerful opponent of the pithecoid theory—and the theory of
evolution in general—during the last thirty years (until his death in
September, 1902) was the famous Berlin anatomist, Rudolf Virchow. In
the speeches which he delivered every year at various congresses and
meetings on this question, he was never tired of attacking the hated
“ape theory.” His constant categorical position was: “It is quite
certain that man does not descend from the ape or any other animal.”
This has been repeated incessantly by opponents of the theory,
especially theologians and philosophers. In the inaugural speech that
he delivered in 1894 at the Anthropological Congress at Vienna, he said
that “man might just as well have descended from a sheep or an elephant
as from an ape.” Absurd expressions like this only show that the famous
pathological anatomist, who did so much for medicine in the
establishment of cellular pathology, had not the requisite attainments
in comparative anatomy and ontogeny, systematic zoology and
paleontology, for sound judgment in the province of anthropology. The
Strassburg anatomist, Gustav Schwalbe, deserved great praise for having
the moral courage to oppose this dogmatic and ungrounded teaching of
Virchow, and showing its untenability. The recent admirable works of
Schwalbe on the Pithecanthropus, the earliest races of men, and the
Neanderthal skull (1897–1901) will supply any candid and judicious
reader with the empirical material with which he can convince himself
of the baselessness of the erroneous dogmas of Virchow and his clerical
friends (J. Ranke, J. Bumüller, etc.).

As the Pithecanthropus walked erect, and his brain (judging from the
capacity of his skull, Fig. 283) was midway between the lowest men and
the anthropoid apes, we must assume that the next great step in the
advance from the Pithecanthropus to man was the further development of
human speech and reason.

Comparative philology has recently shown that human speech is
polyphyletic in origin; that we must distinguish several (probably
many) different primitive tongues that were developed independently.
The evolution of language also teaches us (both from its ontogeny in
the child and its phylogeny in the race) that human speech proper was
only gradually developed after the rest of the body had attained its
characteristic form. It is probable that language was not evolved until
after the dispersal of the various species and races of men, and this
probably took place at the commencement of the Quaternary or Diluvial
period. The speechless ape-men or _Alali_ certainly existed towards the
end of the Tertiary period, during the Pliocene, possibly even the
Miocene, period.

The third, and last, stage of our animal ancestry is the true or
speaking man (_Homo_), who was gradually evolved from the preceding
stage by the advance of animal language into articulate human speech.
As to the time and place of this real “creation of man” we can only
express tentative opinions. It was probably during the Diluvial period
in the hotter zone of the Old World, either on the mainland in tropical
Africa or Asia or on an earlier continent (Lemuria—now sunk below the
waves of the Indian Ocean), which stretched from East Africa
(Madagascar, Abyssinia) to East Asia (Sunda Islands, Further India). I
have given fully in my _History of Creation,_ (chapter xxviii) the
weighty reasons for claiming this descent of man from the anthropoid
eastern apes, and shown how we may conceive the spread of the various
races from this “Paradise” over the whole earth. I have also dealt
fully with the relations of the various races and species of men to
each other.

SYNOPSIS OF THE CHIEF SECTIONS OF OUR STEM-HISTORY

First Stage: The Protists


Man’s ancestors are unicellular protozoa, originally unnucleated Monera
like the Chromacea, structureless green particles of plasm; afterwards
real nucleated cells (first plasmodomous _Protophyta,_ like the
Palmella; then plasmophagous _Protozoa,_ like the Amœba).

Second Stage: The Blastæads


Man’s ancestors are round cœnobia or colonies of Protozoa; they consist
of a close association of many homogeneous cells, and thus are
individuals of the second order. They resemble the round
cell-communities of the Magospheræ and Volvocina, equivalent to the
ontogenetic blastula: hollow globules, the wall of which consists of a
single layer of ciliated cells (blastoderm).

Third Stage: The Gastræads

Man’s ancestors are Gastræads, like the simplest of the actual Metazoa
(Prophysema, Olynthus, Hydra, Pemmatodiscus). Their body consists
merely of a primitive gut, the wall of which is made up of the two
primary germinal layers.

Fourth Stage: The Platodes


Man’s ancestors have substantially the organisation of simple Platodes
(at first like the cryptocœlic Platodaria, later like the rhabdocœlic
Turbellaria). The leaf-shaped bilateral-symmetrical body has only one
gut-opening, and develops the first trace of a nervous centre from the
ectoderm in the middle line of the back (Figs. 239, 240).

Fifth Stage: The Vermalia


Man’s ancestors have substantially the organisation of unarticulated
Vermalia, at first Gastrotricha (Ichthydina), afterwards Frontonia
(Nemertina, Enteropneusta). Four secondary germinal layers develop, two
middle layers arising between the limiting layers (cœloma). The dorsal
ectoderm forms the vertical plate, acroganglion (Fig. 243).

Sixth Stage: The Prochordonia


Man’s ancestors have substantially the organisation of a simple
unarticulated Chordonium (Copelata and Ascidia-larvæ). The unsegmented
chorda develops between the dorsal medullary tube and the ventral
gut-tube. The simple cœlom-pouches divide by a frontal septum into two
on each side; the dorsal pouch (episomite) forms a muscle-plate; the
ventral pouch (hyposomite) forms a gonad. Head-gut with gill-clefts.

Seventh Stage: The Acrania


Man’s ancestors are skull-less Vertebrates, like the Amphioxus. The
body is a series of metamera, as several of the primitive segments are
developed. The head contains in the ventral half the branchial gut, the
trunk the hepatic gut. The medullary tube is still simple. No skull,
jaws, or limbs.

Eighth Stage: The Cyclostoma


Man’s ancestors are jaw-less Craniotes (like the Myxinoida and
Petromyzonta). The number of metamera increases. The fore-end of the
medullary tube expands into a vesicle and forms the brain, which soon
divides into five cerebral vesicles. In the sides of it appear the
three higher sense-organs: nose, eyes, and auditory vesicles. No jaws,
limbs, or floating bladder.

Ninth Stage: The Ichthyoda


Man’s ancestors are fish-like Craniotes: (1) Primitive fishes
(Selachii); (2) plated fishes (Ganoida); (3) amphibian fishes
(Dipneusta); (4) mailed amphibia (Stegocephala). The ancestors of this
series develop two pairs of limbs: a pair of fore (breast-fins) and of
hind (belly-fins) legs. The gill-arches are formed between the
gill-clefts: the first pair form the maxillary arches (the upper and
lower jaws). The floating bladder (lung) and pancreas grow out of the
gut.

Tenth Stage: The Amniotes


Man’s ancestors are Amniotes or gill-less Vertebrates: (1) Primitive
Amniotes (Proreptilia); (2) Sauromammals; (3) Primitive Mammals
(Monotremes); (4) Marsupials; (5) Lemurs (Prosimiæ); (6) Western apes
(Platyrrhinæ); (7) Eastern apes (Catarrhinæ): at first tailed
Cynopitheca; then tail-less anthropoids; later speechless ape-men
(Alali); finally speaking man. The ancestors of these Amniotes develop
an amnion and allantois, and gradually assume the mammal, and finally
the specifically human, form.



Chapter XXIV.
EVOLUTION OF THE NERVOUS SYSTEM


The previous chapters have taught us how the human body as a whole
develops from the first simple rudiment, a single layer of cells. The
whole human race owes its origin, like the individual man, to a simple
cell. The unicellular stem-form of the race is reproduced daily in the
unicellular embryonic stage of the individual. We have now to consider
in detail the evolution of the various parts that make up the human
frame. I must, naturally, confine myself to the most general and
principal outlines; to make a special study of the evolution of each
organ and tissue is both beyond the scope of this work, and probably
beyond the anatomic capacity of most of my readers to appreciate. In
tracing the evolution of the various organs we shall follow the method
that has hitherto guided us, except that we shall now have to consider
the ontogeny and phylogeny of the organs together. We have seen, in
studying the evolution of the body as a whole, that phylogeny casts a
light over the darker paths of ontogeny, and that we should be almost
unable to find our way in it without the aid of the former. We shall
have the same experience in the study of the organs in detail, and I
shall be compelled to give simultaneously their ontogenetic and
phylogenetic origin. The more we go into the details of organic
development, and the more closely we follow the rise of the various
parts, the more we see the inseparable connection of embryology and
stem-history. The ontogeny of the organs can only be understood in the
light of their phylogeny, just as we found of the embryology of the
whole body. Each embryonic form is determined by a corresponding
stem-form. This is true of details as well as of the whole.

We will consider first the animal and then the vegetal systems of
organs of the body. The first group consists of the psychic and the
motor apparatus. To the former belong the skin, the nervous system, and
the sense-organs. The motor apparatus is composed of the passive and
the active organs of movement (the skeleton and the muscles). The
second or vegetal group consists of the nutritive and the reproductive
apparatus. To the nutritive apparatus belong the alimentary canal with
all its appendages, the vascular system, and the renal (kidney) system.
The reproductive apparatus comprises the different organs of sex
(embryonic glands, sexual ducts, and copulative organs).

As we know from previous chapters (XI–XIII), the animal systems of
organs (the organs of sensation and presentation) develop for the most
part out of the _outer_ primary germ-layer, or the cutaneous (skin)
layer. On the other hand, the vegetal systems of organs arise for the
most part from the _inner_ primary germ-layer, the visceral layer. It
is true that this antithesis of the animal and vegetal spheres of the
body in man and all the higher animals is by no means rigid; several
parts of the animal apparatus (for instance, the greater part of the
muscles) are formed from cells that come originally from the entoderm;
and a great part of the vegetative apparatus (for instance, the
mouth-cavity and the gonoducts) are composed of cells that come from
the ectoderm.

In the more advanced animal body there is so much interlacing and
displacement of the various parts that it is often very difficult to
indicate the sources of them. But, broadly speaking, we may take it as
a positive and important fact that in man and the higher animals the
chief part of the animal organs comes from the ectoderm, and the
greater part of the vegetative organs from the entoderm. It was for
this reason that Carl Ernst von Baer called the one the animal and the
other the vegetative layer (see p. 16).

The solid foundation of this important thesis is the _gastrula,_ the
most instructive embryonic form in the animal world, which we still
find in the same shape in the most diverse classes of animals. This
form points demonstrably to a
common stem-form of all the Metazoa, the _Gastræa;_ in this
long-extinct stem-form the whole body consisted throughout life of the
two primary germinal layers, as is now the case temporarily in the
gastrula; in the Gastræa the simple cutaneous (skin) layer _actually_
represented all the animal organs and functions, and the simple
visceral (gut) layer all the vegetal organs and functions. This is the
case with the modern Gastræads (Fig. 233); and it is also the case
potentially with the gastrula.

We shall easily see that the gastræa theory is thus able to throw a
good deal of light, both morphologically and physiologically, on some
of the chief features of embryonic development, if we take up first the
consideration of the chief element in the animal sphere, the psychic
apparatus or sensorium and its evolution. This apparatus consists of
two very different parts, which seem at first to have very little
connection with each other—the outer skin, with all its hairs, nails,
sweat-glands, etc., and the nervous system. The latter comprises the
central nervous system (brain and spinal cord), the peripheral,
cerebral, and spinal nerves, and the sense-organs. In the fully-formed
vertebrate body these two chief elements of the sensorium lie far
apart, the skin being external to, and the central nervous system in
the very centre of, the body. The one is only connected with the other
by a section of the peripheral nervous system and the sense-organs.
Nevertheless, as we know from human embryology, the medullary tube is
formed from the cutaneous layer. The organs that discharge the most
advanced functions of the animal body—the organs of the soul, or of
psychic life—develop from the external skin. This is a perfectly
natural and necessary process. If we reflect on the historical
evolution of the psychic and sensory functions, we are forced to
conclude that the cells which accomplish them must originally have been
located on the outer surface of the body. Only elementary organs in
this superficial position could directly receive the influences of the
environment. Afterwards, under the influence of natural selection, the
cellular group in the skin which was specifically “sensitive” withdrew
into the inner and more protected part of the body, and formed there
the foundation of a central nervous organ. As a result of increased
differentiation, the skin and the central nervous system became further
and further separated, and in the end the two were only permanently
connected by the afferent peripheral sensory nerves.


Fig.244. The human skin in vertical section. Fig. 284—The human skin in
vertical section (from _Ecker_), highly magnified, _a_ horny layer of
the epidermis, _b_ mucous layer of the epidermis, _c_ papillæ of the
corium, _d_ blood-vessels of same, _ef_ ducts of the sweat-glands
(_g_), _h_ fat-glands in the corium, _i_ nerve, passing into a tactile
corpuscle above.


The observations of the comparative anatomist are in complete accord
with this view. He tells us that large numbers of the lower animals
have no nervous system, though they exercise the functions of sensation
and will like the higher animals. In the unicellular Protozoa, which do
not form germinal layers, there is, of course, neither nervous system
nor skin. But in the second division of the animal kingdom also, the
Metazoa, there is at first no nervous system. Its functions are
represented by the simple cell-layer of the ectoderm, which the lower
Metazoa have inherited from the Gastræa (Fig. 30 _e_). We find this in
the lowest Zoophytes—the Gastræads, Physemaria, and Sponges (Figs.
233–238). The lowest Cnidaria (the hydroid polyps) also are little
superior to the Gastræads in structure. Their vegetative functions are
accomplished by the simple visceral layer, and their animal functions
by the simple cutaneous layer. In these
cases the simple cell-layer of the ectoderm is at once skin, locomotive
apparatus, and nervous system.


Fig.285. Epidermic cells of a human embryo of two months. Fig.
285—Epidermic cells of a human embryo of two months. (From _Kölliker._)


When we come to the higher Metazoa, in which the sensory functions and
their organs are more advanced, we find a division of labour among the
ectodermic cells. Groups of sensitive nerve cells separate from the
ordinary epidermic cells; they retire into the more protected tissue of
the mesodermic under-skin, and form special neural ganglia there. Even
in the Platodes, especially the _Turbellaria,_ we find an independent
nervous system, which has separated from the outer skin. This is the
“upper pharyngeal ganglion,” or _acroganglion,_ situated above the
gullet (Fig. 241 _g_).From this rudimentary structure has been
developed the elaborate central nervous system of the higher animals.
In some of the higher worms, such as the earth-worm, the first rudiment
of the central nervous system (Fig. 74 _n_) is a local thickening of
the skin-sense layer (_hs_), which afterwards separates altogether from
the horny plate. In the earliest Platodes (_Cryptocœla_) and Vermalia
(_Gastrotricha_) the acroganglion remains in the epidermis. But the
medullary tube of the Vertebrates originates in the same way. Our
embryology has taught us that this first structure of the central
nervous system also develops originally from the outer germinal layer.

Let us now examine more closely the evolution of the human skin, with
its various appendages, the hairs and glands. This external covering
has, physiologically, a double and important part to play. It is, in
the first place, the common integument that covers the whole surface of
the body, and forms a protective envelope for the other organs. As such
it also effects a certain exchange of matter between the body and the
surrounding atmosphere (exhalation, perspiration). In the second place,
it is the earliest and original sense organ, the common organ of
feeling that experiences the sensation of the temperature of the
environment and the pressure or resistance of bodies that come into
contact.

The human skin (like that of all the higher animals) is composed of two
layers, the outer and the inner or underlying skin. The outer skin or
_epidermis,_ consists of simple ectodermic cells, and contains no
blood-vessels (Fig. 284 _a, b_). It develops from the outer germinal
layer, or skin-sense layer. The underlying skin (_corium_ or
_hypodermis_) consists chiefly of connective tissue, contains numerous
blood-vessels and nerves, and has a totally different origin. It comes
from the outermost parietal stratum of the middle germinal layer, or
the skin-fibre layer. The corium is much thicker than the epidermis. In
its deeper strata (the _subcutis_) there are clusters of fat-cells
(Fig. 284 _h_). Its uppermost stratum (the cutis proper, or the
papillary stratum) forms, over almost the whole surface of the body, a
number of conical microscopic papillæ (something like warts), which
push into the overlying epidermis (_c_). These tactile or sensory
particles contain the finest sensory organs of the skin, the touch
corpuscles. Others contain merely end-loops of the blood-vessels that
nourish the skin (_c, d_). The various parts of the corium arise by
division of labour from the originally homogeneous cells of the
cutis-plate, the outermost lamina of the mesodermic skin-fibre layer
(Fig. 145 _hpr,_ and Figs. 161, 162 _cp_).

In the same way, all the parts and appendages of the epidermis develop
by differentiation from the homogeneous cells of this horny plate (Fig.
285). At an early stage the simple cellular layer of this horny plate
divides into two. The inner and softer stratum (Fig. 284 _b_) is known
as the mucous stratum, the outer and harder (_a_) as the horny
(corneous) stratum. This horny layer is being constantly used up and
rubbed away at the surface; new layers of cells grow up in their place
out of the underlying mucous stratum. At first the epidermis is a
simple covering of the surface of the body. Afterwards various
appendages develop from it, some internally, others externally. The
internal appendages are the cutaneous glands—sweat, fat, etc.
The external appendages are the hairs and nails.

The cutaneous glands are originally merely solid cone-shaped growths of
the epidermis, which sink into the underlying corium (Fig. 286 _1_).
Afterwards a canal (_2, 3_) is formed inside them, either by the
softening and dissolution of the central cells or by the secretion of
fluid internally. Some of the glands, such as the sudoriferous, do not
ramify (Fig. 284 _efg_). These glands, which secrete the perspiration,
are very long, and have a spiral coil at the end, but they never
ramify; so also the wax-glands of the ears. Most of the other cutaneous
glands give out buds and ramify; thus, for instance, the lachrymal
glands of the upper eye-lid that secrete tears (Fig. 286), and the
sebaceous glands which secrete the fat in the skin and generally open
into the hair-follicles. Sudoriferous and sebaceous glands are found
only in mammals. But we find lachrymal glands in all the three classes
of Amniotes—reptiles, birds, and mammals. They are wanting in the lower
aquatic vertebrates.


Fig.286. Rudimentary lachrymal glands from a human embryo of four
months. Fig. 286—Rudimentary lachrymal glands from a human embryo of
four months. (From _Kölliker._) _1_ earliest structure, in the shape of
a simple solid cone, _2_ and _3_ more advanced structures, ramifying
and hollowing out. _a_ solid buds, _e_ cellular coat of the hollow
buds, _f_ structure of the fibrous envelope, which afterwards forms the
corium about the glands.


The mammary glands (Figs. 287, 288) are very remarkable; they are found
in all mammals, and in these alone. They secrete the milk for the
feeding of the new-born mammal. In spite of their unusual size these
structures are nothing more than large sebaceous glands in the skin.
The milk is formed by the liquefaction of the fatty milk-cells inside
the branching mammary-gland tubes (Fig. 287 _c_), in the same way as
the skin-grease or hair-fat, by the solution of fatty cells inside the
sebaceous glands. The outlets of the mammary glands enlarge and form
sac-like mammary ducts (_b_); these narrow again (_a_), and open in the
teats or nipples of the breast by sixteen to twenty-four fine
apertures. The first structure of this large and elaborate gland is a
very simple cone in the epidermis, which penetrates into the corium and
ramifies. In the new-born infant it consists of twelve to eighteen
radiating lobes (Fig. 288). These gradually ramify, their ducts become
hollow and larger, and rich masses of fat accumulate between the lobes.
Thus is formed the prominent female breast (_mamma_), on the top of
which rises the teat or nipple (_mammilla_). The latter is only
developed later on, when the mammary gland is fully-formed; and this
ontogenetic phenomenon is extremely interesting, because the earlier
mammals (the stem-forms of the whole class) have no teats. In them the
milk comes out through a flat portion of the ventral skin that is
pierced like a sieve, as we still find in the lowest living mammals,
the oviparous Monotremes of Australia. The young animal licks the milk
from the mother instead of sucking it. In many of the lower mammals we
find a number of milk-glands at different parts of the ventral surface.
In the human female there is usually only one pair of glands, at the
breast; and it is the same with the apes, bats, elephants, and several
other mammals. Sometimes, however, we find two successive pairs of
glands (or even more) in the human female. Some women have four or five
pairs of breasts, like pigs and hedgehogs (Fig. 103). This polymastism
points back to an older stem-form. We often find these accessory
breasts in the male also (Fig. 103 _D_). Sometimes, moreover, the
normal mammary glands are fully developed and can suckle in the male;
but as a rule they are merely rudimentary organs without functions in
the male. We have already (Chapter XI) dealt with this remarkable and
interesting instance of atavism.

While the cutaneous glands are inner growths of the epidermis, the
appendages
which we call hairs and nails are external local growths in it. The
nails (_Ungues_) which form important protective structures on the back
of the most sensitive parts of our limbs, the tips of the fingers and
toes, are horny growths of the epidermis, which we share with the apes.
The lower mammals usually have claws instead of them; the ungulates,
hoofs. The stem-form of the mammals certainly had claws; we find them
in a rudimentary form even in the salamander. The horny claws are
highly developed in most of the reptiles (Fig. 264), and the mammals
have inherited them from the earliest representatives of this class,
the stem-reptiles (_Tocosauria_). Like the hoofs (_ungulæ_) of the
Ungulates, the nails of apes and men have been evolved from the claws
of the older mammals. In the human embryo the first rudiment of the
nails is found (between the horny and the mucous stratum of the
epidermis) in the fourth month. But their edges do not penetrate
through until the end of the sixth month.


Fig.287. The female breast (mamma) in vertical section. Fig. 287—The
female breast (_mamma_) in vertical section. _c_ racemose glandular
lobes, _b_ enlarged milk-ducts, a narrower outlets, which open into the
nipple. (From _H. Meyer._)


The most interesting and important appendages of the epidermis are the
hairs; on account of their peculiar composition and origin we must
regard them as highly characteristic of the whole mammalian class. It
is true that we also find hairs in many of the lower animals, such as
insects and worms. But these hairs, like the hairs of plants, are
thread-like appendages of the surface, and differ entirely from the
hairs of the mammals in the details of their structure and development.

The embryology of the hairs is known in all its details, but there are
two different views as to their phylogeny. On the older view the hairs
of the mammals are equivalent or homologous to the feathers of the bird
or the horny scales of the reptile. As we deduce all three classes of
Amniotes from a common stem-group, we must assume that these Permian
stem-reptiles had a complete scaly coat, inherited from their
Carboniferous ancestors, the mailed amphibia (_Stegocephala_); the bony
scales of their corium were covered with horny scales. In passing from
aquatic to terrestrial life the horny scales were further developed,
and the bony scales degenerated in most of the reptiles. As regards the
bird’s feathers, it is certain that they are modifications of the horny
scales of their reptilian ancestors. But it is otherwise with the hairs
of the mammals. In their case the hypothesis has lately been advanced
on the strength of very extensive research, especially by Friedrich
Maurer, that they have been evolved from the cutaneous sense-organs of
amphibian ancestors by modification of functions; the epidermic
structure is very similar in both in its embryonic rudiments. This
modern view, which had the support of the greatest expert on the
vertebrates, Carl Gegenbaur, can be harmonised with the older theory to
an extent, in the sense that both formations, scales and hairs, were
very closely connected originally. Probably the conical budding of the
skin-sense layer grew up _under the protection of the horny scale,_ and
became an organ of touch subsequently by the cornification of the
hairs; many hairs are still sensory organs (tactile hairs on the muzzle
and cheeks of many mammals: pubic hairs).

This middle position of the genetic connection of scales and hairs was
advanced in my _Systematic Phylogeny of the Vertebrates_ (p. 433). It
is confirmed by the similar arrangement of the two cutaneous
formations. As Maurer pointed out, the hairs, as well as the cutaneous
sense-organs and the scales, are at first arranged in regular
longitudinal series, and they afterwards break into alternate groups.
In the embryo of a bear two
inches long, which I owe to the kindness of Herr von Schmertzing (of
Arva Varallia, Hungary), the back is covered with sixteen to twenty
alternating longitudinal rows of scaly protuberances (Fig. 289). They
are at the same time arranged in regular transverse rows, which
converge at an acute angle from both sides towards the middle of the
back. The tip of the scale-like wart is turned inwards. Between these
larger hard scales (or groups of hairs) we find numbers of rudimentary
smaller hairs.

The human embryo is, as a rule, entirely clothed with a thick coat of
fine wool during the last three or four weeks of gestation. This
embryonic woollen coat (_Lanugo_) generally disappears in part during
the last weeks of fœtal life but in any case, as a rule, it is lost
immediately after birth, and is replaced by the thinner coat of the
permanent hair. These permanent hairs grow out of hair-follicles, which
are formed from the root-sheaths of the disappearing wool-fibres. The
embryonic wool-coat usually, in the case of the human embryo, covers
the whole body, with the exception of the palms of the hands and soles
of the feet. These parts are always bare, as in the case of apes and of
most other mammals. Sometimes the wool-coat of the embryo has a
striking effect, by its colour, on the later permanent hair-coat. Hence
it happens occasionally, for instance, among our Indo-Germanic races,
that children of blond parents seem—to the dismay of the latter—to be
covered at birth with a dark brown or even a black woolly coat. Not
until this has disappeared do we see the permanent blond hair which the
child has inherited. Sometimes the darker coat remains for weeks, and
even months, after birth. This remarkable woolly coat of the human
embryo is a legacy from the apes, our ancient long-haired ancestors.


Fig.288. Mammary gland of a new-born infant. Fig. 288—Mammary gland of
a new-born infant, _a_ original central gland, _b_ small and _c_ large
buds of same. (From _Langer._)


It is not less noteworthy that many of the higher apes approach man in
the thinness of the hair on various parts of the body. With most of the
apes, especially the higher Catarrhines (or narrow-nosed apes), the
face is mostly, or entirely, bare, or at least it has hair no longer or
thicker than that of man. In their case, too, the back of the head is
usually provided with a thicker growth of hair; this is lacking,
however, in the case of the bald-headed chimpanzee (_Anthropithecus
calvus_). The males of many species of apes have a considerable beard
on the cheeks and chin; this sign of the masculine sex has been
acquired by sexual selection. Many species of apes have a very thin
covering of hair on the breast and the upper side of the limbs—much
thinner than on the back or the under side of the limbs. On the other
hand, we are often astonished to find tufts of hair on the shoulders,
back, and extremities of members of our Indo-Germanic and of the
Semitic races. Exceptional hair on the face, as on the whole body, is
hereditary in certain families of hairy men. The quantity and the
quality of the hair on head and chin are also conspicuously transmitted
in families. These extraordinary variations in the total and partial
hairy coat of the body, which are so noticeable, not only in comparing
different races of men, but also in comparing different families of the
same race, can only be explained on the assumption that in man the
hairy coat is, on the whole, a rudimentary organ, a useless inheritance
from the more thickly-coated apes. In this man resembles the elephant,
rhinoceros, hippopotamus, whale, and other mammals of various orders,
which have also, almost entirely or for the most part, lost their hairy
coats by adaptation.

The particular process of adaptation by which man lost the growth of
hair on most parts of his body, and retained or augmented it at some
points, was most probably sexual selection. As Darwin luminously showed
in his _Descent of Man,_ sexual selection has been very active
in this respect. As the male anthropoid apes chose the females with the
least hair, and the females favoured the males with the finest growths
on chin and head, the general coating of the body gradually
degenerated, and the hair of the beard and head was more strongly
developed. The growth of hair at other parts of the body (arm-pit,
pubic region) was also probably due to sexual selection. Moreover,
changes of climate, or habits, and other adaptations unknown to us, may
have assisted the disappearance of the hairy coat.


Fig.289. Embryo of a bear (Ursus arctos). Fig. 289—Embryo of a bear
(_Ursus arctos_). _A_ seen from ventral side, _B_ from the left.


The fact that our coat of hair is inherited directly from the
anthropoid apes is proved in an interesting way, according to Darwin,
by the direction of the rudimentary hairs on our arms, which cannot be
explained in any other way. Both on the upper and the lower part of the
arm they point towards the elbow. Here they meet at an obtuse angle.
This curious arrangement is found only in the anthropoid apes—gorilla,
chimpanzee, orang, and several species of gibbons—besides man (Figs.
203, 207). In other species of gibbon the hairs are pointed towards the
hand both in the upper and lower arm, as in the rest of the mammals. We
can easily explain this remarkable peculiarity of the anthropoids and
man on the theory that our common ancestors were accustomed (as the
anthropoid apes are to-day) to place their hands over their heads, or
across a branch above their heads, during rain. In this position, the
fact that the hairs point downwards helps the rain to run off. Thus the
direction of the hair on the lower part of our arm reminds us to-day of
that useful custom of our anthropoid ancestors.

The nervous system in man and all the other Vertebrates is, when fully
formed, an extremely complex apparatus, that we may compare, in
anatomic structure and physiological function, with an extensive
telegraphic system. The chief station of
the system is the central marrow or central nervous system, the
innumerable ganglionic cells or _neurona_ (Fig. 9)of which are
connected by branching processes with each other and with numbers of
very fine conducting wires. The latter are the peripheral and
ubiquitous nerve-fibres; with their terminal apparatus, the
sense-organs, etc., they constitute the conducting marrow or peripheral
nervous system. Some of them—the sensory nerve-fibres—conduct the
impressions from the skin and other sense-organs to the central marrow;
others—the motor nerve-fibres—convey the commands of the will to the
muscles.

The central nervous system or central marrow (_medulla centralis_) is
the real organ of psychic action in the narrower sense. However we
conceive the intimate connection of this organ and its functions, it is
certain that its characteristic actions, which we call sensation, will,
and thought, are inseparably dependent on the normal development of the
material organ in man and all the higher animals. We must, therefore,
pay particular attention to the evolution of the latter. As it can give
us most important information regarding the nature of the “soul,” it
should be full of interest. If the central marrow develops in just the
same way in the human embryo as in the embryo of the other mammals, the
evolution of the human psychic organ from the central organ of the
other mammals, and through them from the lower vertebrates, must be
beyond question. No one can doubt the momentous bearing of these
embryonic phenomena.


CaFig.290. Human embryo, three months old, from the dorsal side: brain
and spinal cord exposed. Fig. 291. Central marrow of a human embryo,
four months old, from the back.nyon Fig. 290—Human embryo, three months
old, from the dorsal side: brain and spinal cord exposed. (From
_Kölliker._) _h_ cerebral hemispheres (fore brain), _m_ corpora
quadrigemina (middle brain), _c_ cerebellum (hind brain): under the
latter is the triangular medulla oblongata (after brain). Fig.
291—Central marrow of a human embryo, four months old, from the back.
(From _Kölliker._) _h_ large hemispheres, _v_ quadrigemina, _c_
cerebellum, _mo_ medulla oblongata: underneath it the spinal cord.


In order to understand them fully we must first say a word or two of
the general form and the anatomic composition of the mature human
central marrow. Like the central nervous system of all the other
Craniotes, it consists of two parts, the head-marrow or brain (_medulla
capitis_ or _encephalon_) and the spinal-marrow (_medulla spinalis_ or
_notomyelon_). The one is enclosed in the bony skull, the other in the
bony vertebral column. Twelve pairs of cerebral nerves proceed from the
brain, and thirty-one pairs of spinal nerves from the spinal cord, to
the rest of the body (Fig. 171). On general anatomic investigation the
spinal marrow is found to be a cylindrical cord, with a spindle-shaped
bulb both in the region of the neck above (at the last cervical
vertebra) and the region of the loins (at the first lumbar vertebra)
below (Fig. 291). At the cervical bulb the strong nerves of the upper
limbs, and at the lumbar bulb those of the lower limbs, proceed from
the spinal cord. Above, the latter passes into the brain through the
medulla oblongata (Fig. 291 _mo_). The spinal cord seems to be a thick
mass of nervous matter, but it has a narrow canal at its axis, which
passes into the further cerebral ventricles above, and is filled, like
these, with a clear fluid.

The brain is a large nerve-mass, occupying the greater part of the
skull, of most elaborate structure. On general examination it divides
into two parts, the cerebrum and cerebellum. The cerebrum lies in front
and above, and has the familiar characteristic convolutions and furrows
on its surface (Figs. 292, 293). On the upper side it is divided by a
deep longitudinal fissure into two halves, the
cerebral hemispheres; these are connected by the _corpus callosum._ The
large cerebrum is separated from the small cerebellum by a deep
transverse furrow. The latter lies behind and below, and has also
numbers of furrows, but much finer and more regular, with convolutions
between, at its surface. The cerebellum also is divided by a
longitudinal fissure into two halves, the “small hemispheres”; these
are connected by a worm-shaped piece, the _vermis cerebelli,_ above,
and by the broad _pons Varolii_ below (Fig. 292 _VI_).


Fig.292. The human brain, seen from below. Fig. 292—The human brain,
seen from below. (From _H. Meyer._) Above (in front) is the cerebrum
with its extensive branching furrows; below (behind) the cerebellum
with its narrow parallel furrows. The Roman numbers indicate the roots
of the twelve pairs of cerebral nerves in a series towards the rear.


But comparative anatomy and ontogeny teach us that in man and all the
other Craniotes the brain is at first composed, not of these two, but
of three, and afterwards five, consecutive parts. These are found in
just the same form—as five consecutive vesicles—in the embryo of all
the Craniotes, from the Cyclostoma and fishes to man. But, however much
they agree in their rudimentary condition, they differ considerably
afterwards. In man and the higher mammals the first of these
ventricles, the cerebrum, grows so much that in its mature condition it
is by far the largest and heaviest part of the brain. To it belong not
only the large hemispheres, but also the corpus callosum that unites
them, the olfactory lobes, from which the olfactory nerves start, and
most of the structures that are found at the roof and bottom of the
large lateral ventricles inside the two hemispheres, such as the
_corpora striata._ On the other hand, the _optic thalami,_ which lie
between the latter, belong to the second division, which develops from
the “intermediate brain ”; to the same section belong the single third
cerebral ventricle and the structures that are known as the corpora
geniculata, the infundibulum, and the pineal gland. Behind these parts
we find, between the cerebrum and cerebellum, a small ganglion composed
of two prominences, which is called the _corpus quadrigeminum_ on
account of a superficial transverse fissure cutting across (Figs. 290
_m_ and 291 _v_). Although this quadrigeminum is very insignificant in
man and the higher mammals, it forms a special third section, greatly
developed in the lower vertebrates, the “middle brain.” The fourth
section is the “hind-brain” or little brain (cerebellum) in the
narrower sense, with the single median part, the vermis, and the pair
of lateral parts, the “small hemispheres” (Fig. 291 _c_). Finally, we
have the fifth and last section, the medulla oblongata (Fig. 291 _mo_),
which contains the single fourth cerebral cavity and the contiguous
parts (pyramids, olivary bodies, corpora restiformia). The medulla
oblongata passes straight into the medulla spinalis (spinal cord). The
narrow central canal of the spinal cord continues above into the
quadrangular fourth cerebral cavity of the medulla oblongata, the floor
of which is the quadrangular depression. From here a narrow duct,
called “the aqueduct of Sylvius,” passes through the corpus
quadrigeminum to the third cerebral ventricle, which lies between the
two optic thalami; and this in turn is connected with the pairs of
lateral ventricles which lie to the right and left in the large
hemispheres. Thus all the cavities of the central marrow are directly
interconnected. All these parts of the brain have an infinitely complex
structure in detail, but we cannot go into this. Although it is much
more elaborate in man and the higher Vertebrates than in the lower
classes, it develops in them all from the same rudimentary structure,
the five simple cerebral vesicles of the embryonic brain.

But before we consider the development of the complicated structure of
the brain from this simple series of vesicles, let
us glance for a moment at the lower animals, which have no brain. Even
in the skull-less vertebrate, the Amphioxus, we find no independent
brain, as we have seen. The whole central marrow is merely a simple
cylindrical cord which runs the length of the body, and ends equally
simply at both extremities—a plain medullary tube. All that we can
discover is a small vesicular bulb at the foremost part of the tube, a
degenerate rudiment of a primitive brain. We meet the same simple
medullary tube in the first structure of the ascidia larva, in the same
characteristic position, above the chorda. On closer examination we
find here also a small vesicular swelling at the fore end of the tube,
the first trace of a differentiation of it into brain and spinal cord.
It is probable that this differentiation was more advanced in the
extinct Provertebrates, and the brain-bulb more pronounced (Figs.
98–102). The brain is phylogenetically older than the spinal cord, as
the trunk was not developed until after the head. If we consider the
undeniable affinity of the Ascidiæ to the Vermalia, and remember that
we can trace all the Chordonia to lower Vermalia, it seems probable
that the simple central marrow of the former is equivalent to the
simple nervous ganglion, which lies above the gullet in the lower
worms, and has long been known as the “upper pharyngeal ganglion”
(_ganglion pharyngeum superius_); it would be better to call it the
primitive or vertical brain (acroganglion).

Probably this upper pharyngeal ganglion of the lower worms is the
structure from which the complex central marrow of the higher animals
has been evolved. The medullary tube of the Chordonia has been formed
by the lengthening of the vertical brain on the dorsal side. In all the
other animals the central nervous system has been developed in a
totally different way from the upper pharyngeal ganglion; in the
Articulates, especially, a pharyngeal ring, with ventral marrow, has
been added. The Molluscs also have a pharyngeal ring, but it is not
found in the Vertebrates. In these the central marrow has been
prolonged down the dorsal side; in the Articulates down the ventral
side. This fact proves of itself that there is no direct relationship
between the Vertebrates and the Articulates. The unfortunate attempts
to derive the dorsal marrow of the former from the ventral marrow of
the latter have totally failed (cf. p. 219).


Fig.293. The human brain, seen from the left. Fig. 293—The human brain,
seen from the left. (From _H. Meyer._) The furrows of the cerebrum are
indicated by thick, and those of the cerebellum by finer lines. Under
the latter we can see the medulla oblongata. _f1–f2_ frontal
convolutions, _C_ central convolutions, _S_ fissure of Sylvius, _T_
temporal furrow, _Pa_ parietal lobes, _An_ angular gyrus, _Po_
parieto-occipital fissure.


When we examine the embryology of the human nervous system, we must
start from the important fact, which we have already seen, that the
first structure of it in man and all the higher Vertebrates is the
simple medullary tube, and that this separates from the outer germinal
layer in the middle line of the sole-shaped embryonic shield. As the
reader will remember, the straight medullary furrow first appears in
the middle of the sandal-shaped embryonic shield. At each side of it
the parallel borders curve over in the form of dorsal or medullary
swellings. These bend together with their free borders, and thus form
the closed medullary tube (Figs. 133–137). At first this tube lies
directly underneath the horny plate; but it afterwards travels inwards,
the upper edges of the provertebral plates growing together between the
horny plate and the tube, joining above the latter, and forming a
completely closed canal. As Gegenbaur very properly observes, “this
gradual imbedding in the
inner part of the body is a process acquired with the progressive
differentiation and the higher potentiality that this secures; by this
process the organ of greater value to the organism is buried within the
frame.” (Cf. Figs. 143–146).

In the Cyclostoma—a stage above the Acrania—the fore end of the
cylindrical medullary tube begins early to expand into a pear-shaped
vesicle; this is the first outline of an independent brain. In this way
the central marrow of the Vertebrates divides clearly into its two
chief sections, brain and spinal cord. The simple vesicular form of the
brain, which persists for some time in the Cyclostoma, is found also at
first in all the higher Vertebrates (Fig. 153 _hb_). But in these it
soon passes away, the one vesicle being divided into several successive
parts by transverse constrictions. There are first two of these
constrictions, dividing the brain into three consecutive vesicles (fore
brain, middle brain, and hind brain, Fig. 154 _v, m, h_). Then the
first and third are sub-divided by fresh constrictions, and thus we get
five successive sections (Fig. 155).


Fig.294. Central marrow of the human embryo from the seventh week, 4/5
inch long. Fig. 294. The brain from above. Fig. 295. The brain with the
uppermost part of the cord, from the left. Fig. 296. Back view of the
whole embryo: brain and spinal cord exposed. Fig. 294–296—Central
marrow of the human embryo from the seventh week, 4/5 inch long. (From
_Kölliker._) Fig. 294. The brain from above, _v_ fore brain, _z_
intermediate brain, _m_ middle brain, _h_ hind brain, _n_ after brain.
Fig. 2955. The brain with the uppermost part of the cord, from the
left. Fig. 296. Back view of the whole embryo: brain and spinal cord
exposed.


In all the Craniotes, from the Cyclostoma up to man, the same parts
develop from these five original cerebral vesicles, though in very
different ways. The first vesicle, the fore brain (Fig. 155 _v_), forms
by far the largest part of the cerebrum—namely, the large hemispheres,
the olfactory lobes, the corpora striata, the callosum, and the fornix.
From the second vesicle, the intermediate brain (_z_), originate
especially the optic thalami, the other parts that surround the third
cerebral ventricle, and the infundibulum and pineal gland. The third
vesicle, the middle brain (_m_), produces the corpora quadrigemina and
the aqueduct of Sylvius. From the fourth vesicle, the hind brain (_h_),
develops the greater part of the cerebellum—namely, the vermis and the
two small hemispheres. Finally, the fifth vesicle, the after brain
(_n_), forms the medulla oblongata, with the quadrangular pit (the
floor of the fourth ventricle), the pyramids, olivary bodies, etc.

We must certainly regard it as a comparative-anatomical and ontogenetic
fact of the greatest significance that in all the Craniotes, from the
lowest Cyclostomes and fishes up to the apes and man, the brain
develops in just the same way in the embryo. The first rudiment of it
is always a simple vesicular enlargement of the fore end of the
medullary tube. In every case, first three, then five, vesicles develop
from this bulb, and the permanent brain with all its complex anatomic
structures, of so great a variety in the various classes of
Vertebrates, is formed from the five primitive vesicles. When we
compare the mature brain of a fish, an amphibian, a reptile, a bird,
and a mammal, it seems incredible that we can trace the various parts
of these organs, that differ so much internally and externally, to
common types. Yet all these different Craniote brains have started with
the same rudimentary structure. To convince ourselves of this we have
only to compare the corresponding stages of development of the embryos
of these different animals.

This comparison is extremely instructive. If we extend it through the
whole series of the Craniotes, we soon discover this interesting fact:
In the Cyclostomes (the Myxinoida and Petromyzonta), which we have
recognised as the lowest and earliest Craniotes, the whole brain
remains throughout life at a very low stage, which is very brief and
passing in the embryos of the higher Craniotes; they retain the five
original sections of the brain unchanged. In the fishes we find an
essential and considerable modification of the five vesicles; it is
clearly the brain of the Selachii in the first place, and subsequently
the brain of the Ganoids, from which the brain of the rest of the
fishes on the one hand and of the Dipneusts and Amphibia, and through
these of the higher Vertebrates, on the other hand, must be derived. In
the fishes and Amphibia (Fig. 300) there is a preponderant development
of the middle brain, and also the after brain, the first, second, and
fourth sections remaining very primitive. It is just the reverse in the
higher Vertebrates, in which the first and third sections, the cerebrum
and cerebellum, are exceptionally developed; while the middle brain and
after brain remain small. The corpora quadrigemina are mostly covered
by the cerebrum, and the oblongata by the cerebellum. But we find a
number of stages of development within the higher Vertebrates
themselves. From the Amphibia upwards the brain (and with it the
psychic life) develops in two different directions; one of these is
followed by the reptiles and birds, and the other by the mammals. The
development of the first section, the fore brain, is particularly
characteristic of the mammals. It is only in them that the cerebrum
becomes so large as to cover all the other parts of the brain (Figs.
293, 301–304).


Fig.297. Head of a chick embryo (hatched fifty-eight hours), from the
back. Fig. 297—Head of a chick embryo (hatched fifty-eight hours), from
the back. (From _Mihalkovics._) _vw_ anterior wall of the fore brain.
_vh_ its ventricle. _au_ optic vesicles, _mh_ middle brain, _kh_ hind
brain, _nh_ after brain, _hz_ heart (seen from below), _vw_ vitelline
veins, _us_ primitive segment, _rm_ spinal cord.



Fig.298. Brain of three craniote embryos in vertical section. Fig. 299.
Brain of a shark (Scyllium), back view. Fig. 298—Brain of three
craniote embryos in vertical section. _A_ of a shark (_Heptarchus_),
_B_ of a serpent (_Coluber_), _C_ of a goat (_Capra_). _a_ fore brain,
_b_ intermediate brain, _c_ middle brain, _d_ hind brain, _e_ after
brain, _s_ primitive cleft. (From _Gegenbaur._)
Fig. 299—Brain of a shark (_Scyllium_), back view. _g_ fore-brain, _h_
olfactory lobes, which send the large olfactory nerves to the nasal
capsule (_o_), _d_ intermediate brain, _b_ middle brain; behind this
the insignificant structure of the hind brain, _a_ after brain. (From
_Gegenbaur._)


There are also notable variations in the relative position of the
cerebral vesicles. In the lower Craniotes they lie originally almost in
the same plane. When we examine the brain laterally, we can cut through
all five vesicles with a straight line. But in the Amniotes there is a
considerable curve in the brain along with the bending of the head and
neck; the whole of the upper dorsal surface of the brain develops much
more than the under ventral surface. This causes a curve, so that the
parts come to lie as follows: The fore brain is right in front and
below, the intermediate brain a little higher, and the middle brain
highest of all; the hind brain lies a little lower, and the after brain
lower still. We find this only in the Amniotes—the reptiles, birds, and
mammals.

Thus, while the brain of the mammals agrees a good deal in general
growth with that of the birds and reptiles, there are some striking
differences between the two. In the Sauropsids (birds and reptiles) the
middle brain and the middle part of the hind brain are well developed.
In the mammals these parts do not grow, and the fore-brain develops so
much that it overlies the other vesicles. As it continues to grow
towards the rear, it at last covers the whole of the rest of the brain,
and also encloses the middle parts from the sides (Figs. 301–303). This
process is of great importance, because the fore brain is the organ of
the higher psychic life, and in it those functions of the nerve-cells
are discharged which we sum up in
the word “soul.” The highest achievements of the animal body—the
wonderful manifestations of consciousness and the complex molecular
processes of thought—have their seat in the fore brain. We can remove
the large hemispheres, piece by piece, from the mammal without killing
it, and we then see how the higher functions of consciousness, thought,
will, and sensation, are gradually destroyed, and in the end completely
extinguished. If the animal is fed artificially, it may be kept alive
for a long time, as the destruction of the psychic organs by no means
involves the extinction of the faculties of digestion, respiration,
circulation, urination—in a word, the vegetative functions. It is only
conscious sensation, voluntary movement, thought, and the combination
of various higher psychic functions that are affected.


Fig.300. Brain and spinal cord of the frog. Fig. 300—Brain and spinal
cord of the frog. _A_ from the dorsal, _B_ from the ventral side. _a_
olfactory lobes before the (_b_) fore brain, _i_ infundibulum at the
base of the intermediate brain, _c_ middle brain, _d_ hind brain, _s_
quadrangular pit in the after brain, _m_ spinal cord (very short in the
frog), _m_′ roots of the spinal nerves, _t_ terminal fibres of the
spinal cord. (From _Gegenbaur._)


The fore brain, the organ of these functions, only attains this high
level of development in the more advanced Placentals, and thus we have
the simple explanation of the intellectual superiority of the higher
mammals. The soul of most of the lower Placentals is not much above
that of the reptiles, but among the higher Placentals we find an
uninterrupted gradation of mental power up to the apes and man. In
harmony with this we find an astonishing variation in the degree of
development of their fore brain, not only qualitatively, but also
quantitatively. The mass and weight of the brain are much greater in
modern mammals, and the differentiation of its various parts more
important, than in their extinct Tertiary ancestors. This can be shown
paleontologically in any particular order. The brains of the living
ungulates are (relatively to the size of the body) four to six times
(in the highest groups even eight times) as large as those of their
earlier Tertiary ancestors, the well-preserved skulls of which enable
us to determine the size and weight of the brain.


Fig.301. Brain of an ox-embryo, two inches in length.
Fig. 301—Brain of an ox-embryo, two inches in length. (From
_Mihalkovics._) Left view; the lateral wall of the left hemisphere has
been removed, _st_ corpora striata, _ml_ Monro-foramen, _ag_ arterial
plexus, _ah_ Ammon’s horn, _mh_ middle brain, _kh_ cerebellum, _dv_
roof of the fourth ventricle, _bb_ pons Varolii, _na_ medulla
oblongata.


Fig. 302. Brain of a human embryo, twelve weeks old. Fig. 302—Brain of
a human embryo, twelve weeks old. (From _Mihalkovics._) Seen from
behind and above. _ms_ mantle-furrow, _mh_ corpora quadrigemina (middle
brain), _vs_ anterior medullary ala, _kh_ cerebellum, _vv_ fourth
ventricle, _na_ medulla oblongata.


In the lower mammals the surface of the cerebral hemispheres is quite
smooth and level, as in the rabbit (Fig. 304). Moreover, the fore brain
remains so small that it does not cover the middle brain. At a stage
higher the middle
brain is covered, but the hind brain remains free. Finally, in the apes
and man, the latter also is covered by the fore brain. We can trace a
similar gradual development in the fissures and convolutions that are
found on the surface of the cerebrum of the higher mammals (Figs. 292,
293). If we compare different groups of mammals in regard to these
fissures and convolutions, we find that their development proceeds step
by step with the advance of mental life.


Fig.303. Brain of a human embryo, twenty-four weeks old, halved in the
median plane: right hemisphere seen from inside. Fig. 303—Brain of a
human embryo, twenty-four weeks old, halved in the median plane: right
hemisphere seen from inside. (From _Mihalkovics._) _rn_ olfactory
nerve, _tr_ funnel of the intermediate brain, _vc_ anterior commissure,
_ml_ Monro-foramen, _gw_ fornix, _ds_ transparent sheath, _bl_ corpus
callosum, _br_ fissure at its border, _hs_ occipital fissure, _zh_
cuneus, _sf_ occipital transverse fissure, _zb_ pineal gland, _mh_
corpora quadrigemina, _kh_ cerebellum.


Of late years great attention has been paid to this special branch of
cerebral anatomy, and very striking individual differences have been
detected within the limits of the human race. In all human beings of
special gifts and high intelligence the convolutions and fissures are
much more developed than in the average man; and they are more
developed in the latter than in idiots and others of low mental
capacity. There is a similar gradation among the mammals in the
internal structure of the fore brain. In particular the corpus
callosum, that unites the two cerebral hemispheres, is only developed
in the Placentals. Other structures—for instance, in the lateral
ventricles—that seem at first to be peculiar to man, are also found in
the higher apes, and these alone. It was long thought that man had
certain distinctive organs in his cerebrum which were not found in any
other animal. But careful examination has discovered that this is not
the case, but that the characteristic features of the human brain are
found in a rudimentary form in the lower apes, and are more or less
fully developed in the higher apes. Huxley has convincingly shown, in
his _Man’s Place in Nature_ (1863), that the differences in the
formation of the brain within the ape-group constitute a deeper gulf
between the lower and higher apes than between the higher apes and man.


Fig.304. Brain of the rabbit. Fig. 304—Brain of the rabbit. _A_ from
the dorsal, _B_ from the ventral side, _lo_ olfactory lobes, _I_ fore
brain, _h_ hypophysis at the base of the intermediate brain, _III_
middle brain, _IV_ hind brain, _V_ after brain, _2_ optic nerve, _3_
oculo-motor nerve, _5–8_ cerebral nerves. In _A_ the roof of the right
hemisphere (_I_) is removed, so that we can see the corpora striata in
the lateral ventricle. (From _Gegenbaur._)


The comparative anatomy and physiology of the brain of the higher and
lower mammals are very instructive, and give important information in
connection with the chief questions of psychology.

The central marrow (brain and spinal cord) develops from the medullary
tube in man just as in all the other mammals, and the same applies to
the conducting marrow or “peripheral nervous system.” It consists of
the _sensory_ nerves, which conduct centripetally the impressions from
the skin and the sense-organs to the central marrow, and of the _motor_
nerves, which convey centrifugally the movements of the will from the
central marrow to the muscles. All these
peripheral nerves grow out of the medullary tube (Fig. 171), and are,
like it, products of the skin-sense layer.

The complete agreement in the structure and development of the psychic
organs which we find between man and the highest mammals, and which can
only be explained by their common origin, is of profound importance in
the monistic psychology. This is only seen in its full light when we
compare these morphological facts with the corresponding physiological
phenomena, and remember that every psychic action requires the complete
and normal condition of the correlative brain structure for its full
and normal exercise. The very complex molecular movements inside the
neural cells, which we describe comprehensively as “the life of the
soul,” can no more exist in the vertebrate, and therefore in man,
without their organs than the circulation without the heart and blood.
And as the central marrow develops in man from the same medullary tube
as that of the other vertebrates, and as man shares the characteristic
structure of his cerebrum (the organ of thought) with the anthropoid
apes, his psychic life also must have the same origin as theirs.

If we appreciate the full weight of these morphological and
physiological facts, and put a proper phylogenetic interpretation on
the observations of embryology, we see that the older idea of the
personal immortality of the human soul is scientifically untenable.
Death puts an end, in man as in any other vertebrate, to the
physiological function of the cerebral neurona, the countless
microscopic ganglionic cells, the collective activity of which is known
as “the soul.” I have shown this fully in the eleventh chapter of my
_Riddle of the Universe._



Chapter XXV.
EVOLUTION OF THE SENSE-ORGANS


The sense-organs are indubitably among the most important and
interesting parts of the human body; they are the organs by means of
which we obtain our knowledge of objects in the surrounding world.
_Nihil est in intellectu quod non prius fuerit in sensu._ They are the
first sources of the life of the soul. There is no other part of the
body in which we discover such elaborate anatomical structures,
co-operating with a definite purpose; and there is no other organ in
which the wonderful and purposive structure seems so clearly to compel
us to admit a Creator and a preconceived plan. Hence we find special
efforts made by dualists to draw our attention here to the “wisdom of
the Creator” and the design visible in his works. As a matter of fact,
you will discover, on mature reflection, that on this theory the
Creator is at bottom only playing the part of a clever mechanic or
watch-maker; all these familiar teleological ideas of Creator and
creation are based, in the long run, on a similar childlike
anthropomorphism.

However, we must grant that at the first glance the teleological theory
seems to give the simplest and most satisfactory explanation of these
purposive structures. If we merely examine the structure and functions
of the most advanced sense-organs, it seems impossible to explain them
without postulating a creative act. Yet evolution shows us quite
clearly that this popular idea is totally wrong. With its assistance we
discover that the purposive and remarkable sense-organs were developed,
like all other organs, without any preconceived design—developed by the
same mechanical process of natural selection, the same constant
correlation of adaptation and heredity, by which the other purposive
structures in the animal frame were slowly and gradually brought forth
in the struggle for life.

Like most other Vertebrates, man has six sensory organs, which serve
for eight
different classes of sensations. The skin serves for sensations of
pressure and temperature. This is the oldest, lowest, and vaguest of
the sense-organs; it is distributed over the surface of the body. The
other sensory activities are localised. The sexual sense is bound up
with the skin of the external sexual organs, the sense of taste with
the mucous lining of the mouth (tongue and palate), and the sense of
smell with the mucous lining of the nasal cavity. For the two most
advanced and most highly differentiated sensory functions there are
special and very elaborate mechanical structures—the eye for the sense
of sight, and the ear for the sense of hearing and space (equilibrium).

Comparative anatomy and physiology teach us that there are no
differentiated sense-organs in the lower animals; all their sensations
are received by the surface of the skin. The undifferentiated
skin-layer or ectoderm of the Gastræa is the simple stratum of cells
from which the differentiated sense-organs of all the Metazoa
(including the Vertebrates) have been evolved. Starting from the
assumption that necessarily only the superficial parts of the body,
which are in direct touch with the outer world, could be concerned in
the origin of sensations, we can see at once that the sense-organs also
must have arisen there. This is really the case. The chief part of all
the sense-organs originates from the skin-sense layer, partly directly
from the horny plate, partly from the brain, the foremost part, of the
medullary tube, after it has separated from the horny plate. If we
compare the embryonic development of the various sense-organs, we see
that they all make their appearance in the simplest conceivable form;
the wonderful contrivances that make the higher sense-organs among the
most remarkable and elaborate structures in the body develop only
gradually. In the phylogenetic explanation of them comparative anatomy
and ontogeny achieve their greatest triumphs. But at first all the
sense-organs are merely parts of the skin in which sensory nerves
expand. These nerves themselves were originally of a homogeneous
character. The different functions or specific energies of the
differentiated sense-nerves were only gradually developed by division
of labour. At the same time, their simple terminal expansions in the
skin were converted into extremely complex organs.

The great instructiveness of these historical facts in connection with
the life of the soul is not difficult to see. The whole philosophy of
the future will be transformed as soon as psychology takes cognisance
of these genetic phenomena and makes them the basis of its
speculations. When we examine impartially the manuals of psychology
that have been published by the most distinguished speculative
philosophers and are still widely distributed, we are astonished at the
naivete with which the authors raise their airy metaphysical
speculations, regardless of the momentous embryological facts that
completely refute them. Yet the science of evolution, in conjunction
with the great advance of the comparative anatomy and physiology of the
sense-organs, provides the one sound empirical basis of a natural
psychology.


Fig.305. Head of a shark (Scyllium), from the ventral side. Fig.
305—Head of a shark (_Scyllium_), from the ventral side. _m_ mouth, _o_
olfactory pits, _r_ nasal groove, _n_ nasal fold in natural position,
_n′_ nasal fold drawn up. (The dots are openings of the mucous canals.)
(From _Gegenbaur._)


In respect of the terminal expansions of the sensory nerves, we can
distribute the human sense-organs in three groups, which correspond to
three stages of development. The first group comprises those organs the
nerves of which spread out quite simply in the free surface of the skin
itself (organs of the sense of pressure, warmth, and sex). In the
second group the nerves spread out in the mucous coat of cavities which
are at first depressions in or invaginations of the skin (organs of the
sense of smell and taste). The third group is formed of the very
elaborate organs, the nerves of which spread out in an internal
vesicle, separated from the skin (organs of the sense of sight,
hearing, and space).

There is little to be said of the development of the lower
sense-organs. We
have already considered (p. 268) the organ of touch and temperature in
the skin. I need only add that in the corium of man and all the higher
Vertebrates countless microscopic sense-organs develop, but the precise
relation of these to the sensations of pressure or resistance, of
warmth and cold, has not yet been explained. Organs of this kind, in or
on which sensory cutaneous nerves terminate, are the “tactile
corpuscles” (or the Pacinian corpuscles) and end-bulbs. We find similar
corpuscles in the organs of the sexual sense, the male penis and the
female clitoris; they are processes of the skin, the development of
which we will consider later (together with the rest of the sexual
parts, Chapter XXIX). The evolution of the organ of taste, the tongue
and palate, will also be treated later, together with that of the
alimentary canal to which these parts belong (Chapter XXVII). I will
only point out for the present that the mucous coat of the tongue and
palate, in which the gustatory nerve ends, originates from a part of
the outer skin. As we have seen, the whole of the mouth-cavity is
formed, not as a part of the gut-tube proper, but as a pit-like fold in
the outer skin (p. 139). Its mucous lining is therefore formed, not
from the visceral, but from the cutaneous layer, and the taste-cells at
the surface of the tongue and palate are not products of the gut-fibre
layer, but of the skin-sense layer.


Figs. 306 and 307. Head of a chick embryo, three days old. Fig. 308.
Head of a chick embryo, four days old, from below. Figs. 309 and 310.
Heads of chick embryos: 309 from the end of the fourth, 310 from the
beginning of the fifth week. Fig. 306 and 307—Head of a chick embryo,
three days old: 2.306 front view, 2.307 from the right. _n_ rudimentary
nose (olfactory pits), _l_ rudimentary eyes (optic pits), _g_
rudimentary ear (auscultory pit), _v_ fore brain, _gl_ eye-cleft, _o_
process of upper jaw, _u_ process of lower jaw of the first gill-arch.


Fig. 308—Head of a chick embryo, four days old, from below. _n_ nasal
pit, _o_ upper-jaw process of the first gill-arch, _u_ lower-jaw
process of same, _k″_ second gill-arch, _sp_ choroid fissure of eye,
_s_ gullet.

Fig. 309 and 310—Heads of chick embryos: 309 from the end of the
fourth, 310 from the beginning of the fifth week. Letters as in Fig.
308, except: in inner, an outer, nasal process, _nf_ nasal furrow, _st_
frontal process, _m_ mouth. (From _Kölliker._).

This applies also to the mucous lining of the olfactory organ, the
nose. However, the development of this organ is much more interesting.
Although the nose seems superficially to be simple and single, it
really consists, in man and all
other Gnathostomes, of two completely separated halves, the right and
left cavities. They are divided by a vertical partition, so that the
right nostril leads into the right cavity alone and the left nostril
into the left cavity. They open internally (and separately) by the
posterior nasal apertures into the pharynx, so that we can get direct
into the gullet through the nasal passages without touching the mouth.
This is the way the air usually passes in respiration; the mouth being
closed, it goes through the nose into the gullet, and through the
larynx and bronchial tubes into the lungs. The nasal cavities are
separated from the mouth by the horizontal bony palate, to which is
attached behind (as a dependent process) the soft palate with the
uvula. In the upper and hinder parts of the nasal cavities the
olfactory nerve, the first pair of cerebral nerves, expands in the
mucous coat which clothes them. The terminal branches of it spread
partly over the septum (partition), partly on the side walls of the
internal cavities, to which are attached the turbinated bones. These
bones are much more developed in many of the higher mammals than in
man, but there are three of them in all mammals. The sensation of smell
arises by the passage of a current of air containing odorous matter
over the mucous lining of the cavities, and stimulating the olfactory
cells of the nerve-endings.

Man has all the features which distinguish the olfactory organ of the
mammals from that of the lower Vertebrates. In all essential points the
human nose entirely resembles that of the Catarrhine apes, some of
which have quite a human external nose (compare the face of the
long-nosed apes). However, the first structure of the olfactory organ
in the human embryo gives no indication of the future ample proportions
of our catarrhine nose. It has the form in which we find it permanently
in the fishes—a couple of simple depressions in the skin at the outer
surface of the head. We find these blind olfactory pits in all the
fishes; sometimes they lie near the eyes, sometimes more forward at the
point of the muzzle, sometimes lower down, near the mouth (Fig. 249).


Fig.311. Frontal section of the mouth and throat of a human embryo,
neck half-inch long. Fig. 311—Frontal section of the mouth and throat
of a human embryo, neck half-inch long. “Invented” by _Wilhelm His._
The vertical section (in the frontal plane, from left to right) is so
constructed that we see the nasal pits in the upper third of the figure
and the eyes at the sides: in the middle third the primitive gullet
with the gill-clefts (gill-arches in section); in the lower third the
pectoral cavity with the bronchial tubes and the rudimentary lungs.


This first rudimentary structure of the double nose is the same in all
the Gnathostomes; it has no connection with the primitive mouth. But
even in a section of the fishes a connection of this kind begins to
make its appearance, a furrow in the surface of the skin running from
each side of the nasal pit to the nearest corner of the mouth. This
furrow, the nasal groove or furrow (Fig. 305 _r_), is very important.
In many of the sharks, such as the _Scyllium,_ a special process of the
frontal skin, the nasal fold or internal nasal process, is formed
internally over the groove (_n, n″_). In contrast to this the outer
edge of the furrow rises in an “external nasal process.” As the two
processes meet and coalesce over the nasal groove in the Dipneusts and
Amphibia, it is converted into a canal, the nasal canal. Henceforth we
can penetrate from the external pits through the nasal canals direct
into the mouth, which has been formed quite independently. In the
Dipneusts and the lower Amphibia the internal aperture of the nasal
canals lies in front (behind the lips); in the higher Amphibia it is
right behind. Finally, in the three higher classes of Vertebrates the
primary mouth-cavity is divided by the formation of the horizontal
palate-roof into two distinct cavities—the upper (secondary) nasal
cavity and the lower (secondary) mouth-cavity. The nasal cavity in turn
is divided by the construction of the vertical septum into two
halves—right and left.


Fig.312. Diagrammatic section of the mouth-nose cavity. Fig.
312—Diagrammatic section of the mouth-nose cavity. While the
palate-plates (_p_) divide the original mouth-cavity into the lower
secondary mouth (_m_) and the upper nasal cavity, the latter in turn is
divided by the vertical partition (_e_) into two halves (_n, n_). (From
_Gegenbaur._)


Comparative anatomy shows us to-day, in the series of the double-nosed
Vertebrates, from the fishes up to man, all the different stages in the
development of the nose, which the advanced olfactory organ of the
higher mammals has passed through at various periods in the course of
its phylogeny. It first appears in the embryo of man and the higher
Vertebrates, in which the double fish-nose persists throughout life. At
an early stage, before there is any trace of the characteristic human
face, a pair of small pits are formed in the head over the original
mouth-cavity; these were first discovered by Baer, and rightly called
the “olfactory pits” (Figs. 306 _n_, 307 _n_). These primitive nasal
pits are quite separate from the rudimentary mouth, which also
originates as a pit-like depression in the skin, in front of the blind
fore end of the gut. Both the pair of nasal pits and the single
mouth-pit (Fig. 310 _m_) are clothed with the horny plate. The original
separation of the former from the latter is, however, presently
abolished, a process forming above the mouth-pit—the “frontal process”
(Fig. 309 _st_). Its outer edge rises to the right and left in the
shape of two lateral processes; these are the inner nasal processes or
folds (_in_). Opposite to these a parallel ridge is formed on either
side between the eye and the nasal pit; these are the outer nasal
processes (_an_). Thus between the inner and outer nasal processes a
groove-like depression is formed on either side, which leads from the
nasal pit towards the mouth-pit (_m_); this groove is, as the reader
will guess, the same nasal furrow or groove that we have already seen
in the shark (Fig. 305 _r_). As the parallel edges of the inner and
outer nasal processes bend towards each other and join above the nasal
groove, this is converted into a tube, the primitive nasal canal. Hence
the nose of man and all the other Amniotes consists at this embryonic
stage of a couple of narrow tubes, the nasal canals, which lead from
the outer surface of the forehead into the rudimentary mouth. This
transitory condition resembles that in which we find the nose
permanently in the Dipneusts and Amphibia.

A cone-shaped structure, which grows from below towards the lower ends
of the two nasal processes and joins with them, plays an important part
in the conversion of the open nasal groove into the closed canal. This
is the upper-jaw process (Figs. 306–310 _o_). Below the mouth-pit are
the gill-arches, which are separated by the gill-clefts. The first of
these gill-arches, and the most important for our purpose, which we may
call the maxillary (jaw) arch, forms the skeleton of the jaws. Above at
the basis a small process grows out of this first gill-arch; this is
the upper-jaw process. The first gill-arch itself develops a cartilage
at one of its inner sides, the “Meckel cartilage” (named after its
discoverer), on the outer surface of which the lower jaw is formed
(Figs. 306–310 _u_). The upper-jaw process forms the chief part of the
skeleton of that jaw, the palate bone, and the pterygoid bone. On its
outer side is afterwards formed the upper-jaw bone, in the narrower
sense, while the middle part of the skeleton of the upper jaw, the
intermaxillary, develops from the foremost part of the frontal process.

The two upper-jaw processes are of great importance in the further
development of the face. From them is formed, growing into the
primitive mouth-cavity, the important horizontal partition (the palate)
that divides the former into two distinct cavities. The upper cavity,
into which the nasal canals open, now develops into the nasal cavity,
the air-passage and the organ of smell. The lower cavity forms the
permanent secondary mouth (Fig. 312 _m_), the food-passage and the
organ of taste. Both the upper and lower cavities open behind into the
gullet (pharynx). The hard
palate that separates them is formed by the joining of two lateral
halves, the horizontal plates of the two upper-jaw processes, or the
palate-plates (_p_). When these do not, sometimes, completely join in
the middle, a longitudinal cleft remains, through which we can
penetrate from the mouth straight into the nasal cavity. This is the
malformation known as “wolf’s throat.” “Hare-lip” is the lesser form of
the same defect. At the same time as the horizontal partition of the
hard palate a vertical partition is formed by which the single nasal
cavity is divided into two sections—a right and left half (Fig. 312 _n,
n_).


Figs. 313 and 314. Upper part of the body of a human embryo, two-thirds
of an inch long, of the sixth week; Fig. 313 from the left, Fig. 314
from the front. Figs. 313 and 314—Upper part of the body of a human
embryo, two-thirds of an inch long, of the sixth week; Fig. 313 from
the left, Fig. 314 from the front. The origin of the nose and the upper
lip from two lateral and originally separate halves can be clearly
seen. Nose and upper lip are large in proportion to the rest of the
face, and especially to the lower lip. (From _Kollmann._)


The double nose has now acquired the characteristic form that man
shares with the other mammals. Its further development is easy to
follow; it consists of the formation of the inner and outer processes
of the walls of the two cavities. The external nose is not formed until
long after all these essential parts of the internal organ of smell.
The first traces of it in the human embryo are found about the middle
of the second month (Figs. 313–316). As can be seen in any human embryo
during the first month, there is at first no trace of the external
nose. It only develops afterwards from the foremost nasal part of the
primitive skull, growing forwards from behind. The characteristic human
nose is formed very late. Much stress is at times laid on this organ as
an exclusive privilege of man. But there are apes that have similar
noses, such as the long-nosed ape.

The evolution of the eye is not less interesting and instructive than
that of the nose. Although this noblest of the sensory organs is one of
the most elaborate and purposive on account of its optic perfection and
remarkable structure, it nevertheless develops, without preconceived
design, from a simple process of the outer germinal layer. The
fully-formed human eye is a round capsule, the eye-ball (Fig. 317).
This lies in the bony cavity of the skull, surrounded by protective fat
and motor muscles. The greater part of it is taken up with a
semi-fluid, transparent gelatinous substance, the _corpus vitreum._ The
crystalline lens is fitted into the anterior surface of the ball (Fig.
317 _l_). It is a lenticular, bi-convex, transparent body, the most
important of the refractive media in the eye. Of this group we have,
besides the corpus vitreum and the lens, the watery fluid (_humor
aqueus_) that is found in front of the lens (at the letter _m_ in Fig.
317). These three transparent refractive media, by which the rays of
light that
enter the eye are broken up and re-focussed, are enclosed in a solid
round capsule, composed of several different coats, something like the
concentric layers of an onion. The outermost and thickest of these
envelopes is the white sclerotic coat of the eye. It consists of tough
white connective tissue. In front of the lens a circular,
strongly-curved, transparent plate is fitted into the sclerotic, like
the glass of a watch—the _cornea_ (_b_). At its outer surface the
cornea is covered with a very thin layer of the epidermis; this is
known as the _conjunctiva._ It goes from the cornea over the inner
surface of the eye-lids, the upper and lower folds which we draw over
the eye in closing it. At the inner corner of the eye we have a
rudimentary organ in the shape of the relic of a third (inner) eye-lid,
which is greatly developed, as “nictitating (winking) membrane,” in the
lower Vertebrates (p. 5). Underneath the upper eye-lid are the
lachrymal glands, the product of which, the lachrymal fluid, keeps the
outer surface of the eye smooth and clean.


Fig.315. Face of a human embryo, seven weeks old. Fig. 315—Face of a
human embryo, seven weeks old. (From _Kollmann._) Joining of the nasal
processes (_e_ outer, _i_ inner) with the upper-jaw process (_o_), _n_
nasal wall, _a_ ear-opening.


Immediately under the sclerotic we find a very delicate, dark-red
membrane, very rich in blood-vessels—the _choroid coat_—and inside this
the retina (_o_), the expansion of the optic nerve (_i_). The latter is
the second cerebral nerve. It proceeds from the optic thalami (the
second cerebral vesicle) to the eye; penetrates its outer envelopes,
and then spreads out like a net between the choroid and the corpus
vitreum. Between the retina and the choroid there is a very delicate
membrane, which is usually (but wrongly) associated with the latter.
This is the black pigment-membrane (_n_). It consists of a single
stratum of graceful, hexagonal, regularly-joined cells, full of
granules of black colouring matter. This pigment membrane clothes, not
only the inner surface of the choroid proper, but also the hind surface
of its anterior muscular continuation, which covers the edge of the
lens in front as a circular membrane, and arrests the rays of light at
the sides. This is the well-known _iris_ of the eye (_h_), coloured
differently in different individuals (blue, grey, brown, etc.); it
forms the anterior border of the choroid. The circular opening that is
left in the middle is the _pupil,_ through which the rays of light
penetrate into the eye. At the point where the iris leaves the anterior
border of the choroid proper the latter is very thick, and forms a
delicate crown of folds (_g_), which surrounds the edge of the lens
with about seventy large and many smaller rays (_corona ciliaris._)


Fig.316. Face of a human embryo, eight weeks old. Fig. 316—Face of a
human embryo, eight weeks old. (From _Ecker._)


At a very early stage a couple of pear-shaped vesicles develop from the
foremost part of the first cerebral vesicle in the embryo of man and
the other Craniotes (Figs. 155 _a_, 297 _au_). These growths are the
primary optic vesicles. They are at first directed outwards and
forwards, but presently grow downward, so that, after the complete
separation of the five cerebral vesicles, they lie at the base of the
intermediate brain. The inner cavities of these pear-shaped vesicles,
which soon attain a considerable size, are openly connected with the
ventricle of the intermediate brain by their hollow stems. They are
covered externally by the epidermis.

At the point where this comes into direct contact with the most curved
part of the primary optic vesicle there is a thickening (_l_) and also
a depression (_o_) of the horny plate (Fig. 318, _I_). This pit, which
we may call the lens-pit, is converted into a closed sac, the thick-
walled lens-vesicle (_2, l_), the thick edges of the pit joining
together above it. In the same way in which the medullary tube
separates from the outer germinal layer, we now see this lens-sac sever
itself entirely from the horny plate (_h_), its source of origin. The
hollow of the sac is afterwards filled with the cells of its thick
walls, and thus we get the solid crystalline lens. This is, therefore,
a purely epidermic structure. Together with the lens the small
underlying piece of corium-plate also separates from the skin.

As the lens separates from the corneous plate and grows inwards, it
necessarily hollows out the contiguous primary optic vesicle (Fig. 318,
_1–3_). This is done in just the same way as the invagination of the
blastula, which gives rise to the gastrula in the amphioxus (Fig. 38
_C–F_). In both cases the hollowing of the closed vesicle on one side
goes so far that at last the inner, folded part touches the outer, not
folded part, and the cavity disappears. As in the gastrula the first
part is converted into the entoderm and the latter into the ectoderm,
so in the invagination of the primary optic vesicle the retina (_r_) is
formed from the first (inner) part, and the black pigment membrane
(_u_) from the latter (outer, non-invaginated) part. The hollow stem of
the primary optic vesicle is converted into the optic nerve. The lens
(_l_), which has so important a part in this process, lies at first
directly on the invaginated part, or the retina (_r_). But they soon
separate, a new structure, the corpus vitreum (_gl_), growing between
them. While the lenticular sac is being detached and is causing the
invagination of the primary optic vesicle, another invagination is
taking place from below; this proceeds from the superficial part of the
skin-fibre layer—the corium of the head. Behind and under the lens a
last-shaped process rises from the cutis-plate (Fig. 319 _g_), hollows
out the cup-shaped optic vesicle from below, and presses between the
lens (_l_) and the retina (_i_). In this way the optic vesicle acquires
the form of a hood.


Fig.317. The human eye in section. Fig. 317—The human eye in section.
_a_ sclerotic coat, _b_ cornea, _c_ conjunctiva, _d_ circular veins of
the iris, _e_ choroid coat, _f_ ciliary muscle, _g_ corona ciliaris,
_h_ iris, _i_ optic nerve, _k_ anterior border of the retina, _l_
crystalline lens, _m_ inner covering of the cornea (aqueous membrane),
_n_ pigment membrane, _o_ retina, _p_ Petit’s canal, _q_ yellow spot of
the retina. (From _Helmholtz._)


Finally, a complete fibrous envelope, the fibrous capsule of the
eye-ball, is formed about the secondary optic vesicle and its stem (the
secondary optic nerve). It originates from the part of the head-plates
which immediately encloses the eye. This fibrous envelope takes the
form of a closed round vesicle, surrounding the whole of the ball and
pushing between the lens and the horny plate at its outer side. The
round wall of the capsule soon divides into two different membranes by
surface-cleavage. The inner membrane becomes the choroid or vascular
coat, and in front the ciliary corona and iris. The outer membrane is
converted into the white protective or sclerotic coat—in front, the
transparent cornea. The eye is now formed in all its essential parts.
The further development—the complicated differentiation and composition
of the various parts—is a matter of detail.

The chief point in this remarkable evolution of the eye is the
circumstance that the optic nerve, the retina, and the pigment membrane
originate really from a part of the brain—an outgrowth of the
intermediate brain—while the lens, the chief refractive body, develops
from the outer skin. From the skin—the horny
plate—also arises the delicate conjunctiva, which afterwards covers the
outer surface of the eyeball. The lachrymal glands are ramified growths
from the conjunctiva (Fig. 286). All these important parts of the eye
are products of the outer germinal layer. The remaining parts—the
corpus vitreum (with the vascular capsule of the lens), the choroid
(with the iris), and the sclerotic (with the cornea)—are formed from
the middle germinal layer.


Fig.318. Eye of the chick embryo in longitudinal section (1. from an
embryo sixty-five hours old; 2. from a somewhat older embryo; 3. from
an embryo four days old). Fig. 318—Eye of the chick embryo in
longitudinal section (_1._ from an embryo sixty-five hours old; _2._
from a somewhat older embryo; _3._ from an embryo four days old). _h_
horny plate, _o_ lens-pit, _l_ lens (in _1._ still part of the
epidermis, in _2._ and _3._ separated from it), _x_ thickening of the
horny plate at the point where the lens has severed itself, _gl_ corpus
vitreum, _r_ retina, _u_ pigment membrane. (From _Remak._)


The outer protection of the eye, the eye-lids, are merely folds of the
skin, which are formed in the third month of human embryonic life. In
the fourth month the upper eye-lid reaches the lower, and the eye
remains covered with them until birth. As a rule, they open wide
shortly before birth (sometimes only after birth). Our craniote
ancestors had a third eye-lid, the nictitating membrane, which was
drawn over the eye from its inner angle. It is still found in many of
the Selachii and Amniotes. In the apes and man it has degenerated, and
there is now only a small relic of it at the inner corner of the eye,
the semi-lunar fold, a useless rudimentary organ (cf. p. 32). The apes
and man have also lost the Harderian gland that opened under the
nictitating membrane; we find this in the rest of the mammals, and the
birds, reptiles, and amphibia.

The peculiar embryonic development of the vertebrate eye does not
enable us to draw any definite conclusions as to its obscure phylogeny;
it is clearly cenogenetic to a great extent, or obscured by the
reduction and curtailment of its original features. It is probable that
many of the earlier stages of its phylogeny have disappeared without
leaving a trace. It can only be said positively that the peculiar
ontogeny of the complicated optic apparatus in man follows just the
same laws as in all the other Vertebrates. Their eye is a part of the
fore brain, which has grown forward towards the skin, not an original
cutaneous sense-organ, as in the Invertebrates.


Fig.319. Horizontal transverse section of the eye of a human embryo,
four weeks old. Fig. 319—Horizontal transverse section of the eye of a
human embryo, four weeks old. (From _Kölliker._) _t_ lens (the dark
wall of which is as thick as the diameter of the central cavity), _g_
corpus vitreum (connected by a stem, _g,_ with the corium), _v_
vascular loop (pressing behind the lens inside the corpus vitreum by
means of this stem _g_), _i_ retina (inner thicker, invaginated layer
of the primary optic vesicle), _a_ pigment membrane (outer, thin,
non-invaginated layer of same), _h_ space between retina and pigment
membrane (remainder of the cavity of the primary optic vesicle).


The vertebrate ear resembles the eye and nose in many important
respects, but is different in others, in its development. The
auscultory organ in the fully-developed man is like that of the other
mammals, and especially the apes, in the main features. As in them, it
consists of two chief parts—an apparatus for conducting sound (external
and middle ear) and an apparatus for the sensation of sound (internal
ear). The external ear opens in the shell at the side of the head (Fig.
320 _a_). From this point the external passage (_b_), about an inch in
length, leads into the head. The inner end of it is closed by the
tympanum, a vertical, but not quite upright, thin membrane of an oval
shape (_c_). This tympanum separates the external passage from the
tympanic cavity (_d_). This is a small cavity, filled with air, in the
temporal bone; it is connected with the mouth by a special tube. This
tube is rather longer, but much narrower, than the outer passage, leads
inwards obliquely from the anterior wall of the tympanic cavity, and
opens in the throat below, behind the nasal
openings. It is called the Eustachian tube (_e_); it serves to equalise
the pressure of the air within the tympanic cavity and the outer
atmosphere that enters by the external passage. Both the Eustachian
tube and the tympanic cavity are lined with a thin mucous coat, which
is a direct continuation of the mucous lining of the throat. Inside the
tympanic cavity there are three small bones which are known (from their
shape) as the hammer, anvil, and stirrup (Fig. 320, _f, g, h_). The
hammer (_f_) is the outermost, next to the tympanum. The anvil (_g_)
fits between the other two, above and inside the hammer. The stirrup
(_h_) lies inside the anvil, and touches with its base the outer wall
of the internal ear, or auscultory vesicle. All these parts of the
external and middle ear belong to the apparatus for conducting sound.
Their chief task is to convey the waves of sound through the thick wall
of the head to the inner-lying auscultory vesicle. They are not found
at all in the fishes. In these the waves of sound are conveyed directly
by the wall of the head to the auscultory vesicle.


Fig.320. The human ear (left ear, seen from the front). Fig. 320—The
human ear (left ear, seen from the front), _a_ shell of ear, _b_
external passage, _c_ tympanum, _d_ tympanic cavity, _e_ Eustachian
tube, _f, g, h_ the three bones of the ear (_f_ hammer, _g_ anvil, _h_
stirrup), _i_ utricle, _k_ the three semi-circular canals, _l_ the
sacculus, _m_ cochlea, _n_ auscultory nerve.


Fig. 321. The bony labyrinth of the human  ear (left side). Fig.
321—The bony labyrinth of the human ear (left side). _a_ vestibulum,
_b_ cochlea, _c_ upper canal, _d_ posterior canal, _e_ outer canal, _f_
oval fenestra, _g_ round fenestra. (From _Meyer._)


The internal apparatus for the sensation of sound, which receives the
waves of sound from the conducting apparatus, consists in man and all
other mammals of a closed auscultory vesicle filled with fluid and an
auditory nerve, the ends of which expand over the wall of this vesicle.
The vibrations of the sound-waves are conveyed by these media to the
nerve-endings. In the labyrinthic water that fills the auscultory
vesicle there are small stones at the points of entry of the acoustic
nerves, which are composed of groups of microscopic calcareous crystals
(otoliths). The auscultory organ of most of the Invertebrates has
substantially the same composition. It usually consists of a closed
vesicle, filled with fluid, and containing otoliths, with the acoustic
nerve expanding on its wall. But, while the auditory vesicle is usually
of a simple round or oval shape in the Invertebrates, it has in the
Vertebrates a special and curious structure, the labyrinth. This
thin-membraned labyrinth is enclosed in a bony capsule of the same
shape, the osseous labyrinth (Fig. 321), and this lies in the middle of
the petrous bone of the skull. The labyrinth is divided into two
vesicles in all the Gnathostomes. The larger one is called the
_utriculus,_ and has three arched appendages, called the “semi-circular
canals” (_c, d, e_). The smaller vesicle is called the sacculus, and is
connected with a peculiar appendage, with (in man and the higher
mammals) a spiral form something like a snail’s shell, and therefore
called the _cochlea_ (= snail, _b_). On the thin wall of this delicate
labyrinth the acoustic nerve, which comes from the after-brain, spreads
out in most elaborate fashion. It divides into two main branches—a
cochlear nerve (for the cochlea) and a vestibular nerve (for the rest
of the labyrinth). The former seems to have more to do with the
quality, the latter with the quantity, of the acoustic sensations.
Through the cochlear nerves we learn the height and timbre, through the
vestibular nerves the intensity, of tones.

The first structure of this highly elaborate organ is very simple in
the embryo of man and all the other Craniotes; it is a
pit-like depression in the skin. At the back part of the head at both
sides, near the after brain, a small thickening of the horny plate is
formed at the upper end of the second gill-cleft (Fig. 322 _A fl_).
This sinks into a sort of pit, and severs from the epidermis, just as
the lens of the eye does. In this way is formed at each side, directly
under the horny plate of the back part of the head, a small vesicle
filled with fluid, the primitive auscultory vesicle, or the primary
labyrinth. As it separates from its source, the horny plate, and
presses inwards and backwards into the skull, it changes from round to
pear-shaped (Figs. 322 _B lv_, 323 _o_). The outer part of it is
lengthened into a thin stem, which at first still opens outwards by a
narrow canal. This is the labyrinthic appendage (Fig. 322 _lr_). In the
lower Vertebrates it develops into a special cavity filled with
calcareous crystals, which remains open permanently in some of the
primitive fishes, and opens outwards in the upper part of the skull.
But in the mammals the labyrinthic appendage degenerates. In these it
has only a phylogenetic interest as a rudimentary organ, with no actual
physiological significance. The useless relic of it passes through the
wall of the petrous bone in the shape of a narrow canal, and is called
the vestibular aqueduct.


Fig.322. Development of the auscultory labyrinth of the chick, in five
successive stages (A to E). Fig. 322—Development of the auscultory
labyrinth of the chick, in five successive stages (_A–E_). (Vertical
transverse sections of the skull.) _fl_ auscultory pits, _lv_
auscultory vesicles, _lr_ labyrinthic appendage, _c_ rudimentary
cochlea, _csp_ posterior canal, _cse_ external canal, _jv_ jugular
vein. (From _Reissner._)


It is only the inner and lower bulbous part of the separated auscultory
vesicle that develops into the highly complex and differentiated
structure that is afterwards known as the secondary labyrinth. This
vesicle divides at an early stage into an upper and larger and a lower
and smaller section. From the one we get the _utriculus_ with the
semi-circular canals; from the other the _sacculus_ and the cochlea
(Fig. 320 _c_). The canals are formed in the shape of simple pouch-like
involutions of the utricle (_cse_ and _csp_). The edges join together
in the middle part of each fold, and separate from the utricle, the two
ends remaining in open connection with its cavity. All the Gnathostomes
have these three canals like man, whereas among the Cyclostomes the
lampreys have only two and the hag-fishes only one. The very complex
structure of the cochlea, one of the most elaborate and wonderful
outcomes of adaptation in the mammal body, develops originally in very
simple fashion as a flask-like projection from the sacculus. As Hasse
and Retzius have pointed out, we find the successive ontogenetic stages
of its growth represented permanently in the series of the higher
Vertebrates. The cochlea is wanting even in the Monotremes, and is
restricted to the rest of the mammals and man.

The auditory nerve, or eighth cerebral nerve, expands with one branch
in the cochlea, and with the other in the remaining parts of the
labyrinth. This nerve is, as Gegenbaur has shown, the sensory dorsal
branch of a cerebro-spinal nerve, the motor ventral branch of which
acts for the muscles of the face (_nervus facialis_). It has therefore
originated phylogenetically from an ordinary cutaneous nerve, and so is
of quite different origin from the optic and olfactory nerves, which
both represent direct outgrowths of the brain. In this respect the
auscultory organ is essentially different from the organs of sight and
smell. The acoustic nerve is formed from ectodermic cells of the hind
brain, and develops from the nervous structure that appears at its
dorsal limit. On the other hand, all the membranous, cartilaginous, and
osseous coverings of the labyrinth are formed from the mesodermic
head-plates.

The apparatus for conducting sound which we find in the external and
middle ear of mammals develops quite separately from the apparatus for
the sensation of sound. It is both phylogenetically and ontogenetically
an independent secondary formation, a later accession
to the primary internal ear. Nevertheless, its development is not less
interesting, and is explained with the same ease by comparative
anatomy. In all the fishes and in the lowest Vertebrates there is no
special apparatus for conducting sound, no external or middle ear; they
have only a labyrinth, an internal ear, which lies within the skull.
They are without the tympanum and tympanic cavity, and all its
appendages. From many observations made in the last few decades it
seems that many of the fishes (if not all) cannot distinguish tones;
their labyrinth seems to be chiefly (if not exclusively) an organ for
the sense of space (or equilibrium). If it is destroyed, the fishes
lose their balance and fall. In the opinion of recent physiologists
this applies also to many of the Invertebrates (including the nearer
ancestors of the Vertebrates). The round vesicles which are considered
to be their auscultory vesicles, and which contain an otolith, are
supposed to be merely organs of the sense of space (“static vesicles or
statocysts”).


Fig.323. Primitive skull of the human embryo, four weeks old, vertical
section, left half seen internally. Fig. 323—Primitive skull of the
human embryo, four weeks old, vertical section, left half seen
internally. _v, z, m, h, n_ the five pits of the cranial cavity, in
which the five cerebral vesicles lie (fore, intermediate, middle, hind,
and after brains), _o_ pear-shaped primary auscultory vesicle
(appearing through), _a_ eye (appearing through), _no_ optic nerve, _p_
canal of the hypophysis, _t_ central prominence of the skull. (From
_Kölliker._)


The middle ear makes its first appearance in the amphibian class, where
we find a tympanum, tympanic cavity, and Eustachian tube; these
animals, and all terrestrial Vertebrates, certainly have the faculty of
hearing. All these essential parts of the middle ear originate from the
first gill-cleft and its surrounding part; in the Selachii this remains
throughout life an open squirting-hole, and lies between the first and
second gill-arch. In the embryo of the higher Vertebrates it closes up
in the centre, and thus forms the tympanic membrane. The outlying
remainder of the first gill-cleft is the rudiment of the external
meatus. From its inner part we get the tympanic cavity, and, further
inward still, the Eustachian tube. Connected with this is the
development of the three bones of the mammal ear from the first two
gill-arches; the hammer and anvil are formed from the first, the
stirrup from the upper end of the second, gill-arch.


Fig.324. The rudimentary muscles of the ear in the human skull. Fig.
324—The rudimentary muscles of the ear in the human skull. _a_ raising
muscle (_M. attollens_), _b_ drawing muscle (_M. attrahens_), _c_
withdrawing muscle (_M. retrahens_), _d_ large muscle of the helix (_M.
helicis major_), _e_ small muscle of the helix (_M. helicis minor_),
_f_ muscle of the angle of the ear (_M. tragicus_), _g_ anti-angular
muscle (_M. antitragicus_). (From _H. Meyer._)


Finally, the shell (pinna or concha) and external meatus (passage to
the tympanum) of the outer ear are developed in a very simple fashion
from the skin that borders the external aperture of the first
gill-cleft. The shell rises in the shape of a circular fold of the
skin, in which cartilage and muscles are afterwards formed (Figs. 313,
315). This organ is only found in the mammalian class. It is very
rudimentary in the lowest section, the Monotremes. In the others it is
found at very different stages of development, and sometimes of
degeneration. It is degenerate in most of the aquatic mammals. The
majority of them have lost it altogether—for instance, the walruses and
whales and most of the seals. On the other hand, the pinna is well
developed in the great majority of the Marsupials and Placentals; it
receives and collects the waves of sound, and is equipped with a very
elaborate muscular apparatus, by means of which the pinna
can be turned freely in any direction and its shape be altered. It is
well known how readily domestic animals—horses, cows, dogs, hares,
etc.—point their ears and move them in different directions. Most of
the apes do the same, and our earlier ape ancestors were also able to
do it. But our later simian ancestors, which we have in common with the
anthropoid apes, abandoned the use of these muscles, and they gradually
became rudimentary and useless. However, we possess them still (Fig.
324). In fact, some men can still move their ears a little backward and
forward by means of the drawing and withdrawing muscles (_b_ and _c_);
with practice this faculty can be much improved. But no man can now
lift up his ears by the raising muscle (_a_), or change the shape of
them by the small inner muscles (_d, e, f, g_). These muscles were very
useful to our ancestors, but are of no consequence to us. This applies
to most of the anthropoid apes as well.

We also share with the higher anthropoid apes (gorilla, chimpanzee, and
orang) the characteristic form of the human outer ear, especially the
folded border, the helix and the lobe. The lower apes have pointed
ears, without folded border or lobe, like the other mammals. But Darwin
has shown that at the upper part of the folded border there is in many
men a small pointed process, which most of us do not possess. In some
individuals this process is well developed. It can only be explained as
the relic of the original point of the ear, which has been turned
inwards in consequence of the curving of the edge. If we compare the
pinna of man and the various apes in this respect, we find that they
present a connected series of degenerate structures. In the common
catarrhine ancestors of the anthropoids and man the degeneration set in
with the folding together of the pinna. This brought about the helix of
the ear, in which we find the significant angle which represents the
relic of the salient point of the ear in our earlier simian ancestors.
Here again, therefore, comparative anatomy enables us to trace with
certainty the human ear to the similar, but more developed, organ of
the lower mammals. At the same time, comparative physiology shows that
it was a more or less useful implement in the latter, but it is quite
useless in the anthropoids and man. The conducting of the sound has
scarcely been affected by the loss of the pinna. We have also in this
the explanation of the extraordinary variety in the shape and size of
the shell of the ear in different men; in this it resembles other
rudimentary organs.



Chapter XXVI.
EVOLUTION OF THE ORGANS OF MOVEMENT


The peculiar structure of the locomotive apparatus is one of the
features that are most distinctive of the vertebrate stem. The chief
part of this apparatus is formed, as in all the higher animals, by the
active organs of movement, the muscles; in consequence of their
contractility they have the power to draw up and shorten themselves.
This effects the movement of the various parts of the body, and thus
the whole body is conveyed from place to place. But the arrangement of
these muscles and their relation to the solid skeleton are different in
the Vertebrates from the Invertebrates.

In most of the lower animals, especially the Platodes and Vermalia, we
find that the muscles form a simple, thin layer of flesh immediately
underneath the skin. This muscular layer is very closely connected with
the skin itself; it is the same in the Mollusc stem. Even in the large
division of the Articulates, the classes of crabs, spiders, myriapods,
and

insects, we find a similar feature, with the difference that in this
case the skin forms a solid armour—a rigid cutaneous skeleton made of
chitine (and often also of carbonate of lime). This external chitine
coat undergoes a very elaborate articulation both on the trunk and the
limbs of the Articulates, and in consequence the muscular system also,
the contractile fibres of which are attached inside the chitine tubes,
is highly articulated. The Vertebrates form a direct contrast to this.
In these alone a solid internal skeleton is developed, of cartilage or
bone, to which the muscles are attached. This bony skeleton is a
complex lever apparatus, or _passive_ apparatus of movement. Its rigid
parts, the arms of the levers, or the bones, are brought together by
the actively mobile muscles, as if by drawing-ropes. This admirable
locomotorium, especially its solid central axis, the vertebral column,
is a special feature of the Vertebrates, and has given the name to the
group.


Fig.325. The human skeleton from the right. Fig. 326. The human
skeleton. Front. Fig. 325—The human skeleton. From the right. Fig.
326—The human skeleton. Front.


Fig.327. The human vertebral column (standing upright, from the right
side). Fig. 327—The human vertebral column (standing upright, from the
right side). (From _H. Meyer._)


Fig.328. A piece of the axial rod (chorda dorsalis), from a sheep
embryo. Fig. 328—A piece of the axial rod (_chorda dorsalis_), from a
sheep embryo. _a_ cuticular sheath, _b_ cells. (From _Kölliker._)


In order to get a clear idea of the chief features of the development
of the human skeleton, we must first examine its composition in the
adult frame (Fig. 325, the human skeleton seen from the right; Fig.
326, front view of the whole skeleton). As in other mammals, we
distinguish first between the axial or dorsal skeleton and the skeleton
of the limbs. The axial skeleton consists of the vertebral column (the
skeleton of the trunk) and the skull (skeleton of the head); the latter
is a peculiarly modified part of the former. As appendages of the
vertebral column we have the ribs, and of the skull we have the hyoid
bone, the lower jaw, and the other products of the gill-arches.

The skeleton of the limbs or extremities is composed of two groups of
parts—the skeleton of the extremities proper and the zone-skeleton,
which connects these with the vertebral column. The zone-skeleton of
the arms (or fore legs) is the shoulder-zone; the zone-skeleton of the
legs (or hind legs) is the pelvic zone.

The vertebral column (Fig. 327) in man is composed of thirty-three to
thirty-five ring-shaped bones in a continuous series (above each other,
in man’s upright position). These _vertebræ_ are separated from each
other by elastic ligaments, and at the same time connected by joints,
so that the whole column forms a firm and solid, but flexible and
elastic, axial skeleton, moving freely in all directions. The vertebræ
differ in shape and connection at the various parts of the trunk, and
we distinguish the following groups in the series, beginning at the
top: Seven cervical vertebræ, twelve dorsal vertebræ, five lumbar
vertebræ, five sacral vertebræ, and four to six caudal vertebræ. The
uppermost, or those next to the skull, are the cervical vertebræ (Fig.
327); they have a hole in each of the lateral processes. There are
seven of these vertebræ in man and almost all the other mammals, even
if the neck is as long as that of the camel or giraffe, or as short as
that of the mole or hedgehog. This constant number, which has few
exceptions (due to adaptation), is a strong proof of the common descent
of the mammals; it can only be explained by faithful heredity from a
common stem-form, a primitive mammal with seven cervical vertebræ. If
each species had been created separately, it would have been better to
have given the long-necked mammals more, and the short-necked animals
less, cervical vertebræ. Next to these come the dorsal (or pectoral)
vertebræ, which number twelve to thirteen (usually twelve) in man and
most of the other mammals. Each dorsal vertebra (Fig. 165) has at the
side, connected by joints, a couple of ribs, long bony arches that lie
in and protect the wall of the chest. The twelve pairs of ribs,
together with the connecting intercostal muscles and the sternum, which
joins the ends of the right and left ribs in front, form the chest
(_thorax_). In this strong and elastic frame are the lungs, and between
them the heart. Next to the dorsal vertebræ comes a short but stronger
section of the column, formed of five large vertebræ. These are the
lumbar vertebræ (Fig. 166); they have no ribs and no holes in the
transverse processes. To these succeeds the sacral bone, which is
fitted between the two halves of the pelvic zone. The sacrum is formed
of five vertebræ, completely blended together. Finally, we have at the
end a small rudimentary caudal column, the _coccyx._ This consists of a
varying number (usually four, more rarely three, or five or six) of
small degenerated vertebræ, and is a useless rudimentary organ with no
actual physiological significance. Morphologically, however, it is of
great interest as an irrefragable proof of the descent of man and the
anthropoids from long-tailed apes. On no other theory can we explain
the existence of this rudimentary tail. In the earlier stages of
development the tail of the human embryo protrudes considerably. It
afterwards atrophies; but the relic of the atrophied caudal vertebræ
and of the rudimentary muscles that once moved it remains permanently.
Sometimes, in fact, the external tail is preserved. The older
anatomists say that the tail is usually one vertebra longer in the
human female than in the male (or four against five); Steinbach says it
is the reverse.


Fig.329. Three dorsal vertebræ, from a human embryo, eight weeks old,
in lateral longitudinal section. Fig. 329—Three dorsal vertebræ, from a
human embryo, eight weeks old, in lateral longitudinal section. _v_
cartilaginous vertebral body, _li_ inter-vertebral disks, _ch_ chorda.
(From _Kölliker._)


Fig.330. A dorsal vertebra of the same embryo, in lateral transverse
section. Fig. 330—A dorsal vertebra of the same embryo, in lateral
transverse section. _cv_ cartilaginous vertebral body, _ch_ chorda,
_pr_ transverse process, _a_ vertebral arch (upper arch), _c_ upper end
of the rib (lower arch). (From _Kölliker._)


In the human vertebral column there are usually thirty-three vertebræ.
It is interesting to find, however, that the number often changes, one
or two vertebræ dropping out or an additional one appearing. Often,
also, a mobile rib is formed at the last cervical or the first lumbar
vertebra, so that there are then thirteen dorsal vertebræ, besides six
cervical and four lumbar. In this way the contiguous vertebræ of the
various sections of the column may take each other’s places.

In order to understand the embryology of the human vertebral column we
must first carefully consider the shape and connection of the vertebræ.
Each vertebra has, in general, the shape of a seal-ring (Figs.
164–166). The thicker portion, which is turned towards the ventral
side, is called the body of the vertebra, and forms a short osseous
disk; the thinner part forms a semi-circular arch, the _vertebral
arch,_ and is turned towards the back. The arches of the successive
vertebræ are connected by thin intercrural ligaments in such a way that
the cavity they collectively enclose represents a long canal. In this
vertebral canal we find the trunk part of the central nervous system,
the spinal cord. Its head part, the brain, is enclosed by the skull,
and the skull itself is merely the uppermost part of the vertebral
column, distinctively modified. The base or ventral side of the
vesicular cranial capsule corresponds originally to a number of
developed vertebral bodies; its vault or dorsal side to their combined
upper vertebral arches.

While the solid, massive bodies of the vertebræ represent the real
central axis of the skeleton, the dorsal arches serve to protect the
central marrow they enclose. But similar arches develop on the ventral
side for the protection of the viscera in the breast and belly. These
lower or
ventral vertebral arches, proceeding from the ventral side of the
vertebral bodies, form, in many of the lower Vertebrates, a canal in
which the large blood-vessels are enclosed on the lower surface of the
vertebral column (aorta and caudal vein). In the higher Vertebrates the
majority of these vertebral arches are lost or become rudimentary. But
at the thoracic section of the column they develop into independent
strong osseous arches, the ribs (_costæ_). In reality the ribs are
merely large and independent lower vertebral arches, which have lost
their original connection with the vertebral bodies.


Fig.331. Intervertebral disk of a new-born infant, transverse section.
Fig. 331—Intervertebral disk of a new-born infant, transverse section.
_a_ rest of the chorda. (From _Kölliker._)


If we turn from this anatomic survey of the composition of the column
to the question of its development, I may refer the reader to earlier
pages with regard to the first and most important points (pp. 145–148).
It will be remembered that in the human embryo and that of the other
vertebrates we find at first, instead of the segmented column, only a
simple unarticulated cartilaginous rod. This solid but flexible and
elastic rod is the axial rod (or the _chorda dorsalis_). In the lowest
Vertebrate, the Amphioxus, it retains this simple form throughout life,
and permanently represents the whole internal skeleton (Fig. 210 _i_).
In the Tunicates, also, the nearest Invertebrate relatives of the
Vertebrates, we meet the same chorda—transitorily in the passing larva
tail of the Ascidia, permanently in the Copelata (Fig. 225 _c_).
Undoubtedly both the Tunicates and Acrania have inherited the chorda
from a common unsegmented stem-form; and these ancient, long-extinct
ancestors of all the chordonia are our hypothetical Prochordonia.

Long before there is any trace of the skull, limbs, etc., in the embryo
of man or any of the higher Vertebrates—at the early stage in which the
whole body is merely a sole-shaped embryonic shield—there appears in
the middle line of the shield, directly under the medullary furrow, the
simple chorda. (Cf. Figs. 131–135 _ch_). It follows the long axis of
the body in the shape of a cylindrical axial rod of elastic but firm
composition, equally pointed at both ends. In every case the chorda
originates from the dorsal wall of the primitive gut; the cells that
compose it (Fig. 328 _b_) belong to the entoderm (Figs. 216–221). At an
early stage the chorda develops a transparent structureless sheath,
which is secreted from its cells (Fig. 328 _a_). This _chordalemma_ is
often called the “inner chorda-sheath,” and must not be confused with
the real external sheath, the mesoblastic perichorda.


Fig. 332. Human skull. Fig. 332—Human skull.


But this unsegmented primary axial skeleton is soon replaced by the
segmented secondary axial skeleton, which we know as the vertebral
column. The provertebral plates (Fig. 124 _s_) differentiate from the
innermost, median part of the visceral layer of the cœlom-pouches at
each side of the chorda. As they grow round the chorda and enclose it
they form the skeleton plate or skeletogenetic layer—that is to say,
the skeleton-forming stratum of cells, which provides the mobile
foundation of the permanent vertebral column and skull (scleroblast).
In the head-half of the embryo the skeletal plate remains a continuous,
simple, undivided layer of tissue, and presently enlarges into a
thin-walled capsule enclosing the brain, the primordial skull. In the
trunk-half the provertebral
plate divides into a number of homogeneous, cubical, successive pieces;
these are the several primitive vertebræ. They are not numerous at
first, but soon increase as the embryo grows longer (Figs. 153–155).


Fig. 333. Skull of a new-born child. Fig. 333—Skull of a new-born
child. (From _Kollmann._) Above, in the three bones of the roof of the
skull, we see the lines that radiate from the central points of
ossification; in front, the frontal bone; behind, the occipital bone;
between the two the large parietal bone, _p. s_ the scurf bone, _w_
mastoid fontanelle, _f_ petrous bone, _t_ tympanic bone, _l_ lateral
part, _b_ bulla, _j_ cheek-bone, _a_ large wing of cuneiform bone, _k_
fontanelle of cuneiform bone.


In all the Craniotes the soft, indifferent cells of the mesoderm, which
originally compose the skeletal plate, are afterwards converted for the
most part into cartilaginous cells, and these secrete a firm and
elastic intercellular substance between them, and form cartilaginous
tissue. Like most of the other parts of the skeleton, the membranous
rudiments of the vertebræ soon pass into a cartilaginous state, and in
the higher Vertebrates this is afterwards replaced by the hard osseous
tissue with its characteristic stellate cells (Fig. 6). The primary
axial skeleton remains a simple chorda throughout life in the Acrania,
the Cyclostomes, and the lowest fishes. In most of the other
Vertebrates the chorda is more or less replaced by the cartilaginous
tissue of the secondary perichorda that grows round it. In the lower
Craniotes (especially the fishes) a more or less considerable part of
the chorda is preserved in the bodies of the vertebræ. In the mammals
it disappears for the most part. By the end of the second month in the
human embryo the chorda is merely a slender thread, running through the
axis of the thick, cartilaginous vertebral column (Figs. 182 _ch,_ 329
_ch_). In the cartilaginous vertebral bodies themselves, which
afterwards ossify, the slender remnant of the chorda presently
disappears (Fig. 330 _ch_). But in the elastic inter-vertebral disks,
which develop from the skeletal plate between each pair of vertebral
bodies (Fig. 329 _li_), a relic of the chorda remains permanently. In
the new-born child there is a large pear-shaped cavity in each
intervertebral disk, filled with a gelatinous mass of cells (Fig. 331
_a_).


Fig.334. Head-skeleton of a primitive fish. Fig. 334—Head-skeleton of a
primitive fish. _n_ nasal pit, _eth_ cribriform bone region, _orb_
orbit of eye, _la_ wall of auscultory labyrinth, _occ_ occipital region
of primitive skull, _cv_ vertebral column, _a_ fore, _bc_ hind-lip
cartilage, _o_ primitive upper jaw (_palato-quadratum_), _u_ primitive
lower jaw, _II_ hyaloid bone, _III–VIII_ first to sixth branchial
arches. (From _Gegenbaur._)


Though less sharply defined, this gelatinous nucleus of the elastic
cartilaginous disks persists throughout life in the mammals, but in the
birds and most reptiles the last trace of the chorda disappears. In the
subsequent ossification of the cartilaginous vertebra the first deposit
of bony matter (“first osseous nucleus”) takes place in the vertebral
body immediately round the remainder of the chorda, and soon displaces
it altogether. Then there is a special osseous nucleus formed in each
half of the vertebral arch. The ossification does not reach the point
at which the three nuclei are joined until after birth. In the first
year the two osseous halves of the arches unite; but it is much
later—in the second to the eighth year—

that they connect with the osseous vertebral bodies.


Fig.345. Roofs of the skulls of nine Primates (Cattarrhines), seen from
above and reduced to a common size. Fig. 335—Roofs of the skulls of
nine Primates (_Cattarrhines_), seen from above and reduced to a common
size. _1_ European, _2_ Brazilian, _3_ Pithecanthropus, _4_ Gorilla,
_5_ Chimpanzee, _6_ Orang, _7_ Gibbon, _8_ Tailed ape, _9_ Baboon.


The bony skull (_cranium_), the head-part of the secondary axial
skeleton, develops in just the same way as the vertebral column. The
skull forms a bony envelope for the brain, just as the vertebral canal
does for the spinal cord; and as the brain is only a peculiarly
differentiated part of the head, while the spinal cord represents the
longer trunk-section of the originally homogeneous medullary tube, we
shall expect to find that the osseous coat of the one is a special
modification of the osseous envelope of the other. When we examine the
adult human skull in itself (Fig. 332), it is difficult to conceive how
it can be merely the modified fore part of the vertebral column. It is
an elaborate and extensive bony structure, composed of no less than
twenty bones of different shapes and sizes. Seven of them form the
spacious shell that surrounds the brain, in which we distinguish the
solid ventral base below and the curved dorsal vault above. The other
thirteen bones form the facial skull, which is especially the bony
envelope of the higher sense-organs, and at the same time encloses the
entrance of the alimentary canal. The lower jaw is articulated at the
base of the skull (usually regarded as the XXI cranial bone). Behind
the lower jaw we find the hyoid bone at the root of the tongue, also
formed from the gill-arches, and a part of the lower arches that have
developed as “head-ribs” from the ventral side of the base of the
cranium.


Fig.336. Skeleton of the breast-fin of Ceratodus (biserial feathered
skeleton). Fig. 337. Skeleton of the breast-fin of an early Selachius
(Acanthias). Fig. 338. Skeleton of the breast-fin of a young Selachius.
Fig. 336—Skeleton of the breast-fin of Ceratodus (biserial feathered
skeleton). _A, B,_ cartilaginous series of the fin-stem. _rr_
cartilaginous fin-radii. (From _Gunther._)
Fig. 337—Skeleton of the breast-fin of an early Selachius
(_Acanthias_). The radii of the median fin-border (_B_) have
disappeared for the most part; a few only (_R_) are left. _R, R,_ radii
of the lateral fin-border, _mt_ metapterygium, _ms_ mesopterygium, _p_
propterygium. (From _Gegenbaur._) Fig. 338—Skeleton of the breast-fin
of a young Selachius. The radii of the median fin-border have wholly
disappeared. The shaded part on the right is the section that persists
in the five-fingered hand of the higher Vertebrates. (_b_ the three
basal pieces of the fin: _mt_ metapterygium, rudiment of the humerus,
_ms_ mesopterygium, _p_ propterygium.) (From _Gegenbaur._)


Although the fully-developed skull of the higher Vertebrates, with its
peculiar shape, its enormous size, and its complex composition, seems
to have nothing in common with the ordinary vertebræ, nevertheless even
the older comparative anatomists came to recognise at the end of the
eighteenth century that it is really nothing else originally than a
series of modified vertebræ. When Goethe in 1790 “picked up the skull
of a slain victim from the sand of the Jewish cemetery at Venice, he
noticed at once
that the bones of the face also could be traced to vertebræ (like the
three hind-most cranial vertebræ).” And when Oken (without knowing
anything of Goethe’s discovery) found at Ilenstein, “a fine bleached
skull of a hind, the thought flashed across him like lightning: ‘It is
a vertebral column.’”

This famous vertebral theory of the skull has interested the most
distinguished zoologists for more than a century: the chief
representatives of comparative anatomy have devoted their highest
powers to the solution of the problem, and the interest has spread far
beyond their circle. But it was not until 1872 that it was happily
solved, after seven years’ labour, by the comparative anatomist who
surpassed all other experts of this science in the second half of the
nineteenth century by the richness of his empirical knowledge and the
acuteness and depth of his philosophic speculations. Carl Gegenbaur has
shown, in his classic _Studies of the Comparative Anatomy of the
Vertebrates_ (third section), that we find the most solid foundation
for the vertebral theory of the skull in the head-skeleton of the
Selachii. Earlier anatomists had wrongly started from the mammal skull,
and had compared the several bones that compose it with the several
parts of the vertebra (Fig. 333) they thought they could prove in this
way that the fully-formed mammal skull was made of from three to six
vertebræ.



Fig.339. Skeleton of the fore leg of an amphibian. Fig. 340. Skeleton
of gorilla’s hand. Fig. 341. Skeleton of human hand, back. Fig.
339—Skeleton of the fore leg of an amphibian. _h_ upper-arm (humerus),
_ru_ lower arm (_r_ radius, _u_ ulna), _rcicu′,_ wrist-bones of first
series (_r_ radiale, _i_ intermedium, _c_ centrale, _u′_ ulnare). _1,
2, 3, 4, 5_ wrist-bones of the second series. (From _Gegenbaur._)
Fig. 340—Skeleton of gorilla’s hand. (From _Huxley._)
Fig. 341—Skeleton of human hand, back. (From _Meyer._)


The older theory was refuted by simple and obvious facts, which were
first pointed out by Huxley. Nevertheless, the fundamental idea of
it—the belief that the skull is formed from the head-part of the
perichordal axial skeleton, just as the brain is from the simple
medullary tube, by differentiation and modification—remained. The work
now was to discover the proper way of supplying this philosophic theory
with an empirical foundation, and it was reserved for Gegenbaur to
achieve this. He first opened out the phylogenetic path which here, as
in all morphological questions, leads most confidently to the goal. He
showed that the primitive fishes (Figs. 249–251), the ancestors of all
the Gnathostomes, still preserve permanently in the form of their skull
the structure out of which the transformed skull of the higher
Vertebrates, including man, has been evolved. He further showed that
the branchial arches of the Selachii prove that their skull originally
consisted of a large number of (at least nine or ten) provertebræ, and
that the cerebral nerves that proceed from the base of the brain
entirely confirm this. These cerebral nerves are (with the exception of
the first and second pair, the olfactory and optic nerves) merely
modifications of spinal nerves, and are essentially similar to them in
their peripheral expansion. The comparative anatomy of these cerebral
nerves, their origin and their expansion, furnishes one of the
strongest arguments for the new vertebral theory of the skull.


Fig.342. Skeleton of the hand or fore foot of six mammals. I man, II
dog, III pig, IV ox, V tapir, VI horse. Fig. 342—Skeleton of the hand
or fore foot of six mammals. _I_ man, _II_ dog, _III_ pig, _IV_ ox, _V_
tapir, _VI_ horse. _r_ radius, _u_ ulna, _a_ scaphoideum, _b_ lunare,
_a_ triquetrum, _d_ trapezium, _e_ trapezoid, _f_ capitatum, _g_
hamatum, _p_ pisiforme. _1_ thumb, _2_ index finger, _3_ middle finger,
_4_ ring finger, _5_ little finger. (From _Gegenbaur._)


We have not space here to go into the details of Gegenbaur’s theory of
the skull. I must be content to refer the reader to the great work I
have mentioned, in which it is thoroughly established from the
empirico-philosophical point of view. He has also given a comprehensive
and up-to-date treatment of the subject in his _Comparative Anatomy of
the Vertebrates_ (1898). Gegenbaur indicates as original “cranial
ribs,” or “lower arches of the cranial vertebræ,” at each side of the
head of the Selachii (Fig. 334), the following pairs of arches: _I_ and
_II,_ two lip-cartilages, the anterior (_a_) of which is composed of an
upper piece only, the posterior (_bc_) from an upper and lower piece;
_III,_ the maxillary arches, also consisting of two pieces on each
side—the primitive upper jaw (_os palato-quadratum, o_) and the
primitive lower jaw (_u_); _IV,_ the hyaloid bone (_II_); finally,
_V–X,_ six branchial arches in the narrower sense (_III–VIII_). From
the anatomic features of these nine to ten cranial ribs or “lower
vertebral arches” and the cranial nerves that spread over them, it is
clear that the apparently simple cartilaginous primitive skull of the
Selachii was originally formed from so many (at least nine) somites or
provertebræ. The blending of these primitive segments into a single
capsule is, however, so ancient that, in virtue of the law of curtailed
heredity, the original division seems to have disappeared; in the
embryonic development it is very difficult to detect it in isolated
traces, and in some respects quite impossible. It is claimed that
several (three to six) traces of provertebræ have been discovered in
the anterior (pre-chordal) part of the Selachii-skull; this would bring
up the number of cranial somites to twelve or sixteen, or even more.

In the primitive skull of man (Fig. 323) and the higher Vertebrates,
which has been evolved from that of the Selachii, five consecutive
sections are discoverable at a certain early period of development, and
one might be induced to trace these to five primitive vertebræ; but
these sections are due entirely to adaptation to

the five primitive cerebral vesicles, and correspond, like these, to a
large number of metamera. That we have in the primitive skull of the
mammals a greatly modified and transformed organ, and not at all a
primitive formation, is clear from the circumstance that its original
soft membranous form only assumes the cartilaginous character for the
most part at the base and the sides, and remains membranous at the
roof. At this part the bones of the subsequent osseous skull develop as
external coverings over the membranous structure, without an
intermediate cartilaginous stage, as there is at the base of the skull.
Thus a large part of the cranial bones develop originally as covering
bones from the corium, and only secondarily come into close touch with
the primitive skull (Fig. 333). We have previously seen how this very
rudimentary beginning of the skull in man is formed ontogenetically
from the “head-plates,” and thus the fore end of the chorda is enclosed
in the base of the skull. (Cf. Fig. 145 and pp. 138, 144, and 149.)


Fig.343-345. Arm and hand of three anthropoids. Fig. 343. Chimpanzee
(Anthropithecus niger). Fig. 344. Veddah of Ceylon (Homo veddalis).
Fig. 345. European (Homo mediterraneus). Figs. 343–345—Arm and hand of
three anthropoids. Fig. 343—Chimpanzee (_Anthropithecus niger_). Fig.
344—Veddah of Ceylon (_Homo veddalis_). Fig. 345—European (_Homo
mediterraneus_). (From _Paul_ and _Fritz Sarasin._)


The phylogeny of the skull has made great progress during the last
three decades through the joint attainments of comparative anatomy,
ontogeny, and paleontology. By the judicious and comprehensive
application of the phylogenetic method (in the sense of Gegenbaur) we
have found the key to the great and important problems that arise from
the thorough comparative study of the skull. Another school of
research, the school of what is called “exact craniology” (in the sense
of Virchow), has, meantime, made fruitless efforts to obtain this
result. We may gratefully acknowledge all that this descriptive school
has done in the way of accurately describing the various forms and
measurements of the human skull, as compared with those of other
mammals. But the vast empirical material that it has accumulated in its
extensive literature is mere dead and sterile erudition until it is
vivified and illumined by phylogenetic speculation.

Virchow confined himself to the most careful analysis of large numbers
of human skulls and those of anthropoid mammals. He saw only the
differences between them, and sought to express these in figures.


Fig.346. Transverse section of a fish’s tail (from the tunny). Fig.
346—Transverse section of a fish’s tail (from the tunny). (From
_Johannes Müller._) _a_ upper (dorsal) lateral muscles, _a′, b′_ lower
(ventral) lateral muscles, _d_ vertebral bodies, _b_ sections of
incomplete conical mantle, _B_ attachment lines of the inter-muscular
ligaments (from the side).


Without adducing a single solid reason, or offering any alternative
explanation, he rejected evolution as an unproved hypothesis. He played
a most unfortunate part in the controversy as to the significance of
the fossil human skulls of Spy and Neanderthal, and the comparison of
them with the skull of the Pithecanthropus (Fig. 283). All the
interesting features of these skulls that clearly indicated the
transition from the anthropoid to the man were declared by Virchow to
be chance pathological variations. He said that the roof of the skull
of Pithecanthropus (Fig. 335, _3_) must have belonged to an ape,
because so pronounced an _orbital stricture_ (the horizontal
constriction between the outer edge of the eye-orbit and the temples)
is not found in any human being. Immediately afterwards Nehring showed
in the skull of a Brazilian Indian (Fig. 335, _2_), found in the
Sambaquis of Santos, that this stricture can be even deeper in man than
in many of the apes. It is very instructive in this connection to
compare the roofs of the skulls (seen from above) of different
primates. I have, therefore, arranged nine such skulls in Fig. 335, and
reduced them to a common size.

We turn now to the branchial arches, which were regarded even by the
earlier natural philosophers as “head-ribs.” (Cf. Figs. 167–170). Of
the four original gill-arches of the mammals the first lies between the
primitive mouth and the first gill-cleft. From the base of this arch is
formed the upper-jaw process, which joins with the inner and outer
nasal processes on each side, in the manner we have previously
explained, and forms the chief parts of the skeleton of the upper jaw
(palate bone, pterygoid bone, etc.) (Cf. p. 284.) The remainder of the
first branchial arch, which is now called, by
way of contrast, the “upper-jaw process,” forms from its base two of
the ear-ossicles (hammer and anvil), and as to the rest is converted
into a long strip of cartilage that is known, after its discoverer, as
“Meckel’s cartilage,” or the _promandibula._ At the outer surface of
the latter is formed from the cellular matter of the corium, as
covering or accessory bone, the permanent bony lower jaw. From the
first part or base of the second branchial arch we get, in the mammals,
the third ossicle of the ear, the stirrup; and from the succeeding
parts we get (in this order) the muscle of the stirrup, the styloid
process of the temporal bone, the styloid-hyoid ligament, and the
little horn of the hyoid bone. The third branchial arch is only
cartilaginous at the foremost part, and here the body of the hyoid bone
and its larger horn are formed at each side by the junction of its two
halves. The fourth branchial arch is only found transitorily in the
mammal embryo as a rudimentary organ, and does not develop special
parts; and there is no trace in the embryo of the higher Vertebrates of
the posterior branchial arches (fifth and sixth pair), which are
permanent in the Selachii. They have been lost long ago. Moreover, the
four gill-clefts of the human embryo are only interesting as
rudimentary organs, and they soon close up and disappear. The first
alone (between the first and second branchial arches) has any permanent
significance; from it are developed the tympanic cavity and the
Eustachian tube. (Cf. Figs. 169, 320.)

It was Carl Gegenbaur again who solved the difficult problem of tracing
the skeleton of the limbs of the Vertebrates to a common type. Few
parts of the vertebrate body have undergone such infinitely varied
modifications in regard to size, shape, and adaptation of structure as
the limbs or extremities; yet we are in a position to reduce them all
to the same hereditary standard. We may generally distinguish three
groups among the Vertebrates in relation to the formation of their
limbs. The lowest and earliest Vertebrates, the Acrania and
Cyclostomes, had, like their invertebrate ancestors, no pairs of limbs,
as we see in the Amphioxus and the Cyclostomes to-day (Figs. 210, 247).
The second group is formed of the two classes of the true fishes and
the Dipneusts; here there are always two pairs of limbs at first, in
the shape of many-toed fins—one pair of breast-fins or fore legs, and
one pair of belly-fins or hind legs (Figs. 248–259). The third group
comprises the four higher classes of Vertebrates—the amphibia,
reptiles, birds, and mammals; in these quadrupeds there are at first
the same two pairs of limbs, but in the shape of five-toed feet.
Frequently we find less than five toes, and sometimes the feet are
wholly atrophied (as in the serpents). But the original stem-form of
the group had five toes or fingers before and behind (Figs. 263–265).

The true primitive form of the pairs of limbs, such as they were found
in the primitive fishes of the Silurian period, is preserved for us in
the Australian dipneust, the remarkable _Ceratodus_ (Fig. 257). Both
the breast-fin and the belly-fin are flat oval paddles, in which we
find a biserial cartilaginous skeleton (Fig. 336). This consists,
firstly, of a much segmented fin-rod or “stem” (_A, B_), which runs
through the fin from base to tip; and secondly of a double row of thin
articulated fin-radii (_r, r_), which are attached to both sides of the
fin-rod, like the feathers of a feathered leaf. This primitive fin,
which Gegenbaur first recognised, is attached to the vertebral column
by a simple zone in the shape of a cartilaginous arch. It has probably
originated from the branchial arches.[31]

 [31] While Gegenbaur derives the fins from two pairs of posterior
 separated branchial arches, Balfour holds that they have been
 developed from segments of a pair of originally continuous lateral
 fins or folds of the skin.)


We find the same biserial primitive fin more or less preserved in the
fossilised remains of the earliest Selachii (Fig. 248), Ganoids (Fig.
253), and Dipneusts (Fig. 256). It is also found in modified form in
some of the actual sharks and pikes. But in the majority of the
Selachii it has already degenerated to the extent that the radii on one
side of the fin-rod have been partly or entirely lost, and are retained
only on the other (Fig. 337). We thus get the uniserial fin, which has
been transmitted from the Selachii to the rest of the fishes (Fig.
338).

Gegenbaur has shown how the five-toed leg of the Amphibia, that has
been inherited by the three classes of Amniotes, was evolved from the
uniserial fish-fin.[32]


 [32] The limb of the four higher classes of Vertebrates is now
 explained in the sense that the original fin-rod passes along its
 outer (ulnar or fibular) side, and ends in the fifth toe. It was
 formerly believed to go along the inner (radial or tibial) side, and
 end in the first toe, as Fig. 339 shows.) In the dipneust ancestors of
 the Amphibia the radii gradually atrophy, and are lost, for the most
 part, on the other side of the fin-rod as well (the lighter cartilages
 in Fig. 338). Only the four lowest radii (shaded in the illustration)
 are preserved; and these are the four inner toes of the foot (first to
 fourth). The little or fifth toe is developed from the lower end of
 the fin-rod. From the middle and upper part of the fin-rod was
 developed the long stem of the limb—the important radius and ulna
 (Fig. 339 _r_ and _u_) and humerus (_h_) of the higher Vertebrates.



Fig.347. Human skeleton. Fig. 348. Skeleton of the giant gorilla. Fig.
347—Human skeleton. (Cf. Figure 326.) Fig. 348—Skeleton of the giant
gorilla. (Cf. Figure 209.)


In this way the five-toed foot of the Amphibia, which we first meet in
the Carboniferous Stegocephala (Fig. 260), and which was inherited from
them by the reptiles on one side and the mammals on the other, was
formed by gradual degeneration and differentiation from the many-toed
fish-fin (Fig. 341). The reduction of the radii to four was accompanied
by a further differentiation of the fin-rod, its transverse
segmentation into upper and lower halves, and the formation of the zone
of the limb, which is composed originally of three limbs before and
behind in the higher Vertebrates. The simple arch of the original
shoulder-zone divides on each side into an upper (dorsal) piece, the
shoulder-blade (_scapula_), and a lower (ventral) piece; the anterior
part of the latter forms the primitive clavicle (_procoracoideum_), and
the posterior part the _coracoideum._ In the same way the simple arch
of the pelvic zone breaks up into an upper (dorsal) piece, the
iliac-bone (_os ilium_), and a lower (ventral) piece; the anterior part
of the latter forms the pubic bone (_os pubis_), and the posterior the
ischial bone (_os ischii_).

There is also a complete agreement between the fore and hind limb in
the stem or shaft. The first section of the stem is supported by a
single strong bone—the humerus in the fore, the femur in the hind limb.
The second section contains two bones: in front the radius (_r_) and
ulna (_u_), behind the tibia and fibula. (Cf. the skeletons in Figs.
260, 265, 270, 278–282, and 348.) The succeeding numerous small bones
of the wrist (_carpus_) and ankle (_tarsus_) are also similarly
arranged in the fore and hind extremities, and so are the five bones of
the middle-hand (_metacarpus_) and middle-foot (_metatarsus_). Finally,
it is the same with the toes themselves, which have a similar
characteristic composition from a series of bony pieces before and
behind. We find a complete parallel in all the parts of the fore leg
and the hind leg.

When we thus learn from comparative anatomy that the skeleton of the
human limbs is composed of just the same bones, put together in the
same way, as the skeleton in the four higher classes of Vertebrates, we
may at once infer a common descent of them from a single stem-form.
This stem-form was the earliest amphibian that had five toes on each
foot. It is particularly the outer parts of the limbs that have been
modified by adaptation to different conditions. We need only recall the
immense variations they offer within the mammal class. We have the
slender legs of the deer and the strong springing legs of the kangaroo,
the climbing feet of the sloth and the digging feet of the mole, the
fins of the whale and the wings of the bat. It will readily be granted
that these organs of locomotion differ as much in regard to size,
shape, and special function as can be conceived. Nevertheless, the bony
skeleton is substantially the same in every case. In the different
limbs we always find the same characteristic bones in essentially the
same rigidly hereditary connection; this is as splendid a proof of the
theory of evolution as comparative anatomy can discover in any organ of
the body. It is true that the skeleton of the limbs of the various
mammals undergoes many distortions and degenerations besides the
special adaptations (Fig. 342). Thus we find the first finger or the
thumb atrophied in the fore-foot (or hand) of the dog (_II_). It has
entirely disappeared in the pig (_III_) and tapir (_V_). In the
ruminants (such as the ox, _IV_) the second and fifth toes are also
atrophied, and only the third and fourth are well developed (_VI, 3_).
Nevertheless, all these different fore-feet, as well as the hand of the
ape (Fig. 340) and of man (Fig. 341), were originally developed from a
common pentadactyle stem-form. This is proved by the rudiments of the
degenerated toes, and by the similarity of the arrangement of the
wrist-bones in all the pentanomes (Fig. 342 _a–p_).

If we candidly compare the bony skeleton of the human arm and hand with
that of the nearest anthropoid apes, we find an almost perfect
identity. This is especially true of the chimpanzee. In regard to the
proportions of the various
parts, the lowest living races of men (the Veddahs of Ceylon, Fig. 344)
are midway between the chimpanzee (Fig. 343) and the European (Fig.
345). More considerable are the differences in structure and the
proportions of the various parts between the different genera of
anthropoid apes (Figs. 278–282); and still greater is the morphological
distance between these and the lowest apes (the _Cynopitheca_). Here,
again, impartial and thorough anatomic comparison confirms the accuracy
of Huxley’s pithecometra principle p. 171.

The complete unity of structure which is thus revealed by the
comparative anatomy of the limbs is fully confirmed by their
embryology. However different the extremities of the four-footed
Craniotes may be in their adult state, they all develop from the same
rudimentary structure. In every case the first trace of the limb in the
embryo is a very simple protuberance that grows out of the side of the
hyposoma. These simple structures develop directly into fins in the
fishes and Dipneusts by differentiation of their cells. In the higher
classes of Vertebrates each of the four takes the shape in its further
growth of a leaf with a stalk, the inner half becoming narrower and
thicker and the outer half broader and thinner. The inner half (the
stalk of the leaf) then divides into two sections—the upper and lower
parts of the limb. Afterwards four shallow indentations are formed at
the free edge of the leaf, and gradually deepen; these are the
intervals between the five toes (Fig. 174). The toes soon make their
appearance. But at first all five toes, both of fore and hind feet, are
connected by a thin membrane like a swimming-web; they remind us of the
original shaping of the foot as a paddling fin. The further development
of the limbs from this rudimentary structure takes place in the same
way in all the Vertebrates according to the laws of heredity.

The embryonic development of the muscles, or _active_ organs of
locomotion, is not less interesting than that of the skeleton, or
_passive_ organs. But the comparative anatomy and ontogeny of the
muscular system are much more difficult and inaccessible, and
consequently have hitherto been less studied. We can therefore only
draw some general phylogenetic conclusions therefrom.

It is incontestable that the musculature of the Vertebrates has been
evolved from that of lower Invertebrates; and among these we have to
consider especially the unarticulated Vermalia. They have a simple
cutaneous muscular layer, developing from the mesoderm. This was
afterwards replaced by a pair of internal lateral muscles, that
developed from the middle wall of the cœlom-pouches; we still find the
first rudiments of the muscles arising from the muscle-plate of these
in the embryos of all the Vertebrates (cf. Figs. 124, 158–160, 222–224
_mp_). In the unarticulated stem-forms of the Chordonia, which we have
called the Prochordonia, the two cœlom-pouches, and therefore also the
muscle-plates of their walls, were not yet segmented. A great advance
was made in the articulation of them, as we have followed it step by
step in the Amphioxus (Figs. 124, 158). This segmentation of the
muscles was the momentous historical process with which vertebration,
and the development of the vertebrate stem, began. The articulation of
the skeleton came after this segmentation of the muscular system, and
the two entered into very close correlation.

The episomites or dorsal cœlom-pouches of the Acrania, Cyclostomes, and
Selachii (Fig. 161 _h_) first develop from their inner or median wall
(from the cell-layer that lies directly on the skeletal plate [_sk_]
and the medullary tube [_nr_]) a strong muscle-plate (_mp_). By dorsal
growth (_w_) it also reaches the external wall of the cœlom-pouches,
and proceeds from the dorsal to the ventral wall. From these segmental
muscle-plates, which are chiefly concerned in the segmentation of the
Vertebrates, proceed the lateral muscles of the stem, as we find in the
simplest form in the Amphioxus (Fig. 210). By the formation of a
horizontal frontal septum they divide on each side into an upper and
lower series of myotomes, dorsal and ventral lateral muscles. This is
seen with typical regularity in the transverse section of the tail of a
fish (Fig. 346). From these earlier lateral muscles of the trunk
develop the greater part of the subsequent muscles of the trunk, and
also the much later “muscular buds” of the limbs.[33]

 [33] The ontogeny of the muscles is mostly cenogenetic. The greater
 part of the muscles of the head (or the visceral muscles) belong
 originally to the hyposoma of the vertebrate organism, and develop
 from the wall of the hyposomites or ventral cœlom-pouches. This also
 applies originally to the primary muscles of the limbs, as these too
 belong phylogenetically to the hyposoma. (Cf. Chapter XIV.)



Chapter XXVII.
THE EVOLUTION OF THE ALIMENTARY SYSTEM


The chief of the vegetal organs of the human frame, to the evolution of
which we now turn our attention, is the alimentary canal. The gut is
the oldest of all the organs of the metazoic body, and it leads us back
to the earliest age of the formation of organs—to the first section of
the Laurentian period. As we have already seen, the result of the first
division of labour among the homogeneous cells of the earliest
multicellular animal body was the formation of an alimentary cavity.
The first duty and first need of every organism is self-preservation.
This is met by the functions of the nutrition and the covering of the
body. When, therefore, in the primitive globular _Blastæa_ the
homogeneous cells began to effect a division of labour, they had first
to meet this twofold need. One half were converted into alimentary
cells and enclosed a digestive cavity, the gut. The other half became
covering cells, and formed an envelope round the alimentary tube and
the whole body. Thus arose the primary germinal layers—the inner,
alimentary, or vegetal layer, and the outer, covering, or animal layer.
(Cf. pp. 214–17.)

When we try to construct an animal frame of the simplest conceivable
type, that has some such primitive alimentary canal and the two primary
layers constituting its wall, we inevitably come to the very remarkable
embryonic form of the gastrula, which we have found with extraordinary
persistence throughout the whole range of animals, with the exception
of the unicellulars—in the Sponges, Cnidaria, Platodes, Vermalia,
Molluscs, Articulates, Echinoderms, Tunicates, and Vertebrates. In all
these stems the gastrula recurs in the same very simple form. It is
certainly a remarkable fact that the gastrula is found in various
animals as a larva-stage in their individual development, and that this
gastrula, though much disguised by cenogenetic modifications, has
everywhere essentially the same palingenetic structure (Figs. 30–35).
The elaborate alimentary canal of the higher animals develops
ontogenetically from the same simple primitive gut of the _gastrula._

This gastræa theory is now accepted by nearly all zoologists. It was
first supported and partly modified by Professor Ray-Lankester; he
proposed three years afterwards (in his essay on the development of the
Molluscs, 1875) to give the name of _archenteron_ to the primitive gut
and _blastoporus_ to the primitive mouth.

Before we follow the development of the human alimentary canal in
detail, it is necessary to say a word about the general features of its
composition in the fully-developed man. The mature alimentary canal in
man is constructed in all its main features like that of all the higher
mammals, and particularly resembles that of the Catarrhines, the
narrow-nosed apes of the Old World. The entrance into it, the mouth, is
armed with thirty-two teeth, fixed in rows in the upper and lower jaws.
As we have seen, our dentition is exactly the same as that of the
Catarrhines, and differs from that of all other animals p. 257. Above
the mouth-cavity is the double nasal cavity; they are separated by the
palate-wall. But we saw that this separation is not there from the
first, and that originally there is a common mouth-nasal cavity in the
embryo; and this is only divided afterwards by the hard palate into
two—the nasal cavity above and that of the mouth below (Fig. 311).

At the back the cavity of the mouth is half closed by the vertical
curtain that we call the soft palate, in the middle of which is the
uvula. A glance into a mirror with the mouth wide open will show its
shape. The uvula is interesting because, besides man, it is only found
in the ape. At each side of the soft palate are the tonsils. Through
the curved opening that we find
underneath the soft palate we penetrate into the gullet or pharynx
behind the mouth-cavity. Into this opens on either side a narrow canal
(the Eustachian tube), through which there is direct communication with
the tympanic cavity of the ear (Fig. 320 _e_). The pharynx is continued
in a long, narrow tube, the œsophagus ( _sr_). By this the food passes
into the stomach when masticated and swallowed. Into the gullet also
opens, right above, the trachea ( _lr_), that leads to the lungs. The
entrance to it is covered by the epiglottis, over which the food
slides. The cartilaginous epiglottis is found only in the mammals, and
has developed from the fourth branchial arch of the fishes and
amphibia. The lungs are found, in man and all the mammals, to the right
and left in the pectoral cavity, with the heart between them. At the
upper end of the trachea there is, under the epiglottis, a specially
differentiated part, strengthened by a cartilaginous skeleton, the
larynx. This important organ of human speech also develops from a part
of the alimentary canal. In front of the larynx is the thyroid gland,
which sometimes enlarges and forms goitre.

The œsophagus descends into the pectoral cavity along the vertebral
column, behind the lungs and the heart, pierces the diaphragm, and
enters the visceral cavity. The diaphragm is a membrano-muscular
partition that completely separates the thoracic from the abdominal
cavity in all the mammals (and these alone). This separation is not
found in the beginning; there is at first a common breast-belly cavity,
the cœloma or pleuro-peritoneal cavity. The diaphragm is formed later
on as a muscular horizontal partition between the thoracic and
abdominal cavities. It then completely separates the two cavities, and
is only pierced by several organs that pass from the one to the other.
One of the chief of these organs is the œsophagus. After this has
passed through the diaphragm, it expands into the gastric sac in which
digestion chiefly takes place. The stomach of the adult man (Fig. 349)
is a long, somewhat oblique sac, expanding on the left into a blind
sac, the fundus of the stomach ( _b′_), but narrowing on the right, and
passing at the pylorus ( _e_) into the small intestine. At this point
there is a valve, the pyloric valve ( _d_), between the two sections of
the canal; it opens only when the pulpy food passes from the stomach
into the intestine. In man and the higher Vertebrates the stomach
itself is the chief organ of digestion, and is especially occupied with
the solution of the food; this is not the case in many of the lower
Vertebrates, which have no stomach, and discharge its function by a
part of the gut farther on. The muscular wall of the stomach is
comparatively thick; it has externally strong muscles that accomplish
the digestive movements, and internally a large quantity of small
glands, the peptic glands, which secrete the gastric juice.


Fig.349. Human stomach and duodenum, longitudinal section. Fig.
349—Human stomach and duodenum, longitudinal section. _a_ cardiac (end
of œsophagus), _b_ fundus (blind sac of the left side), _c_
pylorus-fold, _d_ pylorus-valves, _e_ pylorus-cavity, _fgh_ duodenum,
_i_ entrance of the gall-duct and the pancreatic duct. (From _Meyer._)


Next to the stomach comes the longest section of the alimentary canal,
the middle gut or small intestine. Its chief function is to absorb the
peptonised fluid mass of food, or the chyle, and it is subdivided into
several sections, of which the first (next to the stomach) is called
the duodenum (Fig. 349 _fgh_). It is a short, horseshoe-shaped loop of
the gut. The largest glands of the alimentary canal open into it—the
liver, the chief digestive gland, that secretes the gall, and the
pancreas, which secretes the pancreatic juice. The two glands pour
their secretions, the bile and pancreatic juice, close together into
the duodenum ( _i_). The opening of the gall-duct is of particular
phylogenetic importance, as it is the same in all the Vertebrates, and
indicates the principal point of the hepatic or trunk-gut (Gegenbaur).
The liver, phylogenetically older than the stomach, is a large gland,
rich in blood, in the adult man, immediately under the diaphragm on the
left
side, and separated by it from the lungs. The pancreas lies a little
further back and more to the left. The remaining part of the small
intestine is so long that it has to coil itself in many folds in order
to find room in the narrow space of the abdominal cavity. It is divided
into the jejunum above and the ileum below. In the last section of it
is the part of the small intestine at which in the embryo the yelk-sac
opens into the gut. This long and thin intestine then passes into the
large intestine, from which it is cut off by a special valve.
Immediately behind this “Bauhin-valve” the first part of the large
intestine forms a wide, pouch-like structure, the cæcum. The atrophied
end of the cæcum is the famous rudimentary organ, the vermiform
appendix. The large intestine ( _colon_) consists of three parts—an
ascending part on the right, a transverse middle part, and a descending
part on the left. The latter finally passes through an S-shaped bend
into the last section of the alimentary canal, the rectum, which opens
behind by the anus. Both the large and small intestines are equipped
with numbers of small glands, which secrete mucous and other fluids.


Fig.350. Median section of the head of a hare-embryo, one-fourth of an
inch in length. Fig. 350—Median section of the head of a hare-embryo,
one-fourth of an inch in length. (From _Mihalcovics._) The deep
mouth-cleft ( _hp_) is separated by the membrane of the throat ( _rh_)
from the blind cavity of the head-gut ( _kd_). _hz_ heart, _ch_ chorda,
_hp_ the point at which the hypophysis develops from the mouth-cleft,
_vh_ ventricle of the cerebrum, _v3_ , third ventricle (intermediate
brain), _v4_ fourth ventricle (hind brain), _ck_ spinal canal.


For the greater part of its length the alimentary canal is attached to
the inner dorsal surface of the abdominal cavity, or to the lower
surface of the vertebral column. The fixing is accomplished by means of
the thin membranous plate that we call the mesentery.

Although the fully-formed alimentary canal is thus a very elaborate
organ, and although in detail it has a quantity of complex structural
features into which we cannot enter here, nevertheless the whole
complicated structure has been historically evolved from the very
simple form of the primitive gut that we find in our
gastræad-ancestors, and that every gastrula brings before us to-day. We
have already pointed out (Chapter IX) how the epigastrula of the
mammals (Fig. 67) can be reduced to the original type of the
bell-gastrula, which is now preserved by the amphioxus alone (Fig. 35).
Like the latter, the human gastrula and that of all other mammals must
be regarded as the ontogenetic reproduction of the phylogenetic form
that we call the Gastræa, in which the whole body is nothing but a
double-walled gastric sac.

We already know from embryology the manner in which the gut develops in
the embryo of man and the other mammals. From the gastrula is first
formed the spherical embryonic vesicle filled with fluid (
_gastrocystis,_ Fig. 106). In the dorsal wall of this the sole-shaped
embryonic shield is developed, and on the under-side of this a shallow
groove appears in the middle line, the first trace of the later,
secondary alimentary tube. The gut-groove becomes deeper and deeper,
and its edges bend towards each other, and finally form a tube.

As we have seen, this simple cylindrical gut-tube is at first
completely closed before and behind in man and in the Vertebrates
generally (Fig. 148); the permanent openings of the alimentary canal,
the mouth and anus, are only formed later on, and from the outer skin.
A mouth-pit appears in the skin in front (Fig. 350 _hp_), and this
grows towards the blind fore-end of the cavity of the head-gut ( _kd_),
and at length breaks into it. In the same way a shallow anus-pit is
formed in the skin behind, which grows deeper and deeper, advances
towards the blind hinder end of the pelvic gut, and at last connects
with it. There is at first, both before and behind, a thin partition
between the external cutaneous pit and the blind end of the gut—the
throat-membrane in front and the anus-membrane behind; these disappear
when the connection takes place.

Directly in front of the anus-opening the allantois develops from the
hind gut; this is the important embryonic structure
that forms into the placenta in the Placentals (including man). In this
more advanced form the human alimentary canal (and that of all the
other mammals) is a slightly bent, cylindrical tube, with an opening at
each end, and two appendages growing from its lower wall: the anterior
one is the umbilical vesicle or yelk-sac, and the posterior the
allantois or urinary sac (Fig. 195).

The thin wall of this simple alimentary tube and its ventral appendages
is found, on microscopic examination, to consist of two strata of
cells. The inner stratum, lining the entire cavity, consists of larger
and darker cells, and is the gut-gland layer. The outer stratum
consists of smaller and lighter cells, and is the gut-fibre layer. The
only exception is in the cavities of the mouth and anus, because these
originate from the skin. The inner coat of the mouth-cavity is not
provided by the gut-gland layer, but by the skin-sense layer; and its
muscular substratum is provided, not by the gut-fibre, but the
skin-fibre, layer. It is the same with the wall of the small
anus-cavity.

If it is asked how these constituent layers of the primitive gut-wall
are related to the various tissues and organs that we find afterwards
in the fully-developed system, the answer is very simple. It can be put
in a single sentence. The epithelium of the gut—that is to say, the
internal soft stratum of cells that lines the cavity of the alimentary
canal and all its appendages, and is immediately occupied with the
processes of nutrition—is formed solely from the gut-gland layer; all
other tissues and organs that belong to the alimentary canal and its
appendages originate from the gut-fibre layer. From the latter is also
developed the whole of the outer envelope of the gut and its
appendages; the fibrous connective tissue and the smooth muscles that
compose its muscular layer, the cartilages that support it (such as the
cartilages of the larynx and the trachea), the blood-vessels and
lymph-vessels that absorb the nutritive fluid from the intestines—in a
word, all that there is in the alimentary system besides the epithelium
of the gut. From the same layer we also get the whole of the mesentery,
with all the organs embedded in it—the heart, the large blood-vessels
of the body, etc.


Fig.351. Scales or cutaneous teeth of a shark (Centrophorus calceus).
Fig. 351—Scales or cutaneous teeth of a shark ( _Centrophorus
calceus_). A three-pointed tooth rises obliquely on each of the
quadrangular bony plates that lie in the corium. (From _Gegenbaur._)


Let us now leave this original structure of the mammal gut for a
moment, in order to compare it with the alimentary canal of the lower
Vertebrates, and of those Invertebrates that we have recognised as
man’s ancestors. We find, first of all, in the lowest Metazoa, the
Gastræads, that the gut remains permanently in the very simple form in
which we find it transitorily in the palingenetic gastrula of the other
animals; it is thus in the Gastremaria ( _Pemmatodiscus_), the
Physemaria ( _Prophysema_), the simplest Sponges ( _Olynthus_), the
freshwater Polyps ( _Hydra_), and the ascula-embryos of many other
Cœlenteria (Figs. 233–238). Even in the simplest forms of the Platodes,
the Rhabdocœla (Fig. 240), the gut is still a simple straight tube,
lined with the entoderm; but with the important difference that in this
case its single opening, the primitive mouth ( _m_), has formed a
muscular gullet ( _sd_) by invagination of the skin.

We have the same simple form in the gut of the lowest Vermalia
(Gastrotricha, Fig. 242, Nematodes, Sagitta, etc.). But in these a
second important opening of the gut has been formed at the opposite end
to the mouth, the anus (Fig. 242 _a_).


We see a great advance in the structure of the vermalian gut in the
remarkable _Balanoglossus_ (Fig. 245), the sole survivor of the
Enteropneust class. Here we have the first appearance of the division
of the alimentary tube into two sections that characterises the
Chordonia. The fore half, the head-gut ( _cephalogaster_), becomes the
organ of respiration (branchial gut, Fig. 245 _k_); the hind half, the
trunk-gut ( _truncogaster_), alone acts as digestive organ (hepatic
gut, _d_). The differentiation of these two parts of the gut in the
Enteropneust is just the same as in all the Tunicates and Vertebrates.


Fig.352. Gut of a human embryo, one-sixth of an inch long. Fig. 352—Gut
of a human embryo, one-sixth of an inch long. (From _His._)


It is particularly interesting and instructive in this connection to
compare the Enteropneusts with the Ascidia and the Amphioxus (Figs.
220, 210)—the remarkable animals that form the connecting link between
the Invertebrates and the Vertebrates. In both forms the gut is of
substantially the same construction; the anterior section forms the
respiratory branchial gut, the posterior the digestive hepatic gut. In
both it develops palingenetically from the primitive gut of the
gastrula, and in both the hinder end of the medullary tube covers the
primitive mouth to such an extent that the remarkable medullary
intestinal duct is formed, the passing communication between the neural
and intestinal tubes ( _canalis neurentericus,_ Figs. 83, 85 _ne_). In
the vicinity of the closed primitive mouth, possibly in its place, the
later anus is developed. In the same way the mouth is a fresh formation
in the Amphioxus and the Ascidia. It is the same with the human mouth
and that of the Craniotes generally. The secondary formation of the
mouth in the Chordonia is probably connected with the development of
the gill-clefts which are formed in the gut-wall immediately behind the
mouth. In this way the anterior section of the gut is converted into a
respiratory organ. I have already pointed out that this modification is
distinctive of the
Vertebrates and Tunicates. The phylogenetic appearance of the
gill-clefts indicates the commencement of a new epoch in the
stem-history of the Vertebrates.

In the further ontogenetic development of the alimentary canal in the
human embryo the appearance of the gill-clefts is the most important
process. At a very early stage the gullet-wall joins with the external
body-wall in the head of the human embryo, and this is followed by the
formation of four clefts, which lead directly into the gullet from
without, on the right and left sides of the neck, behind the mouth.
These are the gill or gullet clefts, and the partitions that separate
them are the gill or gullet-arches (Fig. 171). These are most
interesting embryonic structures. They show us that all the higher
Vertebrates reproduce in their earlier stages, in harmony with the
biogenetic law, the process that had so important a part in the rise of
the whole Chordonia-stem. This process was the differentiation of the
gut into two sections—an anterior respiratory section, the branchial
gut, that was restricted to breathing, and a posterior digestive
section, the hepatic gut. As we find this highly characteristic
differentiation of the gut into two different sections in all the
Vertebrates and all the Tunicates, we may conclude that it was also
found in their common ancestors, the Prochordonia—especially as even
the Enteropneusts have it. (Cf. pp. 119, 151, 227, Figs. 210, 220,
245.) It is entirely wanting in all the other Invertebrates.


Fig.353. Gut of a dog-embryo (shown  in Fig. 202, from Bischoff), seen
from the ventral side. Fig. 354. The same gut seen from the right. Fig.
353—Gut of a dog-embryo (shown in Fig. 202, from _Bischoff_), seen from
the ventral side. _a_ gill-arches (four pairs), _b_ rudiments of
pharynx and larynx, _c_ lungs, _d_ stomach, _f_ liver, _g_ walls of the
open yelk-sac (into which the middle gut opens with a wide aperture),
_h_ rectum.

Fig. 354—The same gut seen from the right. _a_ lungs, _b_ stomach, _c_
liver, _d_ yelk-sac, _e_ rectum.)


There is at first only one pair of gill-clefts in the Amphioxus, as in
the Ascidia and Enteropneusts; and the Copelata (Fig. 225) have only
one pair throughout life. But the number presently increases in the
former. In the Craniotes, however, it decreases still further. The
Cyclostomes have six to eight pairs (Fig. 247); some of the Selachii
six or seven pairs, most of the fishes only four or five pairs. In the
embryo of man, and the higher Vertebrates generally, where they make an
appearance at an early stage, only three or four pairs are developed.
In the fishes they remain throughout life, and form an exit for the
water taken in at the mouth (Figs. 249–251). But they are partly lost
in the amphibia, and entirely in the higher Vertebrates. In these
nothing is left but a relic of the first gill-cleft. This is formed
into a part of the organ of hearing; from it are developed the external
meatus, the tympanic cavity, and the Eustachian tube. We have already
considered these remarkable structures, and need only point here to the
interesting fact that our middle and external ear is a modified
inheritance from the fishes. The branchial arches also, which separate
the clefts, develop into very different parts. In the fishes they
remain gill-arches, supporting the respiratory gill-leaves. It is the
same with the lowest amphibia, but in the higher amphibia they undergo
various modifications; and in the three higher classes of Vertebrates
(including man) the hyoid bone and the ossicles of the ear develop from
them. (Cf. p. 291.)

From the first gill-arch, from the inner surface of which the muscular
tongue proceeds, we get the first structure of the maxillary
skeleton—the upper and lower jaws, which surround the mouth and support
the teeth. These important parts are wholly wanting in the two lowest
classes of Vertebrates, the Acrania and Cyclostoma. They appear first
in the earliest Selachii (Figs. 248–251), and have been transmitted
from this stem-group of the Gnathostomes to the higher
Vertebrates. Hence the original formation of the skeleton of the mouth
can be traced to these primitive fishes, from which we have inherited
it. The teeth are developed from the skin that clothes the jaws. As the
whole mouth cavity originates from the outer integument (Fig. 350), the
teeth also must come from it. As a fact, this is found to be the case
on microscopic examination of the development and finer structure of
the teeth. The scales of the fishes, especially of the shark type (Fig.
351), are in the same position as their teeth in this respect (Fig.
252). The osseous matter of the tooth (dentine) develops from the
corium; its enamel covering is a secretion of the epidermis that covers
the corium. It is the same with the cutaneous teeth or placoid scales
of the Selachii. At first the whole of the mouth was armed with these
cutaneous teeth in the Selachii and in the earliest amphibia.
Afterwards the formation of them was restricted to the edges of the
jaws.


Fig.355. Median section of the head of a Petromyzon-larva. Fig.
355—Median section of the head of a Petromyzon-larva. (From
_Gegenbaur._) _h_ hypobranchial groove (above it in the gullet we see
the internal openings of the seven gill-clefts), _v_ velum, _o_ mouth,
_c_ heart, _a_ auditory vesicle, _n_ neural tube, _ch_ chorda.


Hence our human teeth are, in relation to their original source,
modified fish-scales. For the same reason we must regard the salivary
glands, which open into the mouth, as epidermic glands, as they are
formed, not from the glandular layer of the gut like the rest of the
alimentary glands, but from the epidermis, from the horny plate of the
outer germinal layer. Naturally, in harmony with this evolution of the
mouth, the salivary glands belong genetically to one series with the
sudoriferous, sebaceous, and mammary glands.

Thus the human alimentary canal is as simple as the primitive gut of
the gastrula in its original structure. Later it resembles the gut of
the earliest Vermalia (Gastrotricha). It then divides into two
sections, a fore or branchial gut and a hind or hepatic gut, like the
alimentary canal of the Balanoglossus, the Ascidia, and the Amphioxus.
The formation of the jaws and the branchial arches changes it into a
real fish-gut ( _Selachii_). But the branchial gut, the one
reminiscence of our fish-ancestors, is afterwards atrophied as such.
The parts of it that remain are converted into entirely different
structures.


Fig.356. Transverse section of the head of a Petromyzon-larva. Fig.
356—Transverse section of the head of a Petromyzon-larva. (From
_Gegenbaur._) Beneath the pharynx ( _d_) we see the hypobranchial
groove; above it the chorda and neural tube. _A, B, C_ stages of
constriction.


But, although the anterior section of our alimentary canal thus
entirely loses its original character of branchial gut, it retains the
physiological character of respiratory gut. We are now astonished to
find that the permanent respiratory organ of the higher Vertebrates,
the air-breathing lung, is developed from this first part of the
alimentary canal. Our lungs, trachea, and larynx are formed from the
ventral wall of the branchial gut. The whole of the respiratory
apparatus, which occupies the greater part of the pectoral cavity in
the adult man, is at first merely a small pair of vesicles or sacs,
which grow out of the floor of the head-gut immediately behind the
gills (Figs. 354 _c,_ 147 _l_). These vesicles are found in all the
Vertebrates except the two lowest classes, the Acrania and Cyclostomes.
In the lower Vertebrates they do not develop
into lungs, but into a large air-filled bladder, which occupies a good
deal of the body-cavity and has a quite different purport. It serves,
not for breathing, but to effect swimming movements up and down, and so
is a sort of hydrostatic apparatus—the floating bladder of the fishes (
_nectocystis,_ p. 233). However, the human lungs, and those of all
air-breathing Vertebrates, develop from the same simple vesicular
appendage of the head-gut that becomes the floating bladder in the
fishes.

At first this bladder has no respiratory function, but merely acts as
hydrostatic apparatus for the purpose of increasing or lessening the
specific gravity of the body. The fishes, which have a fully-developed
floating bladder, can press it together, and thus condense the air it
contains. The air also escapes sometimes from the alimentary canal,
through an air-duct that connects the floating bladder with the
pharynx, and is ejected by the mouth. This lessens the size of the
bladder, and so the fish becomes heavier and sinks. When it wishes to
rise again, the bladder is expanded by relaxing the pressure. In many
of the Crossopterygii the wall of the bladder is covered with bony
plates, as in the Triassic _Undina_ (Fig. 254).

This hydrostatic apparatus begins in the Dipneusts to change into a
respiratory organ; the blood-vessels in the wall of the bladder now no
longer merely secrete air themselves, but also take in fresh air
through the air-duct. This process reaches its full development in the
Amphibia. In these the floating bladder has turned into lungs, and the
air-passage into a trachea. The lungs of the Amphibia have been
transmitted to the three higher classes of Vertebrates. In the lowest
Amphibia the lungs on either side are still very simple transparent
sacs with thin walls, as in the common water-salamander, the Triton. It
still entirely resembles the floating bladder of the fishes. It is true
that the Amphibia have two lungs, right and left. But the floating
bladder is also double in many of the fishes (such as the early
Ganoids), and divides into right and left halves. On the other hand,
the lung is single in Ceratodus (Fig. 257).


Fig.357. Thoracic and abdominal viscera of a human embryo of twelve
weeks. Fig. 357—Thoracic and abdominal viscera of a human embryo of
twelve weeks. (From _Kölliker._) The head is omitted. Ventral and
pectoral walls are removed. The greater part of the body-cavity is
taken up with the liver, from the middle part of which the cæcum and
the vermiform appendix protrude. Above the diaphragm, in the middle, is
the conical heart; to the right and left of it are the two small lungs.


In the human embryo and that of all the other Amniotes the lungs
develop from the hind part of the ventral wall of the head-gut (Fig.
149). Immediately behind the single structure of the thyroid gland a
median groove, the rudiment of the trachea, is detached from the
gullet. From its hinder end a couple of vesicles develop—the simple
tubular rudiments of the right and left lungs. They afterwards increase
considerably in size, fill the greater part of the thoracic cavity, and
take the heart between them. Even in the frogs we find that the simple
sac has developed into a spongy body of peculiar froth-like tissue. The
originally short connection of the pulmonary sacs with the head-gut
extends into a long, thin tube. This is the wind-pipe (trachea); it
opens into the gullet above, and divides below into two branches which
go to the two lungs. In the wall of the trachea circular cartilages
develop, and these keep it open. At its upper end, underneath its
pharyngeal opening, the larynx is formed—the organ of voice and speech.
The larynx is found at various stages of development in the Amphibia,
and comparative anatomists are in a position to trace the progressive
growth of this important organ from the rudimentary structure of the
lower Amphibia up to the elaborate and delicate vocal apparatus that we
have in the larynx of man and of the birds.

We must refer here to an interesting rudimentary organ of the
respiratory gut, the thyroid gland, the large gland in front of the
larynx, that lies below the “Adam’s
apple,” and is often especially developed in the male sex. It has a
certain function—not yet fully understood—in the nutrition of the body,
and arises in the embryo by constriction from the lower wall of the
pharynx. In many mining districts the thyroid gland is peculiarly
liable to morbid enlargement, and then forms goitre, a growth that
hangs at the front of the neck. But it is much more interesting
phylogenetically. As Wilhelm Müller, of Jena, has shown, this
rudimentary organ is the last relic of the hypobranchial groove, which
we considered in a previous chapter, and which runs in the middle line
of the gill-crate in the Ascidia and Amphioxus, and conveys food to the
stomach. (Cf. p. 184,Fig. 246). We still find it in its original
character in the larvæ of the Cyclostomes (Figs. 355, 356).

The second section of the alimentary canal, the trunk or hepatic gut,
undergoes not less important modifications among our vertebrate
ancestors than the first section. In tracing the further development of
this digestive part of the gut, we find that most complex and elaborate
organs originate from a very rudimentary original structure. For
clearness we may divide the digestive gut into three sections: the fore
gut (with œsophagus and stomach), the middle gut (duodenum, with liver,
pancreas, jejunum, and ileum, and the hind gut (colon and rectum). Here
again we find vesicular growths or appendages of the originally simple
gut developing into a variety of organs. Two of these embryonic
structures, the yelk-sac and allantois, are already known to us. The
two large glands that open into the duodenum, the liver and pancreas,
are growths from the middle and most important part of the trunk-gut.

Immediately behind the vesicular rudiments of the lungs comes the
section of the alimentary canal that forms the stomach (Figs. 353 _d,_
354 _b_). This sac-shaped organ, which is chiefly responsible for the
solution and digestion of the food, has not in the lower Vertebrates
the great physiological importance and the complex character that it
has in the higher. In the Acrania and Cyclostomes and the earlier
fishes we can scarcely distinguish a real stomach; it is represented
merely by the short piece from the branchial to the hepatic gut. In
some of the other fishes also the stomach is only a very simple
spindle-shaped enlargement at the beginning of the digestive section of
the gut, running straight from front to back in the median plane of the
body, underneath the vertebral column. In the mammals its first
structure is just as rudimentary as it is permanently in the preceding.
But its various parts soon begin to develop. As the left side of the
spindle-shaped sac grows much more quickly than the right, and as it
turns considerably on its axis at the same time, it soon comes to lie
obliquely. The upper end is more to the left, and the lower end more to
the right. The foremost end draws up into the longer and narrower canal
of the œsophagus. Underneath this on the left the blind sac (fundus) of
the stomach bulges out, and thus the later form gradually develops
(Figs. 349, 184 _e_). The original longitudinal axis becomes oblique,
sinking below to the left and rising to the right, and approaches
nearer and nearer to a transverse position. In the outer layer of the
stomach-wall the powerful muscles that accomplish the digestive
movements develop from the gut-fibre layer. In the inner layer a number
of small glandular tubes are formed from the gut-gland layer; these are
the peptic glands that secrete the gastric juice. At the lower end of
the gastric sac is developed the valve that separates it from the
duodenum (the pylorus, Fig. 349 _d_).

Underneath the stomach there now develops the disproportionately long
stretch of the small intestine. The development of this section is very
simple, and consists essentially in an extremely rapid and considerable
growth lengthways. It is at first very short, quite straight, and
simple. But immediately behind the stomach we find at an early stage a
horseshoe-shaped bend and loop of the gut, in connection with the
severance of the alimentary canal from the yelk-sac and the development
of the first mesentery. The thin delicate membrane that fastens this
loop to the ventral side of the vertebral column, and fills the inner
bend of the horseshoe formation, is the first rudiment of the mesentery
(Fig. 147 _g_). We find at an early stage a considerable growth of the
small intestine; it is thus forced to coil itself in a number of loops.
The various sections that we have to distinguish in it are
differentiated in a very simple way—the duodenum (next to the stomach),
the succeeding long jejunum, and the last section of the small
intestine, the ileum.


From the duodenum are developed the two large glands that we have
already mentioned—the liver and pancreas. The liver appears first in
the shape of two small sacs, that are found to the right and left
immediately behind the stomach (Figs. 353 _f,_ 354 _c_). In many of the
lower Vertebrates they remain separate for a long time (in the
Myxinoides throughout life), or are only imperfectly joined. In the
higher Vertebrates they soon blend more or less completely to form a
single large organ. The growth of the liver is very brisk at first. In
the human embryo it grows so much in the second month of development
that in the third it occupies by far the greater part of the
body-cavity (Fig. 357). At first the two halves develop equally;
afterwards the left falls far behind the right. In consequence of the
unsymmetrical development and turning of the stomach and other
abdominal viscera, the whole liver is now pushed to the right side.
Although the liver does not afterwards grow so disproportionately, it
is comparatively larger in the embryo at the end of pregnancy than in
the adult. Its weight relatively to that of the whole body is 1 : 36 in
the adult, and 1 : 18 in the embryo. Hence it is very important
physiologically during embryonic life; it is chiefly concerned in the
formation of blood, not so much in the secretion of bile.

Immediately behind the liver a second large visceral gland develops
from the duodenum, the pancreas or sweetbread. It is wanting in most of
the lowest classes of Vertebrates, and is first found in the fishes.
This organ is also an outgrowth from the gut.

The last section of the alimentary canal, the large intestine, is at
first in the embryo a very simple, short, and straight tube, which
opens behind by the anus. It remains thus throughout life in the lower
Vertebrates. But it grows considerably in the mammals, coils into
various folds, and divides into two sections, the first and longer of
which is the colon, and the second the rectum. At the beginning of the
colon there is a valve (valvula _Bauhini_) that separates it from the
small intestine. Immediately behind this there is a sac-like growth,
which enlarges into the cæcum (Fig. 357 _v_). In the plant-eating
mammals this is very large, but it is very small or completely
atrophied in the flesh-eaters. In man, and most of the apes, only the
first portion of the cæcum is wide; the blind end-part of it is very
narrow, and seems later to be merely a useless appendage of the former.
This “vermiform appendage” is very interesting as a rudimentary organ.
The only significance of it in man is that not infrequently a
cherry-stone or some other hard and indigestible matter penetrates into
its narrow cavity, and by setting up inflammation and suppuration
causes the death of otherwise sound men. Teleology has great difficulty
in giving a rational explanation of, and attributing to a beneficent
Providence, this dreaded appendicitis. In our plant-eating ancestors
this rudimentary organ was much larger and had a useful function.

Finally, we have important appendages of the alimentary tube in the
bladder and urethra, which belong to the alimentary system. These
urinary organs, acting as reservoir and duct for the urine excreted by
the kidneys, originate from the innermost part of the allantoic
pedicle. In the Dipneusts and Amphibia, in which the allantoic sac
first makes its appearance, it remains within the body-cavity, and
functions entirely as bladder. But in all the Amniotes it grows far
outside of the body-cavity of the embryo, and forms the large embryonic
“primitive bladder,” from which the placenta develops in the higher
mammals. This is lost at birth. But the long stalk or pedicle of the
allantois remains, and forms with its upper part the middle
vesico-umbilical ligament, a rudimentary organ that goes in the shape
of a solid string from the vertex of the bladder to the navel. The
lowest part of the allantoic pedicle (or the “urachus”) remains hollow,
and forms the bladder. At first this opens into the last section of the
gut in man as in the lower Vertebrates; thus there is a real cloaca,
which takes off both urine and excrements. But among the mammals this
cloaca is only permanent in the Monotremes, as it is in all the birds,
reptiles, and amphibia. In all the other mammals (marsupials and
placentals) a transverse partition is afterwards formed, and this
separates the urogenital aperture in front from the anus-opening
behind. (Cf. p. 249 and Chapter 29.)



Chapter XXVIII.
EVOLUTION OF THE VASCULAR SYSTEM


The use that we have hitherto made of our biogenetic law will give the
reader an idea how far we may trust its guidance in phylogenetic
investigation. This differs considerably in the various systems of
organs; the reason is that heredity and variability have a very
different range in these systems. While some of them faithfully
preserve the original palingenetic development inherited from earlier
animal ancestors, others show little trace of this rigid heredity; they
are rather disposed to follow new and divergent _cenogenetic_ lines of
development in consequence of adaptation. The organs of the first kind
represent the _conservative_ element in the multicellular state of the
human frame, while the latter represent the _progressive_ element. The
course of historic development is a result of the correlation of the
two tendencies, and they must be carefully distinguished.

There is perhaps no other system of organs in the human body in which
this is more necessary than in that of which we are now going to
consider the obscure development—the vascular system, or apparatus of
circulation. If we were to draw our conclusions as to the original
features in our earlier animal ancestors solely from the phenomena of
the development of this system in the embryo of man and the other
higher Vertebrates, we should be wholly misled. By a number of
important embryonic adaptations, the chief of which is the formation of
an extensive food-yelk, the original course of the development of the
vascular system has been so much falsified and curtailed in the higher
Vertebrates that little or nothing now remains in their embryology of
some of the principal phylogenetic features. We should be quite unable
to explain these if comparative anatomy and ontogeny did not come to
our assistance.

The vascular system in man and all the Craniotes is an elaborate
apparatus of cavities filled with juices or cell-containing fluids.
These “vessels” (_vascula_) play an important part in the nutrition of
the body. They partly conduct the nutritive red blood to the various
parts of the body (blood-vessels); partly absorb from the gut the white
chyle formed in digestion (chyle-vessels); and partly collect the
used-up juices and convey them away from the tissues (lymphatic
vessels). With the latter are connected the large cavities of the body,
especially the body-cavity, or cœloma. The lymphatic vessels conduct
both the colourless lymph and the white chyle into the venous part of
the circulation. The lymphatic glands act as producers of new
blood-cells, and with them is associated the spleen. The centre of
movement for the circulation of the fluids is the heart, a strong
muscular sac, which contracts regularly and is equipped with valves
like a pump. This constant and steady circulation of the blood makes
possible the complex metabolism of the higher animals.

But, however important the vascular system may be to the more advanced
and larger and highly-differentiated animals, it is not at all so
indispensable an element of animal life as is commonly supposed. The
older science of medicine regarded the blood as the real source of
life. Even in the still prevalent confused notions of heredity the
blood plays the chief part. People speak generally of full blood, half
blood, etc., and imagine that the hereditary transmission of certain
characters “lies in the blood.” The incorrectness of these ideas is
clearly seen from the fact that in the act of generation the blood of
the parents is not directly transmitted to the offspring, nor does the
embryo possess blood in its early stages. We have already seen that not
only the differentiation of the four secondary germinal layers, but
also the first structures of the principal organs in the embryo of all
the Vertebrates, take place long before there is any
trace of the vascular system—the heart and the blood. In accordance
with this ontogenetic fact, we must regard the vascular system as one
of the latest organs from the phylogenetic point of view; just as we
have found the alimentary canal to be one of the earliest. In any case,
the vascular system is much later than the alimentary.


Fig.358. Red blood-cells of various Vertebrates. Fig. 359. Vascular
tissues or endothelium (vasalium). A capillary from the mesentery. Fig.
358—Red blood-cells of various Vertebrates (equally magnified). _1._ of
man, _2._ camel, _3._ dove, _4._ proteus, _5._ water-salamander
(_Triton_), _6._ frog, _7._ merlin (_Cobitis_), _8._ lamprey
(_Petromyzon_). _a_ surface-view, _b_ edge-view. (From _Wagner._) Fig.
359—Vascular tissues or endothelium (_vasalium_). A capillary from the
mesentery. _a_ vascular cells, _b_ their nuclei.


The important nutritive fluid that circulates as blood and lymph in the
elaborate canals of our vascular system is not a clear, simple fluid,
but a very complex chemical juice with millions of cells floating in
it. These blood-cells are just as important in the complicated life of
the higher animal body as the circulation of money is to the commerce
of a civilised community. Just as the citizens meet their needs most
conveniently by means of a financial circulation, so the various
tissue-cells, the microscopic citizens of the multicellular human body,
have their food conveyed to them best by the circulating cells in the
blood. These blood cells (_hæmocytes_) are of two kinds in man and all
the other Craniotes—red cells (_rhodocytes_ or _erythrocytes_) and
colourless or lymph cells (_leucocytes_). The red colour of the blood
is caused by the great accumulation of the former, the others circulate
among them in much smaller quantity. When the colourless cells increase
at the expense of the red we get anæmia (or chlorosis).

The lymph-cells (_leucocytes_), commonly called the “white corpuscles”
of the blood, are phylogenetically older and more widely distributed in
the animal world than the red. The great majority of the Invertebrates
that have acquired an independent vascular system have only colourless
lymph-cells in the circulating fluid. There is an exception in the
Nemertines (Fig. 358) and some groups of Annelids. When we examine the
colourless blood of a cray-fish or a snail (Fig. 358) under a high
power of the microscope, we find in each drop numbers of mobile
leucocytes, which behave just like independent Amoebæ (Fig. 17). Like
these unicellular Protozoa, the colourless blood-cells creep slowly
about, their unshapely plasma-body constantly changing its form, and
stretching out finger-like processes first in one direction, then
another. Like the Amoebæ, they take particles into their cell-body. On
account
of this feature these amoeboid plastids are called “eating cells”
(_phagocytes_), and on account of their motions “travelling cells”
(_planocytes_). It has been shown by the discoveries of the last few
decades that these leucocytes are of the greatest physiological and
pathological consequence to the organism. They can absorb either solid
or dissolved particles from the wall of the gut, and convey them to the
blood in the chyle; they can absorb and remove unusable matter from the
tissues. When they pass in large quantities through the fine pores of
the capillaries and accumulate at irritated spots, they cause
inflammation. They can consume and destroy bacteria, the dreaded
vehicles of infectious diseases; but they can also transport these
injurious Monera to fresh regions, and so extend the sphere of
infection. It is probable that the sensitive and travelling leucocytes
of our invertebrate ancestors have powerfully co-operated for millions
of years in the phylogenesis of the advancing animal organisation.


Fig.360. Transverse section of the trunk of a chick-embryo, forty-five
hours old. Fig. 360—Transverse section of the trunk of a chick-embryo,
forty-five hours old. (From _Balfour._) _A_ ectoderm (horny-plate),
_Mc_ medullary tube, _ch_ chorda, _C_ entoderm (gut-gland layer), _Pv_
primitive segment (episomite), _Wd_ prorenal duct, _pp_ cœloma
(secondary body-cavity). _So_ skin-fibre layer, _Sp_ gut-fibre layer,
_v_ blood-vessels in latter, _ao_ primitive aortas, containing red
blood-cells.


The red blood-cells have a much more restricted sphere of distribution
and activity. But they also are very important in connection with
certain functions of the craniote-organism, especially the exchange of
gases or respiration. The cells of the dark red, carbonised or venous,
blood, which have absorbed carbonic acid from the animal tissues, give
this off in the respiratory organs; they receive instead of it fresh
oxygen, and thus bring about the bright red colour that distinguishes
oxydised or arterial blood. The red colouring matter of the blood
(_hæmoglobin_) is regularly distributed in the pores of their
protoplasm. The red cells of most of the Vertebrates are elliptical
flat disks, and enclose a nucleus of the same shape; they differ a good
deal in size (Fig. 358). The mammals are distinguished from the other
Vertebrates by the circular form of their biconcave red cells and by
the absence of a nucleus (Fig. 1); only a few genera still have the
elliptic form inherited from the reptiles (Fig. 2). In the embryos of
the mammals the red cells have a nucleus and the power of increasing by
cleavage (Fig. 10).

The origin of the blood-cells and vessels in the embryo, and their
relation to the germinal layers and tissues, are among the most
difficult problems of ontogeny—those obscure questions on which the
most divergent opinions are still advanced by the most competent
scientists. In general, it is certain that the greater part of the
cells that compose the vessels and their contents come from the
mesoderm—in fact, from the gut-fibre layer; it was on this account that
Baer gave the name of “vascular layer” to this visceral layer of the
coeloma. But other important observers say that a part of these cells
come from other germinal layers, especially from the gut-gland layer.
It seems to be true that blood-cells may be formed from the cells of
the entoderm before the development of the mesoderm. If we examine
sections of chickens, the earliest and most familiar subjects of
embryology, we find at an early stage the “primitive-aortas” we have
already described (Fig. 360 _ao_) in the ventral angle between the
episoma (_Pv_) and hyposoma (_Sp_). The
thin wall of these first vessels of the amniote embryo consists of flat
cells (_endothelia_ or _vascular epithelia_); the fluid within already
contains numbers of red blood-cells; both have been developed from the
gut-fibre layer. It is the same with the vessels of the germinative
area (Fig. 361 _v_), which lie on the entodermic membrane of the
yelk-sac (_c_). These features are seen still more clearly in the
transverse section of the duck-embryo in Fig. 152.In this we see
clearly how a number of stellate cells proceed from the “vascular
layer” and spread in all directions in the “primary body-cavity”—_i.e._
in the spaces between the germinal layers. A part of these travelling
cells come together and line the wall of the larger spaces, and thus
form the first vessels; others enter into the cavity, live in the fluid
that fills it, and multiply by cleavage—the first blood-cells.

But, besides these mesodermic cells of the “vascular layer” proper,
other travelling cells, of which the origin and purport are still
obscure, take part in the formation of blood in the meroblastic
Vertebrates (especially fishes). The chief of these are those that
Ruckert has most aptly denominated “merocytes.” These “eating
yelk-cells” are found in large numbers in the food-yelk of the
Selachii, especially in the yelk-wall—the border zone of the germinal
disk in which the embryonic vascular net is first developed. The nuclei
of the merocytes become ten times as large as the ordinary
cell-nucleus, and are distinguished by their strong capacity for taking
colour, or their special richness in chromatin. Their protoplasmic body
resembles the stellate cells of osseous tissue (astrocytes), and
behaves just like a rhizopod (such as Gromia); it sends out numbers of
stellate processes all round, which ramify and stretch into the
surrounding food-yelk. These variable and very mobile processes, the
pseudopodia of the merocytes, serve both for locomotion and for getting
food; as in the real rhizopods, they surround the solid particles of
food (granules and plates of yelk), and accumulate round their nucleus
the food they have received and digested. Hence we may regard them both
as eating-cells (_phagocytes_) and travelling-cells (_planocytes_).
Their lively nucleus divides quickly and often repeatedly, so that a
number of new nuclei are formed in a short time; as each fresh nucleus
surrounds itself with a mantle of protoplasm, it provides a new cell
for the construction of the embryo. Their origin is still much
disputed.


Fig.361. Merocytes of a shark-embryo, rhizopod-like yelk-cells
underneath the embryonic cavity (B). Fig. 361—Merocytes of a
shark-embryo, rhizopod-like yelk-cells underneath the embryonic cavity
(_B_). (From _Ruckert._) _z_ two embryonic cells, _k_ nuclei of the
merocytes, which wander about in the yelk and eat small yelk-plates
(_d_), _k_ smaller, more superficial, lighter nuclei, _k′_ a deeper
nucleus, in the act of cleavage, _k*_ chromatin-filled border-nucleus,
freed from the surrounding yelk in order to show the numerous
pseudopodia of the protoplasmic cell-body.


Half of the twelve stems of the animal world have no blood-vessels.
They make their first appearance in the Vermalia. Their earliest source
is the primary body-cavity, the simple space between the two primary
germinal layers, which is either a relic of the segmentation-cavity, or
is a subsequent formation. Amoeboid planocytes, which migrate from the
entoderm and reach this fluid-filled primary cavity, live and multiply
there, and form the first colourless blood-cells. We find the vascular
system in this very simple form to-day in the Bryozoa, Rotatoria,
Nematoda, and other lower Vermalia.

The first step in the improvement of this primitive vascular system is
the formation of larger canals or blood-conducting tubes. The spaces
filled with blood, the relics of the primary body-cavity, receive a
special wall. “Blood-vessels” of this kind (in the narrower sense) are
found among the higher worms in various forms, sometimes very simple,
at other times very complex. The form
that was probably the incipient structure of the elaborate vascular
system of the Vertebrates (and of the Articulates) is found in two
primordial principal vessels—a dorsal vessel in the middle line of the
dorsal wall of the gut, and a ventral vessel that runs from front to
rear in the middle line of its ventral wall. From the dorsal vessel is
evolved the aorta (or principal artery), from the ventral vessel the
principal or subintestinal vein. The two vessels are connected in front
and behind by a loop that runs round the gut. The blood contained in
the two tubes is propelled by their peristaltic contractions.


Fig.362. Vascular system of an Annelid (Saenuris), foremost section.
Fig. 362—Vascular system of an Annelid (_Sænuris_), foremost section.
_d_ dorsal vessel, _v_ ventral vessel, _c_ transverse connection of two
(enlarged in shape of heart). The arrows indicate the direction of the
flow of blood. (From _Gegenbaur._


The earliest Vermalia in which we first find this independent vascular
system are the Nemertina (Fig. 244). As a rule, they have three
parallel longitudinal vessels connected by loops, a single dorsal
vessel above the gut and a pair of lateral vessels to the right and
left. In some of the Nemertina the blood is already coloured, and the
red colouring matter is real hæmoglobin, connected with elliptical
discoid cells, as in the Vertebrates. The further evolution of this
rudimentary vascular system can be gathered from the class of the
Annelids in which we find it at various stages of development. First, a
number of transverse connections are formed between the dorsal and
ventral vessels, which pass round the gut ring-wise (Fig. 362). Other
vessels grow into the body-wall and ramify in order to convey blood to
it. In addition to the two large vessels of the middle plane there are
often two lateral vessels, one to the right and one to the left; as,
for instance, in the leech. There are four of these parallel
longitudinal vessels in the Enteropneusts (_Balanoglossus,_ Fig. 245).
In these important Vermalia the foremost section of the gut has already
been converted into a gill-crate, and the vascular arches that rise in
the wall of this from the ventral to the dorsal vessel have become
branchial vessels.


Fig.363. Head of a fish-embryo, with rudimentary vascular system, from
the left. Fig. 363—Head of a fish-embryo, with rudimentary vascular
system, from the left. _dc_ Cuvier’s duct (juncture of the anterior and
posterior principal veins), _sv_ venous sinus (enlarged end of Cuvier’s
duct), _a_ auricle, _v_ ventricle, _abr_ trunk of branchial artery, _s_
gill-clefts (arterial arches between), _ad_ aorta, _c_ carotid artery,
_n_ nasal pit. (From _Gegenbaur._


We have a further important advance in the Tunicates, which we have
recognised as the nearest blood-relatives of our early vertebrate
ancestors. Here we find for the first time a real heart—_i.e._ a
central organ of circulation, driving the blood into the vessels by the
regular contractions of its muscular wall, it is of a very rudimentary
character, a spindle-shaped tube, passing at both ends into a principal
vessel (Fig. 221). By its original position behind the gill-crate, on
ventral side of the Tunicates (sometimes more, sometimes less,
forward), the head shows clearly that it has been formed by the local
enlargement of a section of the ventral vessel. We have already noticed
the remarkable alternation of the direction of the blood stream, the
heart driving it first from one end, then from the other p. 190. This
is very instructive, because in most of the worms (even the
Enteropneust) the blood in the dorsal vessel travels from back to
front, but in the Vertebrates in the opposite direction. As the
Ascidia-heart alternates steadily from one direction to the other, it
shows us permanently, in a sense, the phylogenetic transition from the
earlier forward direction of the dorsal current (in the worms) to the
new backward direction (in the Vertebrates).

As the new direction became permanent in the earlier Prochordonia,
which gave rise to the Vertebrate stem, the two vessels that proceed
from either end of the tubular heart acquired a fixed function.
The foremost section of the ventral vessel henceforth always conveys
blood from the heart, and so acts as an artery; the hind section of the
same vessel brings the blood from the body to the heart, and so becomes
a vein. In view of their relation to the two sections of the gut, we
may call the latter the intestinal vein and the former the branchial
artery. The blood contained in both vessels, and also in the heart, is
venous or carbonised blood—_i.e._ rich in carbonic acid; on the other
hand, the blood that passes from the gills into the dorsal vessel is
provided with fresh oxygen—arterial or oxydised blood. The finest
branches of the arteries and veins pass into each other in the tissues
by means of a network of very fine, ventral, hair-like vessels, or
capillaries (Fig. 359).


Fig.364. The five arterial arches of the Craniotes (1 to 5) in their
original disposition. Fig. 365. The five arterial arches of the birds;
the lighter parts of the structure disappear; only the shaded parts
remain. Fig. 366. The five arterial arches of mammals. Fig. 364—The
five arterial arches of the Craniotes (_1–5_) in their original
disposition. _a_ arterial cone or bulb, _a″_ aorta-trunk, _c_ carotid
artery (foremost continuation of the roots of the aorta). (From
_Rathke._)
Fig. 365—The five arterial arches of the birds; the lighter parts of
the structure disappear; only the shaded parts remain. Letters as in
Fig. 364. _s_ subclavian arteries, _p_ pulmonary artery, _p′_ branches
of same, _c′_ outer carotid, _c″_ inner carotid. (From _Rathke._) Fig.
366—The five arterial arches of mammals; letters as in Fig. 365. _v_
vertebral artery, _b_ Botall’s duct (open in the embryo, closed
afterwards). (From _Rathke._)


When we turn from the Tunicates to the closely-related Amphioxus we are
astonished at first to find an apparent retrogression in the formation
of the vascular system. As we have seen, the Amphioxus has no real
heart; its colourless blood is driven along in its vascular system by
the principal vessel itself, which contracts regularly in its whole
length (cf. Fig. 210). A dorsal vessel that lies above the gut (aorta)
receives the arterial blood from the gills and drives it into the body.
Returning from here, the venous blood gathers in a ventral vessel under
the gut (intestinal vein), and goes back to the gills. A number of
branchial vascular arches, which effect respiration and rise in the
wall of the branchial gut from belly to back, absorb oxygen from the
water and give off carbonic acid; they connect the ventral with the
dorsal vessel. As the same section of the ventral vessel, which also
forms the heart in the Craniotes, has developed in the Ascidia into a
simple tubular heart, we may regard the absence of this in the
Amphioxus as a result of degeneration, a return in this case to the
earlier form of the vascular system, as we find it in many of the
worms. We may assume that the Acrania that really belong to our
ancestral series did not share this retrogression, but inherited the
one-chambered heart of the Prochordonia, and transmitted it directly to
the earliest Craniotes (cf. the ideal Primitive Vertebrate,
_Prospondylus,_ Figs. 98–102).

The further phylogenetic evolution of the vascular system is revealed
to us by the comparative anatomy of the Craniotes. At the lowest stage
of this group, in the Cyclostomes, we find for the first time the
differentiation of the vasorium into two sections: a system of
blood-vessels proper, which convey the _red_ blood about the body, and
a system of lymphatic vessels,
which absorb the colourless lymph from the tissues and convey it to the
blood. The lymphatics that absorb from the gut and pour into the
blood-stream the milky food-fluid formed by digestion are distinguished
by the special name of “chyle-vessels.” While the chyle is white on
account of its high proportion of fatty particles, the lymph proper is
colourless. Both chyle and lymph contain the colourless amœboid cells
(leucocytes, Fig. 12) that we also find distributed in the blood as
colourless blood-cells (or “white corpuscles”); but the blood also
contains a much larger quantity of red cells, and these give its
characteristic colour to the blood of the Craniotes (rhodocytes, Fig.
358). The distinction between lymph, chyle, and blood-vessels which is
found in all the Craniotes may be regarded as an outcome of division of
labour between various sections of our originally simple vascular
system. In the Gnathostomes the spleen makes its first appearance, an
organ rich in blood, the chief function of which is the extensive
formation of new colourless and red cells. It is not found in the
Acrania and Cyclostomes, or any of the Invertebrates. It has been
transmitted from the earliest fishes to all the Craniotes.


Figs. 367-70. Metamorphosis of the five arterial arches in the human
embryo. Figs. 367–70—Metamorphosis of the five arterial arches in the
human embryo (diagram from _Rathke_). _la_ arterial cone, _1, 2, 3, 4,
5_ first to fifth pair of arteries, _ad_ trunk of aorta, _aw_ roots of
aorta. In Fig. 367 only three, in Fig. 368 all five, of the aortic
arches are given (the dotted ones only are developed). In Fig. 369 the
first two pairs have disappeared again. In Fig. 370 the permanent
trunks of the artery are shown; the dotted parts disappear, _s_
subclavian artery, _v_ vertebral, _ax_ axillary, _c_ carotid (_c′_
outer, _c″_ inner carotid), _p_ pulmonary.


The heart also, the central organ of circulation in all the Craniotes,
shows an advance in structure in the Cyclostomes. The simple,
spindle-shaped heart-tube, found in the same form in the embryo of all
the Craniotes, is divided into two sections or chambers in the
Cyclostomes, and these are separated by a pair of valves. The hind
section, the auricle, receives the venous blood from the body and
passes it on to the anterior section, the ventricle. From this it is
driven through the trunk of the branchial artery (the foremost section
of the ventral vessel or principal vein) into the gills.

In the Selachii an arterial cone is developed from the foremost end of
the ventricle, as a special division, cut off by valves. It passes into
the enlarged base of the trunk of the branchial artery (Fig. 363
_abr_). On each side 5–7 arteries proceed from it. These rise between
the gill-clefts (_s_) on the gill-arches, surround the gullet, and
unite above into a common trunk-aorta, the continuation of which over
the gut corresponds to the dorsal vessel of the worms. As the curved
arteries on the gill-arches spread into a network of respiratory
capillaries, they contain venous blood in their lower part (as arches
of the branchial artery) and arterial blood in the upper part (as
arches of the aorta). The junctures of the various aortic arches on the
right and left are called the roots of the aorta. Of an originally
large number of aortic arches there remain at first six, then (owing to
degeneration of the fifth arch) only five, pairs; and from these five
pairs (Fig. 364) the chief parts of the arterial system develop in all
the higher Vertebrates.

The appearance of the lungs and the atmospheric respiration connected
therewith, which we first meet in the Dipneusts, is the next important
step in vascular evolution. In the Dipneusts the auricle of the heart
is divided by an incomplete partition into two halves. Only the right
auricle now receives the venous blood from the veins of the body. The
left auricle receives the arterial blood from the pulmonary veins. The
two auricles have a common opening into the simple ventricle, where the
two kinds of blood mix, and are driven through the arterial cone or
bulb into the arterial arches. From the last arterial arches the
pulmonary arteries arise (Fig. 365 _p_). These force a part of the
mixed blood into the lungs, the other part of it going through the
aorta into the body.


Fig.371. Heart of a rabbit-embryo, from behind. Fig. 372. Heart of the
same embryo (Fig. 371), from the front. Fig. 371—Heart of a
rabbit-embryo, from behind. _a_ vitelline veins, _b_ auricles of the
heart, _c_ atrium, _d_ ventricle, _e_ arterial bulb, _f_ base of the
three pairs of arterial arches. (From _Bischoff._) Fig. 372—Heart of
the same embryo (Fig. 371), from the front. _v_ vitelline veins, _a_
auricle, _ca_ auricular canal, _l_ left ventricle, _r_ right ventricle,
_ta_ arterial bulb. (From _Bischoff._)


From the Dipneusts upwards we now trace a progressive development of
the vascular system, which ends finally with the loss of branchial
respiration and a complete separation of the two halves of the
circulation. In the Amphibia the partition between the two auricles is
complete. In their earlier stages, as tadpoles (Fig. 262), they have
still the branchial respiration and the circulation of the fishes, and
their heart contains venous blood alone. Afterwards the lungs and
pulmonary vessels are developed, and henceforth the ventricle of the
heart contains mixed blood. In the reptiles the ventricle and its
arterial cone begin to divide into two halves by a longitudinal
partition, and this partition becomes complete in the higher reptiles
and birds on the one hand, and the stem-forms of the mammals on the
other. Henceforth, the right half of the heart contains only venous,
and the left half only arterial, blood, as we find in all birds and
mammals. The right auricle receives its carbonised or venous blood from
the veins of the body, and the right ventricle drives it through the
pulmonary arteries into the lungs. From here the blood returns, as
oxydised or arterial blood, through the pulmonary veins to the left
auricle, and is forced by the left ventricle into the arteries of the
body. Between the pulmonary arteries and veins is the capillary system
of the small or pulmonary circulation. Between the body-arteries and
veins is the capillary system of the large or body-circulation. It is
only in the two highest classes of Vertebrates—the birds and
mammals—that we find a complete division of the circulations. Moreover,
this complete separation has been developed quite independently in the
two classes, as the dissimilar formation of the aortas shows of itself.
In the birds the _right_ half of the fourth arterial arch has become
the permanent arch (Fig. 365). In the mammals this has been developed
from the _left_ half of the same fourth arch (Fig. 366).


Fig.373. Heart and head of a dog-embryo, from the front. Fig. 374.
Heart of the same dog-embryo, from behind. Fig. 373—Heart and head of a
dog-embryo, from the front. _a_ fore brain, _b_ eyes, _c_ middle brain,
_d_ primitive lower jaw, _e_ primitive upper jaw, _f_ gill-arches, _g_
right auricle, _h_ left auricle, _i_ left ventricle, _k_ right
ventricle. (From _Bischoff._) Fig. 374—Heart of the same dog-embryo,
from behind. _a_ inosculation of the vitelline veins, _b_ left auricle,
_c_ right auricle, _d_ auricle, _e_ auricular canal, _f_ left
ventricle, _g_ right ventricle, _h_ arterial bulb. (From _Bischoff._)


If we compare the fully-developed arterial system of the various
classes of Craniotes, it shows a good deal of variety, yet it always
proceeds from the same fundamental type. Its development is just the
same in man as in the other mammals; in particular, the modification of
the six pairs of arterial arches is the same in both (Figs. 367–370).
At first there is only a single pair of arches, which
lie on the inner surface of the first pair of gill-arches. Behind this
there then develop a second and third pair of arches (lying on the
inner side of the second and third gill-arches, Fig. 367). Finally, we
get a fourth, fifth, and sixth pair. Of the six primitive arterial
arches of the Amniotes three soon pass away (the first, second, and
fifth); of the remaining three, the third gives the carotids, the
fourth the aortas, and the sixth (number 5 in Figs. 364 and 368) the
pulmonary arteries.


Fig.375. Heart of a human embryo, four weeks old. Fig. 376. Heart of a
human embryo, six weeks old, front view. Fig. 377. Heart of a human
embryo, eight weeks old, back view. Fig. 375—Heart of a human embryo,
four weeks old; _1._ front view, _2._ back view, _3._ opened, and upper
half of the atrium removed. _a′_ left auricle, _a″_ right auricle, _v′_
left ventricle, _v″_ right ventricle, _ao_ arterial bulb, _c_ superior
vena cava (_cd_ right, _cs_ left), _s_ rudiment of the interventricular
wall. (From _Kölliker._)
Fig. 376—Heart of a human embryo, six weeks old, front view. _r_ right
ventricle, _t_ left ventricle, _s_ furrow between ventricles, _ta_
arterial bulb, _af_ furrow on its surface; to right and left are the
two large auricles. (From _Ecker._) Fig. 377—Heart of a human embryo,
eight weeks old, back view. _a′_ left auricle, _a″_ right auricle, _v′_
left ventricle, _v″_ right ventricle, _cd_ right superior vena cava,
_ci_ inferior vena cava. (From _Kölliker._)


The human heart also develops in just the same way as that of the other
mammals (Fig. 378). We have already seen the first rudiments of its
embryology, which in the main corresponds to its phylogeny (Figs. 201,
202). We saw that the palingenetic form of the heart is a
spindle-shaped thickening of the gut-fibre layer in the ventral wall of
the head-gut. The structure is then hollowed out, forms a simple tube,
detaches from its place of origin, and henceforth lies freely in the
cardiac cavity. Presently the tube bends into the shape of an S, and
turns spirally on an imaginary axis in such a way that the hind part
comes to lie on the dorsal surface of the fore part. The united
vitelline veins open into the posterior end. From the anterior end
spring the aortic arches.


Fig.378. Heart of the adult man, fully developed, front view, natural
position. Fig. 378—Heart of the adult man, fully developed, front view,
natural position. _a_ right auricle (underneath it the right
ventricle), _b_ left auricle (under it the left ventricle), _C_
superior vena cava, _V_ pulmonary veins, _P_ pulmonary artery, _d_
Botalli’s duct, _A_ aorta. (From _Meyer._)


This first structure of the human heart, enclosing a very simple
cavity, corresponds to the tunicate-heart, and is a reproduction of
that of the Prochordonia, but it now divides into two, and subsequently
into three, compartments; this reminds us for a time of the heart of
the Cyclostomes and fishes. The spiral turning and bending of the heart
increases, and at the same time two transverse constrictions appear,
dividing it externally into three sections (Figs. 371, 372). The
foremost section, which is turned towards the ventral side, and from
which the aortic arches rise, reproduces the arterial bulb of the
Selachii. The middle section is a simple ventricle, and the hindmost,
the section turned towards the dorsal side, into which the vitelline
veins inosculate, is a simple auricle (or _atrium_). The latter forms,
like the simple atrium of the fish-heart, a pair of lateral
dilatations, the auricles (Fig. 371 _b_); and the constriction between
the atrium and ventricle is called the auricular canal (Fig. 372 _ca_).
The heart of the human embryo is now a complete fish-heart.


In perfect harmony with its phylogeny, the embryonic development of the
human heart shows a gradual transition from the fish-heart, through the
amphibian and reptile, to the mammal form, The most important point in
the transition is the formation of a longitudinal partition—incomplete
at first, but afterwards complete—which separates all three divisions
of the heart into right (venous) and left (arterial) halves (cf. Figs.
373–378). The atrium is separated into a right and left half, each of
which absorbs the corresponding auricle; into the right auricle open
the body-veins (upper and lower vena cava, Figs. 375 _c,_ 377 _c_); the
left auricle receives the pulmonary veins. In the same way a
superficial interventricular furrow is soon seen in the ventricle (Fig.
376 _s_). This is the external sign of the internal partition by which
the ventricle is divided into two—a right venous and left arterial
ventricle. Finally a longitudinal partition is formed in the third
section of the primitive fish-like heart, the arterial bulb, externally
indicated by a longitudinal furrow (Fig. 376 _af_). The cavity of the
bulb is divided into two lateral halves, the pulmonary-artery bulb,
that opens into the right ventricle, and the aorta-bulb, that opens
into the left ventricle. When all the partitions are complete, the
small (pulmonary) circulation is distinguished from the large (body)
circulation; the motive centre of the former is the right half, and
that of the latter the left half, of the heart.


Fig.379. Transverse section of the back of the head of a chick-embryo,
forty hours old. Fig. 379—Transverse section of the back of the head of
a chick-embryo, forty hours old. (From _Kölliker._) _m_ medulla
oblongata, _ph_ pharyngeal cavity (head-gut), _h_ horny plate, _h′_
thicker part of it, from which the auscultory pits afterwards develop,
_hp_ skin-fibre plate, _hh_ cervical cavity (head-cœlom or cardiocœl),
_hzp_ cardiac plate (the outermost mesodermic wall of the heart),
connected by the ventral mesocardium (_uhg_) with the gut-fibre layer
or visceral cœlom-layer (_dfp*prime;_), _Ent_ entoderm, _ihh_ inner
(entodermic?) wall of the heart; the two endothelial cardiac tubes are
still separated by the cenogenetic septum (_s_) of the Amniotes, _g_
vessels.


The heart of all the Vertebrates belongs originally to the hyposoma of
the head, and we accordingly find it in the embryo of man and all the
other Amniotes right in front on the under-side of the head; just as in
the fishes it remains permanently in front of the gullet. It afterwards
descends into the trunk, with the advance in the development of the
neck and breast, and at last reaches the breast, between the two lungs.
At first it lies symmetrically in the middle plane of the body, so that
its long axis corresponds with that of the body. In most of the mammals
it remains permanently in this position. But in the apes the axis
begins to be oblique, and the apex of the heart to move towards the
left side. The displacement is greatest in the anthropoid
apes—chimpanzee, gorilla, and orang—which resemble man in this.

As the heart of all Vertebrates is originally, in the light of
phylogeny, only a local enlargement of the middle principal vein, it is
in perfect accord with the biogenetic law that its first structure in
the embryo is a simple spindle-shaped tube in the ventral wall of the
head-gut. A thin membrane, standing vertically in the middle plane, the
mesocardium, connects the ventral wall of the head-gut with the lower
head-wall. As the cardiac tube extends and detaches from the gut-wall,
it divides the mesocardium into an upper (dorsal) and lower (ventral)
plate (usually called the _mesocardium anterius_ and _posterius_ in
man, Fig. 379 _uhg_). The
mesocardium divides two lateral cavities, Remak’s “neck-cavities” (Fig.
379 _hh_). These cavities afterwards join and form the simple
pericardial cavity, and are therefore called by Kölliker the “primitive
pericardial cavities.”

The double cervical cavity of the Amniotes is very interesting, both
from the anatomical and the evolutionary point of view; it corresponds
to a part of the hyposomites of the head of the lower Vertebrates—that
part of the ventral cœlom-pouches which comes next to Van Wijhe’s
“visceral cavities” below. Each of the cavities still communicates
freely behind with the two cœlom-pouches of the trunk; and, just as
these afterwards coalesce into a simple body-cavity (the ventral
mesentery disappearing), we find the same thing happening in the head.
This simple primary pericardial cavity has been well called by
Gegenbaur the “head-cœloma,” and by Hertwig the “pericardial
breast-cavity.” As it now encloses the heart, it may also be called
_cardiocœl._


Fig.380. Frontal section of a human embryo, one-twelfth of an inch long
in the neck. Fig. 380—Frontal section of a human embryo, one-twelfth of
an inch long in the neck; “invented” by _Wilhelm His._ Seen from
ventral side. _mb_ mouth-fissure, surrounded by the branchial
processes, _ab_ bulbus of aorta, _hm_ middle part of ventricle, _hl_
left lateral part of same, _ho_ auricle, _d_ diaphragm, _vc_ superior
vena cava, _vu_ umbilical vein, _vo_ vitelline space, _lb_ liver, _lg_
hepatic duct.


The cardiocœl, or head-cœlom, is often disproportionately large in the
Amniotes, the simple cardiac tube growing considerably and lying in
several folds. This causes the ventral wall of the amniote embryo,
between the head and the navel, to be pushed outwards as in rupture
(cf. Fig. 180 _h_). A transverse fold of the ventral wall, which
receives all the vein-trunks that open into the heart, grows up from
below between the pericardium and the stomach, and forms a transverse
partition, which is the first structure of the primary diaphragm (Fig.
380 _d_). This important muscular partition, which completely separates
the thoracic and abdominal cavities in the mammals alone, is still very
imperfect here; the two cavities still communicate for a time by two
narrow canals. These canals, which belong to the dorsal part of the
head-cœlom, and which we may call briefly _pleural ducts,_ receive the
two pulmonary sacs, which develop from the hind end of the ventral wall
of the head-gut; they thus become the two pleural cavities.

The diaphragm makes its first appearance in the class of the Amphibia
(in the salamanders) as an insignificant muscular transverse fold of
the ventral wall, which rises from the fore end of the transverse
abdominal muscle, and grows between the pericardium and the liver. In
the reptiles (tortoises and crocodiles) a later dorsal part is joined
to this earlier ventral part of the rudimentary diaphragm, a pair of
subvertebral muscles rising from the vertebral column and being added
as “columns” to the transverse partition. But it was probably in the
Permian sauro-mammals that the two originally separate parts were
united, and the diaphragm became a complete partition between the
thoracic and abdominal cavities in the mammals; as it considerably
enlarges the chest-cavity when it contracts, it becomes an important
respiratory muscle. The ontogeny of the diaphragm in man and the other
mammals reproduces this phylogenetic process to-day, in accordance with
the biogenetic law; in all the mammals the diaphragm is formed by the
secondary conjunction of the two originally separate structures, the
earlier ventral part and the later dorsal part.

Sometimes the blending of the two diaphragmatic structures, and
consequently the severance of the one pleural duct from the abdominal
cavity, is not completed in man. This leads to a diaphragmatic rupture
(_hernia diaphragmatica_). The two cavities then remain in
communication by an open pleural duct, and loops of the intestine may
penetrate by this “rupture opening” into the chest-cavity. This is one
of those
fatal mis-growths that show the great part that blind chance has in
organic development.


Fig.381. Transverse section of the head of a chick-embryo, thirty-six
hours old. Fig. 381—Transverse section of the head of a chick-embryo,
thirty-six hours old. Underneath the medullary tube the two primitive
aortas (_pa_) can be seen in the head-plates (_s_) at each side of the
chorda. Underneath the gullet (_d_) we see the aorta-end of the heart
(_ae_), _hh_ cervical cavity or head cœlom, _hk_ top of heart, _ks_
head-sheath, amniotic fold, _h_ horny plate. (From _Remak._


Thus the thoracic cavity of the mammals, with its important contents,
the heart and lungs, belongs originally to the _head-part_ of the
vertebrate body, and its inclusion in the trunk is secondary. This
instructive and very interesting fact is entirely proved by the
concordant evidence of comparative anatomy and ontogeny. The lungs are
outgrowths of the head-gut; the heart develops from its inner wall. The
pleural sacs that enclose the lungs are dorsal parts of the head-cœlom,
originating from the pleuroducts; the pericardium in which the heart
afterwards lies is also double originally, being formed from ventral
halves of the head-cœlom, which only combine at a later stage. When the
lung of the air-breathing Vertebrates issues from the head-cavity and
enters the trunk-cavity, it follows the example of the floating bladder
of the fishes, which also originates from the pharyngeal wall in the
shape of a small pouch-like out-growth, but soon grows so large that,
in order to find room, it has to pass far behind into the trunk-cavity.
To put it more precisely, the lung of the quadrupeds retains this
hereditary growth-process of the fishes; for the hydrostatic floating
bladder of the latter is the air-filled organ from which the
air-breathing organ of the former has been evolved.


Fig.382. Transverse section of the cardiac region of the same
chick-embryo (behind the preceding). Fig. 382—Transverse section of the
cardiac region of the same chick-embryo (behind the preceding). In the
cervical cavity (_hh_) the heart (_h_) is still connected by a mesocard
(_hg_) with the gut-fibre layer (_pf_). _d_ gut-gland layer, _up_
provertebral plates, _jb_ rudimentary auditory vesicle in the horny
plate, _hp_ first rise of the amniotic fold. (From _Remak._)


There is an interesting cenogenetic phenomenon in the formation of the
heart of the higher Vertebrates that deserves special notice. In its
earliest form the heart is _double,_ as recent observation has shown,
in all the Amniotes, and the simple spindle-shaped cardiac tube, which
we took as our starting-point, is only formed at a later stage, when
the two lateral tubes move backwards, touch each other, and at last
combine in the middle line. In man, as in the rabbit, the two embryonic
hearts are still far apart at the stage when there are already eight
primitive segments (Fig. 134 _h_). So also the two cœlom-pouches of the
head in which they lie are still separated by a broad space. It is not
until the permanent body of the embryo develops and detaches from the
embryonic vesicle that the separate lateral structures join together,
and finally combine in the middle line. As the median partition between
the right and left cardiocœl disappears, the two cervical cavities
freely communicate (Fig. 381), and form, on the ventral side of the
amniote head, a horseshoe-shaped arch, the points of which advance
backwards into the pleuro-ducts or pleural cavities, and from there
into the two peritoneal sacs of the trunk. But even after the
conjunction of the cervical cavities (Fig. 381) the two cardiac tubes
remain separate at first; and even after they have united a delicate
partition in the middle of the simple endothelial tube (Figs. 379 _s,_
382 _h_) indicates the original separation. This _cenogenetic_ “primary
cardiac
septum” presently disappears, and has no relation to the subsequent
permanent partition between the halves of the heart, which, as a
heritage from the reptiles, has a great _palingenetic_ importance.

Thorough opponents of the biogenetic law have laid great stress on
these and similar cenogenetic phenomena, and endeavoured to urge them
as striking disproofs of the law. As in every other instance, careful,
discriminating, comparative-morphological examination converts these
supposed disproofs of evolution into strong arguments in its favour. In
his excellent work, _On the structure of the Heart in the Amphibia_
(1886), Carl Rabl has shown how easily these curious cenogenetic facts
can be explained by the secondary adaptation of the embryonic structure
to the great extension of the food-yelk.

The embryology of all the other parts of the vascular system also gives
us abundant and valuable data for the purposes of phylogeny. But as one
needs a thorough knowledge of the intricate structure of the whole
vascular system in man and the other Vertebrates in order to follow
this with profit, we cannot go into it further here. Moreover, many
important features in the ontogeny of the vascular system are still
very obscure and controverted. The characters of the embryonic
circulation of the Amniotes, which we have previously considered
(Chapter XV), are late acquisitions and entirely cenogenetic. (Cf. pp.
170–171; Figs. 198–202.)



Chapter XXIX.
EVOLUTION OF THE SEXUAL ORGANS


If we measure the importance of the systems of organs in the animal
frame according to the richness and variety of their phenomena and the
physiological interest that this implies, we must regard as one of the
principal and most interesting systems the one which we are now going
to examine—the system of the reproductive organs. Just as nutrition is
the first and most urgent condition for the self-maintenance of the
individual organism, so reproduction alone secures the maintenance of
the species—or, rather, the maintenance of the long series of
generations which the totality of the organic stem represents in their
genealogical connection. No individual organism has the prerogative of
immortality. To each is allotted only a brief span of personal
development, an evanescent moment in the million-year course of the
history of life.

Hence, reproduction and the correlative phenomenon, heredity, have long
been regarded, together with nutrition, as the most important and
fundamental function of living things, and it has been attempted to
distinguish them from “lifeless bodies” on this very score. As a matter
of fact, this division is not so profound and thorough as it seems to
be, and is generally supposed to be. If we examine carefully the nature
of the reproductive process, we soon see that it can be reduced to a
general property that is found in inorganic as well as organic
bodies—growth. Reproduction is a nutrition and growth of the organism
beyond the individual limit, which raises a part of it into the whole.
This is most clearly seen when we study it in the simplest and lowest
organisms, especially the Monera (Figs. 226–228) and the unicellular
Amœbæ (Fig. 17). There the simple individual is a single plastid. As
soon as it has reached a certain limit of size by continuous feeding
and normal growth, it cannot pass it, but divides, by simple cleavage,
into two equal halves. Each of these halves then continues its
independent life, and grows on until it in turn reaches the limit of
growth, and divides. In each of these acts of self-cleavage two new
centres of attraction are formed for the particles of bodies, the
foundations of the two new-formed individuals. There is no such thing
as immortality even in these unicellulars.
The individual as such is annihilated in the act of cleavage (cf. p.
48).

In many other Protozoa reproduction takes place not by cleavage, but by
budding (gemmation). In this case the growth that determines
reproduction is not total (as in segmentation), but partial. Hence in
gemmation also we may oppose the local growth-product, that becomes a
new individual in the bud, as a child-organism to the parent-organism
from which it is formed. The latter is older and larger than the
former. In cleavage the two products are equal in age and morphological
value. Next to gemmation we have, as other forms of asexual
reproduction, the forming of embryonic buds and the forming of
embryonic cells. But the latter leads us at once to sexual generation,
the distinctive feature of which is the separation of the sexes. I have
dealt fully with these various types of reproduction in my _History of
Creation_ (chap. viii) and my _Wonders of Life_ (chap. xi).

The earliest ancestors of man and the higher animals had no faculty of
sexual reproduction, but multiplied solely by asexual means—cleavage,
gemmation, or the formation of embryonic buds or cells, as many
Protozoa still do. The differentiation of the sexes came at a later
stage. We see this most plainly in the Protists, in which the union of
two individuals precedes the continuous cleavage of the unicellular
organism (transitory conjugation and permanent copulation of the
Infusoria). We may say that in this case the growth (the condition of
reproduction) is attained by the coalescence of two full-grown cells
into a single, disproportionately large individual. At the same time,
the mixture of the two plastids causes a rejuvenation of the plasm. At
first the copulating cells are quite homogeneous; but natural selection
soon brings about a certain contrast between them—larger female cells
(_macrospores_) and smaller male cells (_microspores_). It must be a
great advantage in the struggle for life for the new individual to have
inherited different qualities from the two cellular parents. The
further advance of this contrast between the generating cells led to
sexual differentiation. One cell became the female ovum
(_macrogonidion_), and the other the male sperm-cell (_microgonidion_).

The simplest forms of sexual reproduction among the living Metazoa are
seen in the Gastræads p. 233, the lower sponges, the common fresh-water
polyp (_Hydra_), and other Cœlenteria of the lowest rank. Prophysema
(Fig. 234), Olynthus (Fig. 238), Hydra, etc., have very simple tubular
bodies, the thin wall of which consists (as in the original gastrula)
only of the two primary germinal layers. As soon as the body reaches
sexual maturity, a number of the cells in its wall become female ova,
and others male sperm-cells: the former become very large, as they
accumulate a considerable quantity of yelk-granules in their protoplasm
(Fig. 235 _e_); the latter are very small on account of their repeated
cleavage, and change into mobile cone-shaped spermatozoa (Fig. 20).
Both kinds of cells detach from their source of origin, the primary
germinal layers, fall either into the surrounding water or into the
cavity of the gut, and unite there by fusing together. This is the
momentous process of fecundation, which we have examined in Chapter VII
(cf. Figs. 23–29).

From these simplest forms of sexual propagation, as we can observe them
to-day in the lowest Zoophytes, the Gastræads, Sponges, and Polyps, we
gather most important data. In the first place, we learn that, properly
speaking, nothing is required for sexual reproduction except the fusion
or coalescence of two different cells—a female ovum and male
sperm-cell. All other features, and all the very complex phenomena that
accompany the sexual act in the higher animals, are of a subordinate
and secondary character, and are later additions to this simple,
primary process of copulation and fecundation. But if we bear in mind
how extremely important a part this relation of the two sexes plays in
the whole of organic nature, in the life of plants, of animals, and of
man; how the mutual attraction of the sexes, love, is the mainspring of
the most remarkable processes—in fact, one of the chief mechanical
causes of the highest development of life—we cannot too greatly
emphasise this tracing of love to its source, the attractive force of
two erotic cells.

Throughout the whole of living nature the greatest effects proceed from
this very small cause. Consider the part that the flowers, the sexual
organs of the flowering plants, play in nature; or the exuberance of
wonderful phenomena that sexual selection produces in animal life; or
the
momentous influence of love in the life of man. In every case the
fusion of two cells is the sole original motive power; in every case
this invisible process profoundly affects the development of the most
varied structures. We may say, indeed, that no other organic process
can be compared to it for a moment in comprehensiveness and intensity
of action. Are not the Semitic myth of Adam and Eve, the old Greek
legend of Paris and Helena, and so many other famous traditions, only
the poetic expression of the vast influence that love and sexual
selection have exercised over the course of history ever since the
differentiation of the sexes? All the other passions that agitate the
heart of man are far outstripped in their joint influence by this
sense-inflaming and mind-benumbing Eros. On the one hand, we look to
love with gratitude as the source of the greatest artistic
achievements—the noblest creations of poetry, plastic art, and music;
we see in it the chief factor in the moral advance of humanity, the
foundation of family life, and therefore of social advance. On the
other hand, we dread it as the devouring flame that brings destruction
on so many, and has caused more misery, vice, and crime than all the
other evils of human life put together. So wonderful is love and so
momentous its influence on the life of the soul, or on the different
functions of the medullary tube, that here more than anywhere else the
“supernatural” result seems to mock any attempt at natural explanation.
Yet comparative evolution leads us clearly and indubitably to the first
source of love—the affinity of two different erotic cells, the
sperm-cell and ovum.[34]

 [34] The sensual perception (probably related to smell) of the two
 copulating sex-cells, which causes their mutual attraction, is a
 little understood, but very interesting, chemical function of the
 cell-soul (cf. p. 58 and _The Riddle of the Universe,_ chap. ix.)


The lowest Metazoa throw light on this very simple origin of the
intricate phenomena of reproduction, and they also teach us that the
earliest sexual form was hermaphrodism, and that the separation of the
sexes (by division of labour) is a secondary and later phenomenon.
Hermaphrodism predominates in the most varied groups of the lower
animals; each sexually-mature individual, each person, contains female
and male sexual cells, and is therefore able to fertilise itself and
reproduce. Thus we find ova and sperm-cells in the same individual, not
only in the lowest Zoophytes (Gastræads, Sponges, and many Polyps), but
also in many worms (leeches and earthworms), many of the snails (the
common garden and vineyard snails), all the Tunicates, and many other
invertebrate animals. All man’s earlier invertebrate ancestors, from
the Gastræads up to the Prochordonia, were hermaphrodites; possibly
even the earliest Acrania. We have an instructive proof of this in the
remarkable circumstance that many genera of fishes are still
hermaphrodites, and that it is occasionally found in the higher
Vertebrates of all classes (as atavism). We may conclude from this that
gonochorism (separation of the sexes) was a later stage in our
development. At first, male and female individuals differ only in the
possession of one or other kind of gonads; in other respects they were
identical, as we still find in the Amphioxus and the Cyclostomes.
Afterwards, accessory organs (ducts, etc.) are associated with the
primary sexual glands; and much later again sexual selection has given
rise to the secondary sexual characters—those differences between the
sexes which do not affect the sexual organs themselves, but other parts
of the body (such as the man’s beard or the woman’s breast).

The third important fact that we learn from the lower Zoophytes relates
to the earliest origin of the two kinds of sexual cells. As in the
Gastræads (the lowest sponges and hydroids), in which we find the first
beginnings of sexual differentiation, the whole body consists merely of
the two primary germinal layers, it follows that the sexual cells also
must have proceeded from the cells of these primary layers, either the
inner or outer, or from both. This simple fact is extremely important,
because the first trace of the ova as well as the spermatozoa is found
in the middle germinal layer or mesoderm in the higher animals,
especially the Vertebrates. This arrangement is a later development
from the preceding (in connection with the secondary formation of the
mesoderm).

If we trace the phylogeny of the sexual organs in our earliest Metazoa
ancestors, as the comparative anatomy and ontogeny of the lowest
Cœlenteria (_Cnidaria, Platodaria_) exhibit it to us, we find that the
first step in advance is the localisation or concentration of the two
kinds of sexual
cells scattered in the epithelium into definite groups. In the Sponges
and lowest Hydropolyps isolated cells are detached from the cell-strata
of the two primary germinal layers, and become free sexual cells; but
in the Cnidaria and Platodes we find these associated in groups which
we call sexual glands (_gonads_). We can now for the first time speak
of sexual organs in the morphological sense. The female germinative
glands, which in this simplest form are merely groups of homogeneous
cells, are the ovaries (Fig. 241 _c_). The male germinative glands,
which also in their first form consist of a cluster of sperm-cells, are
the testicles (Fig. 241 _h_). In the medusæ, which descend, both
ontogenetically and phylogenetically, from the more simply organised
Polyps, we find these simple sexual glands sometimes as gastric
pouches, sometimes as outgrowths of the radial canals that proceed from
the stomach. Particularly interesting in connection with the question
of the first origin of the gonads are the lowest forms of the Platodes,
the _Cryptocœla_ that have of late been separated as a special class
(_Platodaria_) from the Turbellaria proper (Fig. 239). In these very
primitive Platodes the two pairs of sexual glands are merely two pairs
of rows of differentiated cells in the entodermic wall of the primitive
gut—two median ovaries (_o_) within, and two lateral spermaries (_s_)
without. The mature sexual cells are ejected by the posterior outlets;
the female (_f_) lies in front of the male (_m_).


Fig.383. Embryos of Sagitta, in three earlier stages of development.
Fig. 383—Embryos of Sagitta, in three earlier stages of development.
(From _Hertwig._) _A_ gastrula, _B_ cœlomula with open primitive mouth,
_C_ the same primitive mouth closed, _ua_ primitive gut, _bl_ primitive
mouth, _g_ progonidia (hermaphroditic primitive sexual cells), _cs_
cœlom-pouches, _pm_ parietal layer, _vm_ visceral layer of same, _d_
permanent gut (enteron), _st_ mouth-pit (stomodæum).


In the great majority of the Bilateria or Cœlomaria it is the mesoderm
from which the gonads develop. Probably the first traces of them are
the two large cells that appear at the edge of the primitive mouth
(right and left), as a rule during gastrulation or immediately
afterwards—the important promesoblasts, or “polar cells of the
mesoderm,” or “primitive cells of the middle germinal layer” (p. 194).
In the real Enterocœla, in which the mesoderm appears from the first in
the shape of a couple of cœlom-pouches, these are very probably the
original gonads (p. 194). This is seen very clearly in the arrow-worm
(_Sagitta_). In the gastrula of Sagitta (Fig. 383 _A_) we find at an
early stage a couple of entodermic cells of an unusual size (_g_) at
the base of the primitive gut (_ud_). These primitive sexual cells
(_progonidia_) are symmetrically placed to the right and left of the
middle plane, like the two promesoblasts of the bilateral gastrula of
the Amphioxus (Fig. 38 _p_). A little outwards from them the two cœlom
pouches (_B, cs_) are developed out of the primitive gut, and each
progonidion divides into a male and a female sexual cell (_B, g_). The
two male cells (at first rather the larger) lie close together within,
and are the parent-cells of the testicles (_prospermaria_). The two
female cells lie outwards from these, and are the parent-cells of the
ovary (_protovaria_). Afterwards, when the cœlom-pouches have detached
from the permanent gut (_C, d_) and the primitive mouth (_A, bl_) is
closed, the female cells advance towards the mouth (_C, st_), and the
male towards the rear. The foremost pair of ovaries are then separated
by a transverse partition from the hind pair. Thus the first structures
of the sexual glands of the Sagitta are a couple of hermaphroditic
entodermic cells; each of these divides
into a male and a female cell; and these four cells are the
parent-cells of the four sexual glands. Probably the two promesoblasts
of the Amphioxus-gastrula (Fig. 38) are also hermaphroditic primitive
sexual cells in the same sense, inherited by this earliest vertebrate
from its ancient bilateral gastræad ancestors.


Fig.384. A, Part of the kidneys of Bdellostoma. B Portion of same,
highly magnified. Fig. 384—_A,_ Part of the kidneys of Bdellostoma. _a_
prorenal duct (nephroductus), _b_ segmental or primitive urinary canals
(pronephridia), _c_ renal or Malpighian capsules. _B_ Portion of same,
highly magnified. _c_ renal capsules with the glomerulus, _d_ afferent
artery, _e_ efferent artery. From _Johannes Müller_ (Myxinoides).


The sexually-mature Amphioxus is not hermaphroditic, as its nearest
invertebrate relatives, the Tunicates, are, and as the long-extinct
pre-Silurian Primitive Vertebrate (_Prospondylus,_ Figs. 98–102)
probably was. The actual lancelet has gonochoristic structures of a
very interesting kind. As we saw in the anatomy of the Amphioxus, we
find the ovaries of the female and the spermaries of the male in the
shape of twenty to thirty pairs of elliptical or roundish four-cornered
sacs, which lie on either side of the gut on the parietal surface of
the respiratory pore (Fig. 219 _g_). According to the important
discovery of Rückert (1888), the sexual glands of the earliest fishes,
the Selachii, are similarly arranged. They only unite afterwards to
form a pair of simple gonads. These have been transmitted by heredity
to all the rest of the Craniotes. In every case they lie originally on
each side of the mesentery, underneath the chorda, at the bottom of the
body-cavity. The first traces of them are found in the
cœlom-epithelium, at the spot where the skin-fibre layer and gut-fibre
layer meet in the middle of the mesenteric plate (Fig. 93 _mp_). At
this point we observe at an early stage in all craniote embryos a small
string-like cluster of cells, which we may call, with Waldeyer, the
“germ epithelium,” or (in harmony with the other plate-shaped
rudimentary organs) the _sexual plate_ (Fig. 173 _g_). This germinal or
sexual plate is found in the fifth week in the human embryo, in the
shape of a couple of long whitish streaks, on the inner side of the
primitive kidneys (Fig. 183 _t_). The cells of this sexual plate are
distinguished by their cylindrical form and chemical composition from
the rest of the cœlom-cells; they have a different purport from the
flat cells which line the rest of the body-cavity. As the germ
epithelium of the sexual plate becomes thicker, and supporting tissue
grows into it from the mesoderm, it becomes a rudimentary sexual gland.
This ventral gonad then develops into the ovary in the female
Craniotes, and the testicles in the male.

In the formation of the gonidia or erotic sexual cells and their
conjunction at fecundation we have the sole essential features of
sexual reproduction; but in the great majority of animals we find other
organs taking part in it. The chief of these secondary sexual organs
are the gonoducts, which serve to convey the mature sexual cells out of
the body, and the copulative organs, which bring the fecundating male
sperm into touch with the ovum-bearing female. The latter organs are,
as a rule, only found in the higher animals, and are much less widely
distributed than the gonoducts. But these also are secondary
formations, and are wanting in many animals of the lower groups.

In the lower animals the mature sexual cells are generally ejected
directly from
the body. Sometimes they pass out immediately through the skin (Hydra
and many hydroids); sometimes they fall into the gastric cavity, and
are evacuated by the mouth (gastræads, sponges, many medusæ, and
corals); sometimes they fall into the body-cavity, and are ejected by a
special pore (_porus genitalis_) in the ventral wall. The latter
procedure is found in many of the worms, and also in the lowest
Vertebrates. Amphioxus has the peculiar feature that the mature sexual
products fall first into the mantle-cavity; from there they are either
evacuated by the respiratory pore, or else they pass through the
gill-clefts into the branchial gut, and so out by the mouth (p. 185).
In the Cyclostomes they fall into the body-cavity, and are ejected by a
genital pore in its wall; so also in some of the fishes. From these we
gather the features of our earlier ancestors in this respect. On the
other hand, in all the higher and most of the lower Vertebrates (and
most of the higher Invertebrates) we find in both sexes special tubular
passages of the sexual gland, which are called “gonoducts.” In the
female they conduct the ova from the ovary, and so are called
“oviducts,” or “Fallopian tubes.” In the male they convey the
spermatozoa from the testicles, and are called “spermaducts,” or _vasa
deferentia._


Fig.385. Transverse section of the embryonic shield of a chick,
forty-two hours old. Fig. 385—Transverse section of the embryonic
shield of a chick, forty-two hours old. (From _Kölliker._) _mr_
medullary tube, _ch_ chorda, _h_ horny plate (skin-sense layer), _ung_
nephroduct, _vw_ episomites (dorsal primitive segments), _hp_
skin-fibre layer (parietal layer of the hyposomites), _dfp_ gut-fibre
layer (visceral layer of hyposomites), _ao_ aorta, _g_ vessels. (Cf.
transverse section of duck-embryo, Fig. 152.)


The original and genetic relation of these two kinds of ducts is just
the same in man as in the rest of the higher Vertebrates, and quite
different from what we find in most of the Invertebrates. In the
latter, as a rule, the gonoducts develop directly from the embryonic
glands or from the outer skin; but in the Vertebrates an independent
organic system is employed to convey the sexual products, and this had
originally a totally different function—namely, the system of urinary
organs. These organs have primarily the sole duty of removing unusable
matter from the body in a fluid form. Their liquid excretory product,
the urine, is either evacuated directly through the skin or through the
last section of the gut. It is only at a later stage that the tubular
urinary passages also convey the sexual products from the body. In this
way they become “urogenital ducts.” This remarkable secondary
conjunction of the urinary and sexual organs into a common urogenital
system is very characteristic of the Gnathostomes, the six higher
classes of Vertebrates. It is wanting in the lower classes. In order to
appreciate it fully, we must give a comparative glance at the structure
of the urinary organs.

The renal or urinary system is one of the oldest and most important
systems of organs in the differentiated animal body, as I have pointed
out on several previous occasions (cf. Chapter XVII). We find it not
only in the higher stems, but also very generally distributed in the
earlier group of the Vermalia. Here we meet it in the lowest worms, the
Rotatoria (Gastrotricha, Fig. 242), and in the instructive stem of the
Platodes. It consists of a pair of simple or branching canals, which
are lined with one layer of cells, absorb unusable juices from the
tissue, and eject them by an outlet in the outer skin (Fig. 240 _nm_).
Not only the free-living Turbellaria, but also the parasitic Suctoria,
and even the still more degenerate tapeworms, which have lost their
alimentary canal in consequence of their parasitic life, are equipped
with these renal canals
or nephridia. In the first embryonic structure they are merely a pair
of simple cutaneous glands, or depressions in the ectoderm. They are
generally described as excretory organs in the worms, but formerly
often as “water vessels.” They may be conceived as largely-developed
tubular cutaneous glands, formed by invagination of the cutaneous
layer. According to another view, they owe their origin to a later
rupture of the body-cavity outwards. In most of the Vermalia each
nephridium has an inner opening (with cilia) into the body-cavity and
an outer one on the epidermis.


Fig.386. Rudimentary primitive kidneys of a dog-embryo. Fig.
386—Rudimentary primitive kidneys of a dog-embryo. The hind end of the
embryonic body is seen from the ventral side and covered with the
visceral layer of the yelk-sac, which is torn away and folded down in
front in order to show the nephroducts with the primitive urinary
canals (_a_). _b_ primitive vertebræ, _c_ spinal cord, _d_ entrance
into the pelvic-gut cavity. (From _Bischoff._)


Fig. 387. Primitive kidneys of a human embryo. Fig. 387—Primitive
kidneys of a human embryo. _u_ the urinary canals of the primitive
kidneys, _w_ Wolffian duct, _w′_ uppermost end of the same (Morgagni’s
hydatid), _m_ Mullerian duct. _m′_ uppermost end of same (Fallopian
hydatid), _g_ gonad (sexual gland). (From _Kobelt._)


In these lowest, unsegmented worms, and in the unsegmented Molluscs,
there is only one pair of renal canals. They are more numerous in the
higher Articulates. In the Annelids, the body of which is composed of a
large number of joints, there is a pair of these pronephridia in each
segment (hence they are called segmental canals or organs). Even here
they are still simple tubes; on account of their coiled or looped form
they are often called “looped canals.” In most of the Annelids, and
many of the Vermalia, we can distinguish three sections in the
nephridium—an outer muscular duct, a glandular middle part, and an
inner part that opens by a ciliated funnel into the body-cavity. This
opening is furnished with whirling cilia, and can, therefore, take up
the juices to be excreted directly from the body-cavity and convey them
from the body. But in these worms the sexual cells, which develop in
very primitive form on the inner surface of the body-cavity, also fall
into it when mature, and are sucked up by the funnel-shaped inner
ciliated openings of the renal canals, and ejected with the urine. Thus
the urine-forming looped canals, or pronephridia, serve as oviducts in
the female Annelids and as spermaducts in the male.

The renal system of the Vertebrates is similar to, yet materially
different from, these segmental canals of the Annelids. The peculiar
development of it and its relations to the sexual organs are among the
most difficult problems in the morphology of our stem. If we examine
briefly the vertebrate renal system from the phylogenetic point of
view, as confirmed by recent discoveries, we may distinguish three
forms of it: (1) Fore-kidneys or head-kidneys (_pronephros_); (2)
primitive or middle kidneys (mesonephros); (3) permanent kidneys
(_metanephros_). These three systems of kidneys are not fundamentally
and completely distinct, as earlier students (such as Semper) wrongly
supposed; they represent three different generations of one and the
same excretory apparatus; they correspond to three phylogenetic stages,
and succeed each other in the stem-history of the Vertebrates in such
wise that each younger and more advanced generation develops farther
behind in the body, and replaces the older and less advanced generation
that preceded it in time and space. The _fore kidneys,_ first
accurately described by Wilhelm Müller in 1875 in the Cyclostomes and
Ichthyoda, form the sole excretory organ of the Acrania (Amphioxus);
they continue in the Cyclostomes and some of the fishes, but are found
only in slight traces and for a time in the embryos of the six other
classes of Vertebrates. The _primitive kidneys_ are first found in the
Cyclostomes, behind the fore kidneys; they have been transmitted from
the Selachii to all the Gnathostomes. In the _Anamnia_ they act
permanently as urinary glands; in the _Amniotes_ their anterior part
(“germinal kidneys”) changes into organs of the sexual apparatus, while
the third generation develops from the end of their posterior part
(“urinal kidneys”)—the characteristic after or permanent kidneys of the
three higher classes of Vertebrates. The order in which the three renal
systems succeed each other in the embryo of man and the higher
Vertebrates corresponds to their phylogenetic succession in the history
of our stem, and, consequently, in the natural classification of the
Vertebrates.


Fig.388. Pig-embryo, three-fifths of an inch long, seen from the
ventral side. Fig. 388—Pig-embryo, three-fifths of an inch long, seen
from the ventral side. _a_ fore leg, _z_ hind leg, _b_ ventral wall,
_r_ sexual prominence, _w_ nephroduct, _n_ primitive kidneys, _n1_
their inner part. (From _Oscar Schultze._)


Fig. 389. Human embryo of the fifth week, two-fifths of an inch long,
seen from the ventral side. Fig. 389—Human embryo of the fifth week,
two-fifths of an inch long, seen from the ventral side (the anterior
ventral wall, _b,_ is removed, the body-cavity, _c,_ opened). _d_ gut
(cut off), _f_ frontal process, _g_ cerebrum, _m_ middle brain, _e_
after brain, _h_ heart, _k_ first gill-cleft, _l_ pulmonary sac, _n_
primitive kidneys, _r_ sexual region, _p_ phallus (sexual prominences),
_s_ tail. (From _Kollmann._)


As in the morphology of any other system of organs, so in the case of
the urinary and sexual organs the Amphioxus is the real typical
primitive Vertebrate; it affords the key to the mysteries of the
structure of man and the higher Vertebrates. The kidneys of the
Amphioxus—first discovered by Boveri in 1890—are typical “fore
kidneys,” composed of a double row of short segmental canals (Fig. 217
_x_). The inner aperture of these pronephridia opens into the
mesodermic body-cavity (the middle part of the cœloma, _B_); the
external aperture into the ectodermic mantle or peribranchial cavity
(_C_). Their position, their
structure, and their relation to the branchial vessel make it clear
that these segmental pronephridia correspond to the rudimentary fore
kidneys of the Craniotes. The mantle-cavity into which they open seems
to correspond to the prorenal duct of the latter.


Fig.390, 391, 392. Primitive kidneys and rudimentary sexual organs.
Figs. 390, 391, 392—Primitive kidneys and rudimentary sexual organs.
Figs. 390 and 391 of Amphibia (frog-larvæ); Fig. 390 earlier, 391 later
stage. Fig. 392 of a mammal (ox-embryo). _u_ primitive kidney, _k_
sexual gland (rudiment of testicle and ovary). The primary nephroduct
(_ug_ in Fig. 390) divides (in Figs. 391 and 392) into the two
secondary nephroducts—the Mullerian (_m_) and Wolffian (_ug′_) ducts,
joined together behind in the genital cord (_g_). _l_ ligament of the
primitive kidneys. (From Gegenbaur.)


Fig.393, 394. Urinary and sexual organs of an Amphibian (water
salamander or Triton). Fig. 393 of a female, 394 of a male. Figs. 393,
394—Urinary and sexual organs of an Amphibian (water salamander or
Triton). Fig. 393 of a female, 394 of a male. _r_ primitive kidney,
_ov_ ovary, _od_ oviduct and _c_ Rathke’s duct, both developed from the
Müllerian duct, _u_ primitive ureter (also acting as spermaduct [_ve_]
in the male, opening below into the Wolffian duct [_u_ apostrophe]),
_ms_ mesovarium. (From _Gegenbaur._)


The next higher Vertebrates, the Cyclostomes, yield some very
interesting data. Both orders of this class, the hags and lampreys,
have still the fore kidneys inherited from the Acrania—the former
permanently, the latter in their earlier stages. Behind these the
primitive kidneys soon develop, and in a very characteristic form. The
remarkable structure of the mesonephros of the Cyclostomes, discovered
by Johannes Müller, explains the intricate formation of the kidneys in
the higher Vertebrates. We find in the hag-fishes (_Bdellostoma_) a
long tube, the prorenal duct (_nephroductus,_ Fig. 384 _a_). This opens
with its anterior end into the cœloma by a ciliated aperture, and
externally with its posterior end by an outlet in the skin. Inside it
open a large number of small transverse canals (“segmental or primitive
urinary canals,” _b_). Each of these terminates blindly in a vesicular
capsule (_c_), and this encloses a coil of blood-vessel (_glomerulus,_
an arterial network, Fig. 384 _B, c_). Afferent branches of arteries
conduct arterial blood into the coiled branches of the glomerulus
(_d_), and efferent arterial branches conduct it away from the net
(_c_). The primitive renal canals (mesonephridia) are distinguished by
this net-formation from their predecessors.

In the Selachii also we find a longitudinal row of segmental canals on
each side, which open outwards into the primitive renal ducts
(_nephrotomes,_ p. 149. The segmental canals (a pair in each segment of
the middle part of the body) open internally by a ciliated funnel into
the body-cavity. From the posterior group of these organs a compact
primitive kidney is formed, the anterior group taking part in the
construction of the sexual organs.

In the same simple form that remains
throughout life in the Myxinoides and partly in the Selachii we find
the primitive kidney first developing in the embryo of man and the
higher Craniotes (Figs. 386, 387). Of the two parts that compose the
comb-shaped primitive kidney the longitudinal channel, or nephroduct,
is always the first to appear; afterwards the transverse “canals,” the
excreting nephridia, are formed in the mesoderm; and after this again
the Malpighian capsules with their arterial coils are associated with
these as cœlous outgrowths. The primitive renal duct, which appears
first, is found in all craniote embryos at the early stage in which the
differentiation of the medullary tube takes place in the ectoderm, the
severance of the chorda from the visceral layer in the entoderm, and
the first trace of the cœlom-pouches arises between the limiting layers
(Fig. 385). The nephroduct (_ung_) is seen on each side, directly under
the horny plate, in the shape of a long, thin, thread-like string of
cells. It presently hollows out and becomes a canal, running straight
from front to back, and clearly showing in the transverse section of
the embryo its original position in the space between horny plate
(_h_), primitive segments (_uw_), and lateral plates (_hpl_). As the
originally very short urinary canals lengthen and multiply, each of the
two primitive kidneys assumes the form of a half-feathered leaf (Fig.
387). The lines of the leaf are represented by the urinary canals
(_u_), and the rib by the outlying nephroduct (_w_). At the inner edge
of the primitive kidneys the rudiment of the ventral sexual gland (_g_)
can now be seen as a body of some size. The hindermost end of the
nephroduct opens right behind into the last section of the rectum, thus
making a cloaca of it. However, this opening of the nephroducts into
the intestine must be regarded as a secondary formation. Originally
they open, as the Cyclostomes clearly show, quite independently of the
gut, in the external skin of the abdomen.


Fig.395. Primitive kidneys and germinal glands of a human embryo, three
inches in length (beginning of the sixth week). Fig. 395—Primitive
kidneys and germinal glands of a human embryo, three inches in length
(beginning of the sixth week), magnified. _k_ germinal gland, _u_
primitive kidney, _z_ diaphragmatic ligament of same, _w_ Wolffian duct
(opened on the right), _g_ directing ligament (gubernaculum), _a_
allantoic duct. (From _Kollmann._)


In the Myxinoides the primitive kidneys retain this simple comb-shaped
structure, and a part of it is preserved in the Selachii; but in all
the other Craniotes it is only found for a short time in the embryo, as
an ontogenetic reproduction of the earlier phylogenetic structure. In
these the primitive kidney soon assumes the form (by the rapid growth,
lengthening, increase, and serpentining of the urinary canals) of a
large compact gland, of a long, oval or spindle-shaped character, which
passes through the greater part of the embryonic body-cavity (Figs. 183
_m,_ 184 _m,_ 388 _n_). It lies near the middle line, directly under
the primitive vertebral column, and reaches from the cardiac region to
the cloaca. The right and left kidneys are parallel to each other,
quite close together, and only separated by the mesentery—the thin
narrow layer that attaches the middle gut to the under surface of the
vertebral column. The passage of each primitive kidney, the nephroduct,
runs towards the back on the lower and outer side of the gland, and
opens in the cloaca, close to the starting-point of the allantois; it
afterwards opens into the allantois itself.

The primitive or primordial kidneys of the amniote embryo were formerly
called the “Wolffian bodies,” and sometimes “Oken’s bodies.” They act
for a time as
kidneys, absorbing unusable juices from the embryonic body and
conducting them to the cloaca—afterwards to the allantois. There the
primitive urine accumulates, and thus the allantois acts as bladder or
urinary sac in the embryos of man and the other Amniotes. It has,
however, no genetic connection with the primitive kidneys, but is a
pouch-like growth from the anterior wall of the rectum (Fig. 147 _u_).
Thus it is a product of the visceral layer, whereas the primitive
kidneys are a product of the middle layer. Phylogenetically we must
suppose that the allantois originated as a pouch-like growth from the
cloaca-wall in consequence of the expansion caused by the urine
accumulated in it and excreted by the kidneys. It is originally a blind
sac of the rectum. The real bladder of the vertebrate certainly made
its first appearance among the Dipneusts (in Lepidosiren), and has been
transmitted from them to the Amphibia, and from these to the Amniotes.
In the embryo of the latter it protrudes far out of the not yet closed
ventral wall. It is true that many of the fishes also have a “bladder.”
But this is merely a local enlargement of the lower section of the
nephroducts, and so totally different in origin and composition from
the real bladder. The two structures can be compared from the
physiological point of view, and so are _analogous,_ as they have the
same function; but not from the morphological point of view, and are
therefore not _homologous._ The false bladder of the fishes is a
mesodermic product of the nephroducts; the true bladder of the
Dipneusts, Amphibia, and Amniotes is an entodermic blind sac of the
rectum.


Figs. 396-398. Urinary and sexual organs of ox-embryos. Figs.
396–398—Urinary and sexual organs of ox-embryos. Fig. 396, female
embryo one and a half inches long; Fig. 397, male embryo, one and a
half inches long. Fig. 398 female embryo two and a half inches long.
_w_ primitive kidney, _wg_ Wolffian duct, _m_ Müllerian duct, _m′_
upper end of same (opened at _t_), _i_ lower and thicker part of same
(rudiment of uterus), _g_ genital cord, _h_ testicle, (_h′,_ lower and
_h″,_ upper testicular ligament), _o_ ovary, _o′_ lower ovarian
ligament, _i_ inguinal ligament of primitive kidney, _d_ diaphragmatic
ligament of primitive kidney, _nn_ accessory kidneys, _n_ permanent
kidneys, under them the S-shaped ureters, between these the rectum, _v_
bladder, _a_ umbilical artery. (From _Kölliker._)


In all the Anamnia (the lower amnionless Craniotes, Cyclostomes,
Fishes, Dipneusts, and Amphibia) the urinary organs remain at a lower
stage of development to this extent, that the primitive kidneys
(_protonephri_) act permanently as urinary glands. This is only so as a
passing phase of the early embryonic life in the three higher classes
of Vertebrates, the Amniotes. In these the permanent or after or
secondary (really _tertiary_) kidneys (_renes_ or _metanephri_) that
are distinctive of these three classes soon make their appearance. They
represent the third and last generation of the vertebrate kidneys. The
permanent kidneys do not arise (as was long supposed) as independent
glands from the alimentary tube, but from the last section of the
primitive kidneys and the nephroduct. Here a simple tube, the secondary
renal duct, develops, near the point of its entry into the cloaca; and
this tube grows considerably forward. With its blind upper or anterior
end is connected a glandular renal growth, that owes its origin to a
differentiation of the last part of the primitive kidneys. This
rudiment of the
permanent kidneys consists of coiled urinary canals with Malpighian
capsules and vascular coils (without ciliated funnels), of the same
structure as the segmental mesonephridia of the primitive kidneys. The
further growth of these metanephridia gives rise to the compact
permanent kidneys, which have the familiar bean-shape in man and most
of the higher mammals, but consist of a number of separate folds in the
lower mammals, birds, and reptiles. As the permanent kidneys grow
rapidly and advance forward, their passage, the ureter, detaches
altogether from its birth-place, the posterior end of the nephroduct;
it passes to the posterior surface of the allantois. At first in the
oldest Amniotes this ureter opens into the cloaca together with the
last section of the nephroduct, but afterwards separately from this,
and finally into the permanent bladder apart from the rectum
altogether. The bladder originates from the hindmost and lowest part of
the allantoic pedicle (_urachus_), which enlarges in spindle shape
before the entry into the cloaca. The anterior or upper part of the
pedicle, which runs to the navel in the ventral wall of the embryo,
atrophies subsequently, and only a useless string-like relic of it is
left as a rudimentary organ; that is the single vesico-umbilical
ligament. To the right and left of it in the adult male are a couple of
other rudimentary organs, the lateral vesico-umbilical ligaments. These
are the degenerate string-like relics of the earlier umbilical
arteries.

Though in man and all the other Amniotes the primitive kidneys are thus
early replaced by the permanent kidneys, and these alone then act as
urinary organs, all the parts of the former are by no means lost. The
nephroducts become very important physiologically by being converted
into the passages of the sexual glands. In all the Gnathostomes—or all
the Vertebrates from the fishes up to man—a second similar canal
develops beside the nephroduct at an early stage of embryonic
evolution. The latter is usually called the Müllerian duct, after its
discoverer, Johannes Müller, while the former is called the Wolffian
duct. The origin of the Müllerian duct is still obscure; comparative
anatomy and ontogeny seem to indicate that it originates by
differentiation from the Wolffian duct. Perhaps it would be best to
say: “The original primary nephroduct divides by differentiation (or
longitudinal cleavage) into two secondary nephroducts, the Wolffian and
the Müllerian ducts.” The latter (Fig. 387 _m_) lies just on the inner
side of the former (Fig. 387 _w_). Both open behind into the cloaca.


Fig.399. Female sexual organs of a Monotreme (Ornithorhynchus, Fig.
269). Fig. 399—Female sexual organs of a Monotreme (_Ornithorhynchus,_
Fig. 269). _o_ ovaries, _t_ oviducts, _u_ womb, _sug_ urogenital sinus;
at _u′_ is the outlet of the two wombs, and between them the bladder
(_vu_). _cl_ cloaca. (From _Gegenbaur._)


However uncertain the origin of the nephroduct and its two products,
the Müllerian and the Wolffian ducts, may be, its later development is
clear enough. In all the Gnathostomes the Wolffian duct is converted
into the spermaduct, and the Müllerian duct into the oviduct. Only one
of them is retained in each sex; the other either disappears
altogether, or only leaves relics in the shape of rudimentary organs.
In the male sex, in which the two Wolffian ducts become the
spermaducts, we often find traces of the Müllerian ducts, which I have
called “Rathke’s canals” (Fig. 394 _c_). In the female sex, in which
the two Müllerian ducts form the oviducts, there are relics of the
Wolffian ducts, which are called “the ducts of Gaertner.”

We obtain the most interesting information with regard to this
remarkable evolution of the nephroducts and their association with the
sexual glands from the Amphibia (Figs. 390–395). The first structure of
the nephroduct and its differentiation into Müllerian and Wolffian
ducts are just the same in both sexes in the Amphibia, as in the mammal
embryos (Figs. 392, 396). In the female Amphibia
the Müllerian duct develops on either side into a large oviduct (Fig.
393 _od_), while the Wolffian duct acts permanently as ureter (_u_). In
the male Amphibia the Müllerian duct only remains as a rudimentary
organ without any functional significance, as Rathke’s canal (Fig. 394
_c_); the Wolffian duct serves also as ureter, but at the same time as
spermaduct, the sperm-canals (_ve_) that proceed from the testicles
(_t_) entering the fore part of the primitive kidneys and combining
there with the urinary canals.


Figs. 400, 401. Original position of the sexual glands in the ventral
cavity of the human embryo (three months old). Figs. 400, 401—Original
position of the sexual glands in the ventral cavity of the human embryo
(three months old). Fig. 400, male. _h_ testicles, _gh_ conducting
ligament of the testicles, _wg_ spermaduct, _h_ bladder, _uh_ inferior
vena cava, _nn_ accessory kidneys, _n_ kidneys. Fig. 401, female. _r_
round maternal ligament (underneath it the bladder, over it the
ovaries). _r′_ kidneys, _s_ accessory kidneys, _c_ cæcum, _o_ small
reticle, _om_ large reticle (stomach between the two), _l_ spleen.
(From _Kölliker._)


In the mammals these permanent amphibian features are only seen as
brief phases of the earlier period of embryonic development (Fig. 392).
Here the primitive kidneys, which act as excretory organs of urine
throughout life in the amnion-less Vertebrates, are replaced in the
mammals by the permanent kidneys. The real primitive kidneys disappear
for the most part at an early stage of development, and only small
relics of them remain. In the male mammal the _epididymis_ develops
from the uppermost part of the primitive kidney; in the female a
useless rudimentary organ, the _epovarium,_ is formed from the same
part. The atrophied relic of the former is known as the _paradidymis,_
that of the latter as the _parovarium._


Fig.402. Urogenital system of a human embryo of three inches in length.
Fig. 402—Urogenital system of a human embryo of three inches in length.
_h_ testicles, _wg_ spermaducts, _gh_ conducting ligament, _p_
processus vaginalis, _b_ bladder, _au_ umbilical arteries, _m_
mesorchium, _d_ intestine, _u_ ureter, _n_ kidney, _nn_ accessory
kidney. (From _Kollman._)


The Müllerian ducts undergo very important changes in the female
mammal. The oviducts proper are developed only from their upper part;
the lower part dilates into a spindle-shaped tube with thick muscular
wall, in which the impregnated ovum develops into the embryo. This is
the womb (_uterus_). At first the two wombs (Fig. 399 _u_) are
completely separate, and open into the cloaca on either side of the
bladder (_vu_), as is still the case in the lowest living mammals, the
Monotremes. But in the Marsupials a communication is opened between the
two Müllerian ducts, and in the Placentals they combine below with the
rudimentary Wolffian ducts to form a single “genital cord.” The
original independence of the two wombs and the vaginal canals formed
from their lower ends are retained in many of the lower Placentals, but
in the higher they gradually blend and form a single organ. The
conjunction proceeds from below (or behind) upwards (or forwards). In
many of the Rodents (such as the rabbit and squirrel) two separate
wombs still open into the simple and single vaginal canal; but in
others, and in the Carnivora, Cetacea, and Ungulates, the lower halves
of the wombs have already fused into a single piece, though the upper
halves (or “horns”) are still separate (“two-horned” womb, _uteris
bicornis_). In the bats and lemurs the “horns” are
very short, and the lower common part is longer. Finally, in the apes
and in man the blending of the two halves is complete, and there is
only the one simple, pear-shaped uterine pouch, into which the oviducts
open on each side. This simple uterus is a late evolutionary product,
and is found _only_ in the ape and man.


Figs. 403-406. Origin of human ova in the female ovary. Figs.
403–406—Origin of human ova in the female ovary. Fig. 403. Vertical
section of the ovary of a new-born female infant, _a_ ovarian
epithelium, _b_ rudimentary string of ova, _c_ young ova in the
epithelium, _d_ long string of ova with follicle-formation (Pflüger’s
tube), _e_ group of young follicles, _f_ isolated young follicle, _g_
blood-vessels in connective tissue (stroma) of the ovary. In the
strings the young ova are distinguished by their considerable size from
the surrounding follicle-cells. (From _Waldeyer._) Fig. 404—Two young
Graafian follicles, isolated. In _1_ the follicle-cells still form a
simple, and in _2_ a double, stratum round the young ovum; in _2_ they
are beginning to form the ovolemma or the zona pellucida (_a_). Figs.
405 and 406—Two older Graafian follicles, in which fluid is beginning
to accumulate inside the eccentrically thickened epithelial mass of the
follicle-cells (Fig. 405 with little, 406 with much, follicle-water).
_ei_ the young ovum, with embryonic vesicle and spot, _zp_ ovolemma or
zona pellucida, _dp_ discus proligerus, formed of an accumulation of
follicle-cells, which surround the ovum, _ff_ follicle-liquid (_liquor
folliculi_), gathered inside the stratified follicle-epithelium (_fe_),
_fk_ connective-tissue fibrous capsule of the Graafian follicle (_theca
folliculi_).


In the male mammals there is the same fusion of the Müllerian and
Wolffian ducts at their lower ends. Here again they form a single
genital cord (Fig. 397 _g_), and this opens similarly into the
original urogenital sinus, which develops from the lowest section of
the bladder (_v_). But while in the male mammal the Wolffian ducts
develop into the permanent spermaducts, there are only rudimentary
relics left of the Müllerian ducts. The most notable of these is the
“male womb” (_uterus masculinus_), which originates from the lowest
fused part of the ducts, and corresponds to the female uterus. It is a
small, flask-shaped vesicle without any physiological significance,
which opens into the ureter between the two spermaducts and the
prostate folds (_vesicula prostatica_).


Fig.407. A ripe human Graafian follicle. Fig. 407—A ripe human Graafian
follicle. _a_ the mature ovum, _b_ the surrounding follicle-cells, _c_
the epithelial cells of the follicle, _d_ the fibrous membrane of the
follicle, _e_ its outer surface.


The internal sexual organs of the mammals undergo very distinctive
changes of position. At first the germinal glands of both sexes lie
deep inside the ventral cavity, at the inner edge of the primitive
kidneys (Figs. 386 _g_, 392 _k_), attached to the vertebral column by a
short mesentery (_mesorchium_ in the male, _mesovarium_ in the female).
But this primary arrangement is retained permanently only in the
Monotremes (and the lower Vertebrates). In all other mammals (both
Marsupials and Placentals) they leave their original cradle and travel
more or less far down (or behind), following the direction of a
ligament that goes from the primitive kidneys to the inguinal region of
the ventral wall. This is the inguinal ligament of the primitive
kidneys, known in the male as the Hunterian ligament (Fig. 400 _gh_),
and in the female as the “round maternal ligament” (Fig. 401 _r_). In
woman the ovaries travel more or less towards the small pelvis, or
enter into it altogether. In the male the testicles pass out of the
ventral cavity, and penetrate by the inguinal canal into a sac-shaped
fold of the outer skin. When the right and left folds (“sexual
swellings”) join together they form the _scrotum._ The various mammals
bring before us the successive stages of this displacement. In the
elephant and the whale the testicles descend very little, and remain
underneath the kidneys. In many of the rodents and carnassia they enter
the inguinal canal. In most of the higher mammals they pass through
this into the scrotum. As a rule, the inguinal canal closes up. When it
remains open the testicles may periodically pass into the scrotum, and
withdraw into the ventral cavity again in time of rut (as in many of
the marsupials, rodents, bats, etc.).

The structure of the external sexual organs, the copulative organs that
convey the fecundating sperm from the male to the female organism in
the act of copulation, is also peculiar to the mammals. There are no
organs of this character in most of the other Vertebrates. In those
that live in water (such as the Acrania and Cyclostomes, and most of
the fishes) the ova and sperm-cells are simply ejected into the water,
where their conjunction and fertilisation are left to chance. But in
many of the fishes and amphibia, which are viviparous, there is a
direct conveyance of the male sperm into the female body; and this is
the case with all the Amniotes (reptiles, birds, and mammals). In these
the urinary and sexual organs always open originally into the last
section of the rectum, which thus forms a cloaca
(p. 249). Among the mammals this arrangement is permanent only in the
Monotremes, which take their name from it (Fig. 399 _cl_). In all the
other mammals a frontal partition is developed in the cloaca (in the
human embryo about the beginning of the third month), and this divides
it into two cavities. The anterior cavity receives the urogenital
canal, and is the sole outlet of the urine and the sexual products; the
hind or anus-cavity passes the excrements only.

Even before this partition has been formed in the Marsupials and
Placentals, we see the first trace of the external sexual organs. First
a conical protuberance rises at the anterior border of the
cloaca-outlet—the sexual prominence (_phallus,_ Fig. 402 _A, e, B, e_).
At the tip it is swollen in the shape of a club (“acorn” _glans_). On
its under side there is a furrow, the sexual groove (_sulcus genitalis,
f_), and on each side of this a fold of skin, the “sexual pad” (_torus
genitalis, h l_). The sexual protuberance or phallus is the chief organ
of the sexual sense (p. 282); the sexual nerves spread on it, and these
are the principal organs of the specific sexual sensation. As erectile
bodies (_corpora cavernosa_) are developed in the male phallus by
peculiar modifications of the blood-vessels, it becomes capable of
erecting periodically on a strong accession of blood, becoming stiff,
so as to penetrate into the female vagina and thus effect copulation.
In the male the phallus becomes the penis; in the female it becomes the
much smaller clitoris; this is only found to be very large in certain
apes (_Ateles_). A prepuce (“foreskin”) is developed in both sexes as a
protecting fold on the anterior surface of the phallus.


Fig.408. The human ovum after issuing from the Graafian follicle,
surrounded by the clinging cells of the discus proligerus (in two
radiating crowns). Fig. 408—The human ovum after issuing from the
Graafian follicle, surrounded by the clinging cells of the _discus
proligerus_ (in two radiating crowns). _z_ ovolemma (zona pellucida,
with radial porous canals), _p_ cytosoma (protoplasm of the cell-body,
darker within, lighter without), _k_ nucleus of the ovum (embryonic
vesicle). (From _Nagel._) (Cf. Figs. 1 and 14.)


The external sexual member (_phallus_) is found at various stages of
development within the mammal class, both in regard to size and shape,
and the differentiation and structure of its various parts; this
applies especially to the terminal part of the phallus, the glans, both
the larger _glans penis_ of the male and the smaller _glans clitoridis_
of the female. The part of the cloaca from the upper wall of which it
forms belongs to the _proctodæum,_ the ectodermic invagination of the
rectum (p. 311); hence its epithelial covering can develop the same
horny growths as the corneous layer of the epidermis. Thus the glans,
which is quite smooth in man and the higher apes, is covered with
spines in many of the lower apes and in the cat, and in many of the
rodents with hairs (marmot) or scales (guinea-pig) or solid horny warts
(beaver). Many of the Ungulates have a free conical projection on the
glans, and in many of the Ruminants this “phallus-tentacle” grows into
a long cone, bent hook-wise at the base (as in the goat, antelope,
gazelle, etc.). The different forms of the phallus are connected with
variations in the structure and distribution of the sensory
corpuscles—_i.e._ the real organs of the sexual sense, which develop in
certain papillæ of the corium of the phallus, and have been evolved
from ordinary tactile corpuscles of the corium by erotic adaptation (p.
282).


The formation of the _corpora cavernosa,_ which cause the stiffness of
the phallus and its capability of penetrating the vagina, by certain
special structures of their spongy vascular spaces, also shows a good
deal of variety within the vertebrate stem. This stiffness is increased
in many orders of mammals (especially the carnassia and rodents) by the
ossification of a part of the fibrous body (_corpus fibrosum_). This
penis-bone (_os priapi_) is very large in the badger and dog, and bent
like a hook in the marten; it is also very large in some of the lower
apes, and protrudes far out into the glans. It is wanting in most of
the anthropoid apes; it seems to have been lost in their case (and in
man) by atrophy.

The sexual groove on the under side of the phallus receives in the male
the mouth of the urogenital canal, and is changed into a continuation
of this, becoming a closed canal by the juncture of its parallel edges,
the male urethra. In the female this only takes place in a few cases
(some of the lemurs, rodents, and moles); as a rule, the groove remains
open, and the borders of this “vestibule of the vagina” develop into
the smaller labia (_nymphæ_). The large labia of the female develop
from the sexual pads (_tori genitales_), the two parallel folds of the
skin that are found on each side of the genital groove. They join
together in the male, and form the closed scrotum. These striking
differences between the two sexes cannot yet be detected in the human
embryo of the ninth week. We begin to trace them in the tenth week of
development, and they are accentuated in proportion as the difference
of the sexes develops.

Sometimes the normal juncture of the two sexual pads in the male fails
to take place, and the sexual groove may also remain open
(_hypospadia_). In these cases the external male genitals resemble the
female, and they are often wrongly regarded as cases of hermaphrodism.
Other malformations of various kinds are not infrequently found in the
human external sexual organs, and some of them have a great
morphological interest. The reverse of hypospadia, in which the penis
is split open below, is seen in _epispadia,_ in which the urethra is
open above. In this case the urogenital canal opens above at the dorsal
root of the penis; in the former case down below. These and similar
obstructions interfere with a man’s generative power, and thus
prejudicially affect his whole development. They clearly prove that our
history is not guided by a “kind Providence,” but left to the play of
blind chance.

We must carefully distinguish the rarer cases of real hermaphrodism
from the preceding. This is only found when the essential organs of
reproduction, the genital glands of both kinds, are united in one
individual. In these cases either an ovary is developed on the right
and a testicle on the left (or _vice versa_); or else there are
testicles and ovaries on both sides, some more and others less
developed. As hermaphrodism was probably the original arrangement in
all the Vertebrates, and the division of the sexes only followed by
later differentiation of this, these curious cases offer no theoretical
difficulty. But they are rarely found in man and the higher mammals. On
the other hand, we constantly find the original hermaphrodism in some
of the lower Vertebrates, such as the Myxinoides, many fishes of the
perch-type (_serranus_), and some of the Amphibia (ringed snake, toad).
In these cases the male often has a rudimentary ovary at the fore end
of the testicle; and the female sometimes has a rudimentary, inactive
testicle. In the carp also and some other fishes this is found
occasionally. We have already seen how traces of the earlier
hemaphrodism can be traced in the passages of the Amphibia.

Man has faithfully preserved the main features of his stem-history in
the ontogeny of his urinary and sexual organs. We can follow their
development step by step in the human embryo in the same advancing
gradation that is presented to us by the comparison of the urogenital
organs in the Acrania, Cyclostomes; Fishes, Amphibia, Reptiles, and
then (within the mammal series) in the Monotremes, Marsupials, and the
various Placentals. All the peculiarities of urogenital structure that
distinguish the mammals from the rest of the Vertebrates are found in
man; and in all special structural features he resembles the apes,
particularly the anthropoid apes. In proof of the fact that the special
features of the mammals have been inherited by man, I will, in
conclusion, point out the identical way in which the ova are formed in
the ovary. In all the mammals the mature ova are contained in special
capsules, which are known as the _Graafian_
_follicles,_ after their discoverer, Roger de Graaf (1677). They were
formerly supposed to be the ova themselves; but Baer discovered the ova
within the follicles (p. 16). Each follicle (Fig. 407) consists of a
round fibrous capsule (_d_), which contains fluid and is lined with
several strata of cells (_c_). The layer is thickened like a knob at
one point (_b_); this ovum-capsule encloses the ovum proper (_a_). The
mammal ovary is originally a very simple oval body (Fig. 387 _g_),
formed only of connective tissue and blood-vessels, covered with a
layer of cells, the ovarian epithelium or the female germ epithelium.
From this germ epithelium strings of cells grow out into the connective
tissue or “stroma” of the ovary (Fig. 403 _b_). Some of the cells of
these strings (or Pflüger’s tubes) grow larger and become ova
(primitive ova, _c_); but the great majority remain small, and form a
protective and nutritive stratum of cells round each ovum—the
“follicle-epithelium” (_e_).

The follicle-epithelium of the mammal has at first one stratum (Fig.
404 _1_), but afterwards several (_2_). It is true that in all the
other Vertebrates the ova are enclosed in a membrane, or “follicle,”
that consists of smaller cells. But it is only in the mammals that
fluid accumulates between the growing follicle-cells, and distends the
follicle into a large round capsule, on the inside wall of which the
ovum lies, at one side (Figs. 405, 406). There again, as in the whole
of his morphology, man proves indubitably his descent from the mammals.

In the lower Vertebrates the formation of ova in the germ-epithelium of
the ovary continues throughout life; but in the higher it is restricted
to the earlier stages, or even to the period of embryonic development.
In man it seems to cease in the first year; in the second year we find
no new-formed ova or chains of ova (Pflüger’s tubes). However, the
number of ova in the two ovaries is very large in the young girl; there
are calculated to be 72,000 in the sexually-mature maiden. In the
production of the ova men resemble most of the anthropoid apes.

Generally speaking, the natural history of the human sexual organs is
one of those parts of anthropology that furnish the most convincing
proofs of the animal origin of the human race. Any man who is
acquainted with the facts and impartially weighs them will conclude
from them alone that we have been evolved from the lower Vertebrates.
The larger and the detailed structure, the action, and the
embryological development of the sexual organs are just the same in man
as in the apes. This applies equally to the male and the female, the
internal and the external organs. The differences we find in this
respect between man and the anthropoid apes are much slighter than the
differences between the various species of apes. But all the apes have
certainly a common origin, and have been evolved from a long-extinct
early-Tertiary stem-form, which we must trace to a branch of the
lemurs. If we had this unknown pithecoid stem-form before us, we should
certainly put it in the order of the true apes in the primate system;
but within this order we cannot, for the anatomic and ontogenetic
reasons we have seen, separate man from the group of the anthropoid
apes. Here again, therefore, on the ground of the
pithecometra-principle, comparative anatomy and ontogeny teach with
full confidence the descent of man from the ape.



Chapter XXX.
RESULTS OF ANTHROPOGENY


Now that we have traversed the wonderful region of human embryology and
are familiar with the principal parts of it, it will be well to look
back on the way we have come, and forward to the further path to truth
to which it has led us. We started from the simplest facts of ontogeny,
or the development of the individual—from observations that we can
repeat and verify by microscopic and anatomic study at any moment. The
first and most important of these facts is that every man, like every
other animal, begins his existence as a simple cell. This round ovum
has the same characteristic form and origin as the ovum of any other
mammal. From it is developed in the same manner in all the Placentals,
by repeated cleavage, a multicellular blastula. This is converted into
a gastrula, and this in turn into a blastocystis (or embryonic
vesicle). The two strata of cells that compose its wall are the primary
germinal layers, the skin-layer (ectoderm), and gut-layer (entoderm).
This two-layered embryonic form is the ontogenetic reproduction of the
extremely important phylogenetic stem-form of all the Metazoa, which we
have called the Gastræa. As the human embryo passes through the
gastrula-form like that of all the other Metazoa, we can trace its
phylogenetic origin to the Gastræa.

As we continued to follow the embryonic development of the two-layered
structure, we saw that first a third, or middle layer (mesoderm),
appears between the two primary layers; when this divides into two, we
have the four secondary germinal layers. These have just the same
composition and genetic significance in man as in all the other
Vertebrates. From the skin-sense layer are developed the epidermis, the
central nervous system, and the chief part of the sense-organs. The
skin-fibre layer forms the corium and the motor organs—the skeleton and
the muscular system. From the gut-fibre layer are developed the
vascular system, the muscular wall of the gut, and the sexual glands.
Finally, the gut-gland layer only forms the epithelium, or the inner
cellular stratum of the mucous membrane of the alimentary canal and
glands (lungs, liver, etc.).

The manner in which these different systems of organs arise from the
secondary germinal layers is essentially the same from the start in man
as in all the other Vertebrates. We saw, in studying the embryonic
development of each organ, that the human embryo follows the special
lines of differentiation and construction that are only found otherwise
in the Vertebrates. Within the limits of this vast stem we have
followed, step by step, the development both of the body as a whole and
of its various parts. This higher development follows in the human
embryo the form that is peculiar to the mammals. Finally, we saw that,
even within the limits of this class, the various phylogenetic stages
that we distinguish in a natural classification of the mammals
correspond to the ontogenetic stages that the human embryo passes
through in the course of its evolution. We were thus in a position to
determine precisely the position of man in this class, and so to
establish his relationship to the different orders of mammals.

The line of argument we followed in this explanation of the ontogenetic
facts was simply a consistent application of the biogenetic law. In
this we have throughout taken strict account of the distinction between
palingenetic and cenogenetic phenomena. Palingenesis (or “synoptic
development”) alone enables us to draw conclusions from the observed
embryonic form to the stem-form preserved by heredity. Such inference
becomes more or less precarious when there has been cenogenesis, or
disturbance of development, owing to fresh adaptations. We cannot
understand embryonic development unless we appreciate this very
important distinction. Here we stand at the very limit that separates
the older and the new science or philosophy of nature. The whole of the
results of recent morphological research compel us irresistibly
to recognise the biogenetic law and its far-reaching consequences.
These are, it is true, irreconcilable with the legends and doctrines of
former days, that have been impressed on us by religious education. But
without the _biogenetic law,_ without the distinction between
_palingenesis_ and _cenogenesis,_ and without the theory of _evolution_
on which we base it, it is quite impossible to understand the facts of
organic development; without them we cannot cast the faintest gleam of
explanation over this marvellous field of phenomena. But when we
recognise the causal correlation of ontogeny and phylogeny expressed in
this law, the wonderful facts of embryology are susceptible of a very
simple explanation; they are found to be the necessary mechanical
effects of the evolution of the stem, determined by the laws of
heredity and adaptation. The correlative action of these laws under the
universal influence of the struggle for existence, or—as we may say in
a word, with Darwin—“natural selection,” is entirely adequate to
explain the whole process of embryology in the light of phylogeny. It
is the chief merit of Darwin that he explained by his theory of
selection the correlation of the laws of heredity and adaptation that
Lamarck had recognised, and pointed out the true way to reach a causal
interpretation of evolution.

The phenomenon that it is most imperative to recognise in this
connection is the inheritance of functional variations. Jean Lamarck
was the first to appreciate its fundamental importance in 1809, and we
may therefore justly give the name of Lamarckism to the theory of
descent he based on it. Hence the radical opponents of the latter have
very properly directed their attacks chiefly against the former. One of
the most distinguished and most narrow-minded of these opponents,
Wilhelm His, affirms very positively that “characteristics acquired in
the life of the individual are not inherited.”

The inheritance of acquired characters is denied, not only by thorough
opponents of evolution, but even by scientists who admit it and have
contributed a good deal to its establishment, especially Weismann,
Galton, Ray Lankester, etc. Since 1884 the chief opponent has been
August Weismann, who has rendered the greatest service in the
development of Darwin’s theory of selection. In his work on _The
Continuity of the Germ-plasm,_ and in his recent excellent _Lectures on
the Theory of Descent_ (1902), he has with great success advanced the
opinion that “only those characters can be transmitted to subsequent
generations that were contained in rudimentary form in the embryo.”
However, this germ-plasm theory, with its attempt to explain heredity,
is merely a “provisional molecular hypothesis”; it is one of those
metaphysical speculations that attribute the evolutionary phenomena
exclusively to internal causes, and regard the influence of the
environment as insignificant. Herbert Spencer, Theodor Eimer, Lester
Ward, Hering, and Zehnder have pointed out the untenable consequences
of this position. I have given my view of it in the tenth edition of
the _History of Creation_ (pp. 192, 203). I hold, with Lamarck and
Darwin, that the hereditary transmission of acquired characters is one
of the most important phenomena in biology, and is proved by thousands
of morphological and physiological experiences. It is an indispensable
foundation of the theory of evolution.

Of the many and weighty arguments for the truth of this conception of
evolution I will for the moment merely point to the invaluable evidence
of dysteleology, the science of rudimentary organs. We cannot insist
too often or too strongly on the great morphological significance of
these remarkable organs, which are completely useless from the
physiological point of view. We find some of these useless parts,
inherited from our lower vertebrate ancestors, in every system of
organs in man and the higher Vertebrates. Thus we find at once on the
skin a scanty and rudimentary coat of hair, only fully developed on the
head, under the shoulders, and at a few other parts of the body. The
short hairs on the greater part of the body are quite useless and
devoid of physiological value; they are the last relic of the thicker
hairy coat of our simian ancestors. The sensory apparatus presents a
series of most remarkable rudimentary organs. We have seen that the
whole of the shell of the external ear, with its cartilages, muscles,
and skin, is in man a useless appendage, and has not the physiological
importance that was formerly ascribed to it. It is the degenerate
remainder of the pointed, freely moving, and more advanced mammal ear,
the muscles of which we still have, but cannot work them. We found at
the
inner corner of our eye a small, curious, semi-lunar fold that is of no
use whatever to us, and is only interesting as the last relic of the
nictitating membrane, the third, inner eye-lid that had a distinct
physiological purpose in the ancient sharks, and still has in many of
the Amniotes.

The motor apparatus, in both the skeleton and muscular systems,
provides a number of interesting dysteleological arguments. I need only
recall the projecting tail of the human embryo, with its rudimentary
caudal vertebræ and muscles; this is totally useless in man, but very
interesting as the degenerate relic of the long tail of our simian
ancestors. From these we have also inherited various bony processes and
muscles, which were very useful to them in climbing trees, but are
useless to us. At various points of the skin we have cutaneous muscles
which we never use—remnants of a strongly-developed cutaneous muscle in
our lower mammal ancestors. This “panniculus carnosus” had the function
of contracting and creasing the skin to chase away the flies, as we see
every day in the horse. Another relic in us of this large cutaneous
muscle is the frontal muscle, by which we knit our forehead and raise
our eye-brows; but there is another considerable relic of it, the large
cutaneous muscle in the neck (_platysma myoides_), over which we have
no voluntary control.

Not only in the systems of animal organs, but also in the vegetal
apparatus, we find a number of rudimentary organs, many of which we
have already noticed. In the alimentary apparatus there are the
thymus-gland and the thyroid gland, the seat of goitre and the relic of
a ciliated groove that the Tunicates and Acrania still have in the
gill-pannier; there is also the vermiform appendix to the cæcum. In the
vascular system we have a number of useless cords which represent
relics of atrophied vessels that were once active as blood-canals—the
_ductus Botalli_ between the pulmonary artery and the aorta, the
_ductus venosus Arantii_ between the portal vein and the vena cava, and
many others. The many rudimentary organs in the urinary and sexual
apparatus are particularly interesting. These are generally developed
in one sex and rudimentary in the other. Thus the spermaducts are
formed from the Wolffian ducts in the male, whereas in the female we
have merely rudimentary traces of them in Gaertner’s canals. On the
other hand, in the female the oviducts and womb are developed from the
Mullerian ducts, while in the male only the lowest ends of them remain
as the “male womb” (_vesicula prostatica_). Again, the male has in his
nipples and mammary glands the rudiments of organs that are usually
active only in the female.

A careful anatomic study of the human frame would disclose to us
numbers of other rudimentary organs, and these can only be explained on
the theory of evolution. Robert Wiedersheim has collected a large
number of them in his work on _The Human Frame as a Witness to its
Past._ They are some of the weightiest proofs of the truth of the
mechanical conception and the strongest disproofs of the teleological
view. If, as the latter demands, man or any other organism had been
designed and fitted for his life-purposes from the start and brought
into being by a creative act, the existence of these rudimentary organs
would be an insoluble enigma; it would be impossible to understand why
the Creator had put this useless burden on his creatures to walk a path
that is in itself by no means easy. But the theory of evolution gives
the simplest possible explanation of them. It says: The rudimentary
organs are parts of the body that have fallen into disuse in the course
of centuries; they had definite functions in our animal ancestors, but
have lost their physiological significance. On account of fresh
adaptations they have become superfluous, but are transmitted from
generation to generation by heredity, and gradually atrophy.

We have inherited not only these rudimentary parts, but all the organs
of our body, from the mammals—proximately from the apes. The human body
does not contain a single organ that has not been inherited from the
apes. In fact, with the aid of our biogenetic law we can trace the
origin of our various systems of organs much further, down to the
lowest stages of our ancestry. We can say, for instance, that we have
inherited the oldest organs of the body, the external skin and the
internal coat of the alimentary system, from the Gastræads; the nervous
and muscular systems from the Platodes; the vascular system, the
body-cavity, and the blood from the Vermalia; the chorda and the
branchial gut from the Prochordonia;
the articulation of the body from the Acrania; the primitive skull and
the higher sense-organs from the Cyclostomes; the limbs and jaws from
the Selachii; the five-toed foot from the Amphibia; the palate from the
Reptiles; the hairy coat, the mammary glands, and the external sexual
organs from the Pro-mammals. When we formulated “the law of the
ontogenetic connection of systematically related forms,” and determined
the relative age of organs, we saw how it was possible to draw
phylogenetic conclusions from the ontogenetic succession of systems of
organs.

With the aid of this important law and of comparative anatomy we were
also enabled to determine “man’s place in nature,” or, as we put it,
assign to man his position in the classification of the animal kingdom.
In recent zoological classification the animal world is divided into
twelve stems or phyla, and these are broadly sub-divided into about
sixty classes, and these classes into at least 300 orders. In his whole
organisation man is most certainly, in the first place, a member of one
of these stems, the vertebrate stem; secondly, a member of one
particular class in this stem, the Mammals; and thirdly, of one
particular order, the order of Primates. He has all the characteristics
that distinguish the Vertebrates from the other eleven animal stems,
the Mammals from the other sixty classes, and the Primates from the 300
other orders of the animal kingdom. We may turn and twist as we like,
but we cannot get over this fact of anatomy and classification. Of late
years this fact has given rise to a good deal of discussion, and
especially of controversy as to the particular anatomic relationship of
man to the apes. The most curious opinions have been advanced on this
“ape-question,” or “pithecoid-theory.” It is as well, therefore, to go
into it once more and distinguish the essential from the unessential.
(Cf. pp. 261–5.)

We start from the undisputed fact that man is in any case—whether we
accept or reject his special blood-relationship to the apes—a true
mammal; in fact, a placental mammal. This fundamental fact can be
proved so easily at any moment from comparative anatomy that it has
been universally admitted since the separation of the Placentals from
the lower mammals (Marsupials and Monotremes). But for every consistent
subscriber to the theory of evolution it must follow at once that man
descends from a common stem-form with all the other Placentals, the
stem-ancestor of the Placentals, just as we must admit a common
mesozoic ancestor of all the mammals. This is, however, to settle
decisively the great and burning question of man’s place in nature,
whether or no we go on to admit a nearer or more distant relationship
to the apes. Whether man is or is not a member of the ape-order (or, if
you prefer, the primate-order.) in the phylogenetic sense, in any case
his direct blood-relationship to the rest of the mammals, and
especially the Placentals, is established. It is possible that the
affinities of the various orders of mammals to each other are different
from what we hypothetically assume to-day. But, in any case, the common
descent of man and all the other mammals from one stem-form is beyond
question. This long-extinct Promammal was probably evolved from
Proreptiles during the Triassic period, and must certainly be regarded
as the monotreme and oviparous ancestor of _all_ the mammals.

If we hold firmly to this fundamental and most important thesis, we
shall see the “ape-question” in a very different light from that in
which it is usually regarded. Little reflection is then needed to see
that it is not nearly so important as it is said to be. The origin of
the human race from a series of mammal ancestors, and the historic
evolution of these from an earlier series of lower vertebrate
ancestors, together with all the weighty conclusions that every
thoughtful man deduces therefrom, remain untouched; so far as these are
concerned, it is immaterial whether we regard true “apes” as our
nearest ancestors or not. But as it has become the fashion to lay the
chief stress in the whole question of man’s origin on the “descent from
the apes,” I am compelled to return to it once more, and recall the
facts of comparative anatomy and ontogeny that give a decisive answer
to this “ape-question.”

The shortest way to attain our purpose is that followed by Huxley in
1863 in his able work, which I have already often quoted, _Man’s Place
in Nature_—the way of comparative anatomy and ontogeny. We have to
compare impartially all man’s organs with the same organs in the higher
apes, and then to examine if the differences between the two are
greater
than the corresponding differences between the higher and the lower
apes. The indubitable and incontestable result of this
comparative-anatomical study, conducted with the greatest care and
impartiality, was the pithecometra-principle, which we have called the
Huxleian law in honour of its formulator—namely, that the differences
in organisation between man and the most advanced apes we know are much
slighter than the corresponding differences in organisation between the
higher and lower apes. We may even give a more precise formula to this
law, by excluding the Platyrrhines or American apes as distant
relatives, and restricting the comparison to the narrower family-circle
of the Catarrhines, the apes of the Old World. Within the limits of
this small group of mammals we found the structural differences between
the lower and higher catarrhine apes—for instance, the baboon and the
gorilla—to be much greater than the differences between the anthropoid
apes and man. If we now turn to ontogeny, and find, according to our
“law of the ontogenetic connection of systematically related forms,”
that the embryos of the anthropoid apes and man retain their
resemblance for a longer time than the embryos of the highest and the
lowest apes, we are forced, whether we like it or no, to recognise our
descent from the order of apes. We can assuredly construct an
approximate picture in the imagination of the form of our early
Tertiary ancestors from the foregoing facts of comparative anatomy;
however we may frame this in detail, it will be the picture of a true
ape, and a distinct catarrhine ape. This has been shown so well by
Huxley (1863) that the recent attacks of Klaatsch, Virchow, and other
anthropologists, have completely failed (cf. pp.263–264). All the
structural characters that distinguish the Catarrhines from the
Platyrrhines are found in man. Hence in the genealogy of the mammals we
must derive man immediately from the catarrhine group, and locate the
origin of the human race in the Old World. Only the early root-form
from which both descended was common to them.

It is, therefore, established beyond question for all impartial
scientific inquiry that the human race comes directly from the apes of
the Old World; but, at the same time, I repeat that this is not so
important in connection with the main question of the origin of man as
is commonly supposed. Even if we entirely ignore it, all that we have
learned from the zoological facts of comparative anatomy and ontogeny
as to the placental character of man remains untouched. These prove
beyond all doubt the common descent of man and all the rest of the
mammals. Further, the main question is not in the least affected if it
is said: “It is true that man is a mammal; but he has diverged at the
very root of the class from all the other mammals, and has no closer
relationship to any living group of mammals.” The affinity is more or
less close in any case, if we examine the relation of the mammal class
to the sixty other classes of the animal world. Quite certainly the
whole of the mammals, including man, have had a common origin; and it
is equally certain that their common stem-forms were gradually evolved
from a long series of lower Vertebrates.

The resistance to the theory of a descent from the apes is clearly due
in most men to feeling rather than to reason. They shrink from the
notion of such an origin just because they see in the ape organism a
caricature of man, a distorted and unattractive image of themselves,
because it hurts man’s æsthetic complacency and self-ennoblement. It is
more flattering to think we have descended from some lofty and god-like
being; and so, from the earliest times, human vanity has been pleased
to believe in our origin from gods or demi-gods. The Church, with that
sophistic reversal of ideas of which it is a master, has succeeded in
representing this ridiculous piece of vanity as “Christian humility”;
and the very men who reject with horror the notion of an animal origin,
and count themselves “children of God,” love to prate of their “humble
sense of servitude.” In most of the sermons that have poured out from
pulpit and altar against the doctrine of evolution human vanity and
conceit have been a conspicuous element; and, although we have
inherited this very characteristic weakness from the apes, we must
admit that we have developed it to a higher degree, which is entirely
repudiated by sound and normal intelligence. We are greatly amused at
all the childish follies that the ridiculous pride of ancestry has
maintained from the Middle Ages to our own time; yet there is a large
amount of this empty feeling in
most men. Just as most people much prefer to trace their family back to
some degenerate baron or some famous prince rather than to an unknown
peasant, so most men would rather have as parent of the race a sinful
and fallen Adam than an advancing, and vigorous ape. It is a matter of
taste, and to that extent we cannot quarrel over these genealogical
tendencies. Personally, the notion of ascent is more congenial to me
than that of descent. It seems to me a finer thing to be the advanced
offspring of a simian ancestor, that has developed progressively from
the lower mammals in the struggle for life, than the degenerate
descendant of a god-like being, made from a clod, and fallen for his
sins, and an Eve created from one of his ribs. Speaking of the rib, I
may add to what I have said about the development of the skeleton, that
the number of ribs is just the same in man and woman. In both of them
the ribs are formed from the middle germinal layer, and are, from the
phylogenetic point of view, lower or ventral vertebral arches.

But it is said: “That is all very well, as far as the human body is
concerned; on the facts quoted it is impossible to doubt that it has
really and gradually been evolved from the long ancestral series of the
Vertebrates. But it is quite another thing as regards man’s mind, or
soul; this cannot possibly have been developed from the
vertebrate-soul.”[35] Let us see if we cannot meet this grave stricture
from the well-known facts of comparative anatomy, physiology, and
embryology. It will be best to begin with a comparative study of the
souls of various groups of Vertebrates. Here we find such an enormous
variety of vertebrate souls that, at first sight, it seems quite
impossible to trace them all to a common “Primitive Vertebrate.” Think
of the tiny Amphioxus, with no real brain but a simple medullary tube,
and its whole psychic life at the very lowest stage among the
Vertebrates. The following group of the Cyclostomes are still very
limited, though they have a brain. When we pass on to the fishes, we
find their intelligence remaining at a very low level. We do not see
any material advance in mental development until we go on to the
Amphibia and Reptiles. There is still greater advance when we come to
the Mammals, though even here the minds of the Monotremes and of the
stupid Marsupials remain at a low stage. But when we rise from these to
the Placentals we find within this one vast group such a number of
important stages of differentiation and progress that the psychic
differences between the least intelligent (such as the sloths and
armadillos) and the most intelligent Placentals (such as the dogs and
apes) are much greater than the psychic differences between the lowest
Placentals and the Marsupials or Monotremes. Most certainly the
differences are far greater than the differences in mental power
between the dog, the ape, and man. Yet all these animals are
genetically-related members of a single natural class.

 [35] The English reader will recognise here the curious position of
 Dr. Wallace and of the late Dr. Mivart.—Translator.


We see this to a still more astonishing extent in the comparative
psychology of another class of animals, that is especially interesting
for many reasons—the insect class. It is well known that we find in
many insects a degree of intelligence that is found in man alone among
the Vertebrates. Everybody knows of the famous communities and states
of bees and ants, and of the very remarkable social arrangements in
them, such as we find among the more advanced races of men, but among
no other group of animals. I need only mention the social organisation
and government of the monarchic bees and the republican ants, and their
division into different conditions—queen, drone-nobles, workers,
educators, soldiers, etc. One of the most remarkable phenomena in this
very interesting province is the cattle-keeping of the ants, which rear
plant-lice as milch-cows and regularly extract their honeyed juice.
Still more remarkable is the slave-holding of the large red ants, which
steal the young of the small black ants and bring them up as slaves. It
has long been known that these political and social arrangements of the
ants are due to the deliberate cooperation of the countless citizens,
and that they understand each other. A number of recent observers,
especially Fritz Müller, Sir J. Lubbock (Lord Avebury), and August
Forel, have put the astonishing degree of intelligence of these tiny
Articulates beyond question.

Now, compare with these the mental life of many of the lower,
especially the parasitic insects, as Darwin did. There is, for
instance, the cochineal insect
(_Coccus_), which, in its adult state, has a motionless, shield-shaped
body, attached to the leaves of plants. Its feet are atrophied. Its
snout is sunk in the tissue of the plants of which it absorbs the sap.
The whole psychic life of these inert female parasites consists in the
pleasure they experience from sucking the sap of the plant and in
sexual intercourse with the males. It is the same with the maggot-like
females of the fan-fly (_Strepsitera_), which spend their lives
parasitically and immovably, without wings or feet, in the abdomen of
wasps. There is no question here of higher psychic action. If we
compare these sluggish parasites with the intelligent and active ants,
we must admit that the psychic differences between them are much
greater than the psychic differences between the lowest and highest
mammals, between the Monotremes, Marsupials, and armadillos on the one
hand, and the dog, ape, or man on the other. Yet all these insects
belong to the same class of Articulates, just as all the mammals belong
to one and the same class. And just as every consistent evolutionist
must admit a common stem-form for all these insects, so he must also
for all the mammals.

If we now turn from the comparative study of psychic life in different
animals to the question of the organs of this function, we receive the
answer that in all the higher animals they are always bound up with
certain groups of cells, the ganglionic cells or neurona that compose
the nervous system. All scientists without exception are agreed that
the central nervous system is the organ of psychic life in the animal,
and it is possible to prove this experimentally at any moment. When we
partially or wholly destroy the central nervous system, we extinguish
in the same proportion, partially or wholly, the “soul” or psychic
activity of the animal. We have, therefore, to examine the features of
the psychic organ in man. The reader already knows the incontestable
answer to this question. Man’s psychic organ is, in structure and
origin, just the same organ as in all the other Vertebrates. It
originates in the shape of a simple medullary tube from the outer
membrane of the embryo—the skin-sense layer. The simple cerebral
vesicle that is formed by the expansion of the head-part of this
medullary tube divides by transverse constrictions into five, and these
pass through more or less the same stages of construction in the human
embryo as in the rest of the mammals. As these are undoubtedly of a
common origin, their brain and spinal cord must also have a common
origin.

Physiology teaches us further, on the ground of observation and
experiment, that the relation of the “soul” to its organ, the brain and
spinal cord, is just the same in man as in the other mammals. The one
cannot act at all without the other; it is just as much bound up with
it as muscular movement is with the muscles. It can only develop in
connection with it. If we are evolutionists at all, and grant the
causal connection of ontogenesis and phylogenesis, we are forced to
admit this thesis: The human soul or psyche, as a function of the
medullary tube, has developed along with it; and just as brain and
spinal cord now develop from the simple medullary tube in every human
individual, so the human mind or the psychic life of the whole human
race has been gradually evolved from the lower vertebrate soul. Just as
to-day the intricate structure of the brain proceeds step by step from
the same rudiment in every human individual—the same five cerebral
vesicles—as in all the other Craniotes; so the human soul has been
gradually developed in the course of millions of years from a long
series of craniote-souls. Finally, just as to-day in every human embryo
the various parts of the brain differentiate after the special type of
the ape-brain, so the human psyche has proceeded historically from the
ape-soul.

It is true that this Monistic conception is rejected with horror by
most men, and the Dualistic idea, which denies the inseparable
connection of brain and mind, and regards body and soul as two totally
different things, is still popular. But how can we reconcile this view
with the known facts of evolution? It meets with difficulties equally
great and insuperable in embryology and in phylogeny. If we suppose
with the majority of men that the soul is an independent entity, which
has nothing to do with the body originally, but merely inhabits it for
a time, and gives expression to its experiences through the brain just
as the pianist does through his instrument, we must assign a point in
human embryology at which the soul enters into the brain; and at death
again we must assign a moment at which it abandons the body. As,
further, each human individual has inherited certain
personal features from each parent, we must suppose that in the act of
conception pieces were detached from their souls and transferred to the
embryo. A piece of the paternal soul goes with-the spermatozoon, and a
piece of the mother’s soul remains in the ovum. At the moment of
conception, when portions of the two nuclei of the copulating cells
join together to form the nucleus of the stem-cell, the accompanying
fragments of the immaterial souls must also be supposed to coalesce.

On this Dualistic view the phenomena of psychic development are totally
incomprehensible. Everybody knows that the new-born child has no
consciousness, no knowledge of itself and the surrounding world. Every
parent who has impartially followed the mental development of his
children will find it impossible to deny that it is a case of
biological evolutionary processes. Just as all other functions of the
body develop in connection with their organs, so the soul does in
connection with the brain. This gradual unfolding of the soul of the
child is, in fact, so wonderful and glorious a phenomenon that every
mother or father who has eyes to observe is never tired of
contemplating it. It is only our manuals of psychology that know
nothing of this development; we are almost tempted to think sometimes
that their authors can never have had children themselves. The human
soul, as described in most of our psychological works, is merely the
soul of a learned philosopher, who has read a good many books, but
knows nothing of evolution, and never even reflects that his own soul
has had a development.

When these Dualistic philosophers are consistent they must assign a
moment in the phylogeny of the human soul at which it was first
“introduced” into man’s vertebrate body. Hence, at the time when the
human body was evolved from the anthropoid body of the ape (probably in
the Tertiary period), a specific human psychic element—or, as people
love to say, “a spark of divinity”—must have been suddenly infused or
breathed into the anthropoid brain, and been associated with the
ape-soul already present in it. I need not insist on the enormous
theoretical difficulties of this idea. I will only point out that this
“spark of divinity,” which is supposed to distinguish the soul of man
from that of the other animals, must be itself capable of development,
and has, as a matter of fact, progressively developed in the course of
human history. As a rule, reason is taken to be this “spark of
divinity,” and is supposed to be an exclusive possession of humanity.
But comparative psychology shows us that it is quite impossible to set
up this barrier between man and the brute. Either we take the word
“reason” in the wider sense, and then it is found in the higher mammals
(ape, dog, elephant, horse) just as well as in most men; or else in the
narrower sense, and then it is lacking in most men just as much as in
the majority of animals. On the whole, we may still say of man’s reason
what Goethe’s Mephistopheles said:—

Life somewhat better might content him
But for the gleam of heavenly light that Thou hast given him.
He calls it reason; thence his power’s increased
To be still beastlier than any beast.


If, then, we must reject these popular and, in some respects, agreeable
Dualistic theories as untenable, because inconsistent with the genetic
facts, there remains only the opposite or Monistic conception,
according to which the human soul is, like any other animal soul, a
function of the central nervous system, and develops in inseparable
connection therewith. We see this _ontogenetically_ in every child. The
biogenetic law compels us to affirm it _phylogenetically._ Just as in
every human embryo the skin-sense layer gives rise to the medullary
tube, from the anterior end of which the five cerebral vesicles of the
Craniotes are developed, and from these the mammal brain (first with
the characters of the lower, then with those of the higher mammals);
and as the whole of this ontogenetic process is only a brief,
hereditary reproduction of the same process in the phylogenesis of the
Vertebrates; so the wonderful spiritual life of the human race through
many thousands of years has been evolved step by step from the lowly
psychic life of the lower Vertebrates, and the development of every
child-soul is only a brief repetition of that long and complex
phylogenetic process. From all these facts sound reason must conclude
that the still prevalent belief in the immortality of the soul is an
untenable superstition. I have shown its inconsistency with modern
science in the eleventh chapter of _The Riddle of the Universe._

Here it may also be well to point out
the great importance of anthropogeny, in the light of the biogenetic
law, for the purposes of philosophy. The speculative philosophers who
take cognizance of these ontogenetic facts, and explain them (in
accordance with the law) phylogenetically, will advance the great
questions of philosophy far more than the most distinguished thinkers
of all ages have yet succeeded in doing. Most certainly every clear and
consistent thinker must derive from the facts of comparative anatomy
and ontogeny we have adduced a number of suggestive ideas that cannot
fail to have an influence on the progress of philosophy. Nor can it be
doubted that the candid statement and impartial appreciation of these
facts will lead to the decisive triumph of the philosophic tendency
that we call “Monistic” or “Mechanical,” as opposed to the “Dualistic”
or “Teleological,” on which most of the ancient, medieval, and modern
systems of philosophy are based. The Monistic or Mechanical philosophy
affirms that all the phenomena of human life and of the rest of nature
are ruled by fixed and unalterable laws; that there is everywhere a
necessary causal connection of phenomena; and that, therefore, the
whole knowable universe is a harmonious unity, a _monon._ It says,
further, that all phenomena are due solely to mechanical or efficient
causes, not to final causes. It does not admit free-will in the
ordinary sense of the word. In the light of the Monistic philosophy the
phenomena that we are wont to regard as the freest and most
independent, the expressions of the human will, are subject just as
much to rigid laws as any other natural phenomenon. As a matter of
fact, impartial and thorough examination of our “free” volitions shows
that they are never really free, but always determined by antecedent
factors that can be traced to either heredity or adaptation. We cannot,
therefore, admit the conventional distinction between nature and
spirit. There is spirit everywhere in nature, and we know of no spirit
outside of nature. Hence, also, the common antithesis of natural
science and mental or moral science is untenable. Every science, as
such, is both natural and mental. That is a firm principle of Monism,
which, on its religious side, we may also denominate Pantheism. Man is
not above, but in, nature.

It is true that the opponents of evolution love to misrepresent the
Monistic philosophy based on it as “Materialism,” and confuse the
philosophic tendency of this name with a wholly unconnected and
despicable moral materialism. Strictly speaking, it would be just as
proper to call our system Spiritualism as Materialism. The real
Materialistic philosophy affirms that the phenomena of life are, like
all other phenomena, effects or products of matter. The opposite
extreme, the Spiritualistic philosophy, says, on the contrary, that
matter is a product of energy, and that all material forms are produced
by free and independent forces. Thus, according to one-sided
Materialism, the matter is antecedent to the living force; according to
the equally one-sided view of the Spiritist, it is the reverse. Both
views are Dualistic, and, in my opinion, both are false. For us the
antithesis disappears in the Monistic philosophy, which knows neither
matter without force nor force without matter. It is only necessary to
reflect for some time over the question from the strictly scientific
point of view to see that it is impossible to form a clear idea of
either hypothesis. As Goethe said, “Matter can never exist or act
without spirit, nor spirit without matter.”

The human “spirit” or “soul” is merely a force or form of energy,
inseparably bound up with the material sub-stratum of the body. The
thinking force of the mind is just as much connected with the
structural elements of the brain as the motor force of the muscles with
their structural elements. Our mental powers are functions of the brain
as much as any other force is a function of a material body. We know of
no matter that is devoid of force, and no forces that are not bound up
with matter. When the forces enter into the phenomenon as movements we
call them living or active forces; when they are in a state of rest or
equilibrium we call them latent or potential. This applies equally to
inorganic and organic bodies. The magnet that attracts iron filings,
the powder that explodes, the steam that drives the locomotive, are
living inorganics; they act by living force as much as the sensitive
Mimosa does when it contracts its leaves at touch, or the venerable
Amphioxus that buries itself in the sand of the sea, or man when he
thinks. Only in the latter cases the combinations of the different
forces that appear as “movement” in the
phenomenon are much more intricate and difficult to analyse than in the
former.

Our study has led us to the conclusion that in the whole evolution of
man, in his embryology and in his phylogeny, there are no living forces
at work other than those of the rest of organic and inorganic nature.
All the forces that are operative in it could be reduced in the
ultimate analysis to growth, the fundamental evolutionary function that
brings about the forms of both the organic and the inorganic. But
growth itself depends on the attraction and repulsion of homogeneous
and heterogeneous particles. Seventy-five years ago Carl Ernst von Baer
summed up the general result of his classic studies of animal
development in the sentence: “The evolution of the individual is the
history of the growth of individuality in every respect.” And if we go
deeper to the root of this law of growth, we find that in the long run
it can always be reduced to that attraction and repulsion of animated
atoms which Empedocles called the “love and hatred” of the elements.

Thus the evolution of man is directed by the same “eternal, iron laws”
as the development of any other body. These laws always lead us back to
the same simple principles, the elementary principles of physics and
chemistry. The various phenomena of nature only differ in the degree of
complexity in which the different forces work together. Each single
process of adaptation and heredity in the stem-history of our ancestors
is in itself a very complex physiological phenomenon. Far more
intricate are the processes of human embryology; in these are condensed
and comprised thousands of the phylogenetic processes.

In my _General Morphology,_ which appeared in 1866, I made the first
attempt to apply the theory of evolution, as reformed by Darwin, to the
whole province of biology, and especially to provide with its
assistance a mechanical foundation for the science of organic forms.
The intimate relations that exist between all parts of organic science,
especially the direct causal nexus between the two sections of
evolution—ontogeny and phylogeny—were explained in that work for the
first time by transformism, and were interpreted philosophically in the
light of the theory of descent. The anthropological part of the
_General Morphology_ (Book vii) contains the first attempt to determine
the series of man’s ancestors (vol. ii, p. 428). However imperfect this
attempt was, it provided a starting-point for further investigation. In
the thirty-seven years that have since elapsed the biological horizon
has been enormously widened; our empirical acquisitions in
paleontology, comparative anatomy, and ontogeny have grown to an
astonishing extent, thanks to the united efforts of a number of able
workers and the employment of better methods. Many important biological
questions that then appeared to be obscure enigmas seem to be entirely
settled. Darwinism arose like the dawn of a new day of clear Monistic
science after the dark night of mystic dogmatism, and we can say now,
proudly and gladly, that there is daylight in our field of inquiry.

Philosophers and others, who are equally ignorant of the empirical
sources of our evidence and the phylogenetic methods of utilising it,
have even lately claimed that in the matter of constructing our
genealogical tree nothing more has been done than the discovery of a
“gallery of ancestors,” such as we find in the mansions of the
nobility. This would be quite true if the genealogy given in the second
part of this work were merely the juxtaposition of a series of animal
forms, of which we gathered the genetic connection from their external
physiognomic resemblances. As we have sufficiently proved already, it
is for us a question of a totally different thing—of the morphological
and historical proof of the phylogenetic connection of these ancestors
on the basis of their identity in internal structure and embryonic
development; and I think I have sufficiently shown in the first part of
this work how far this is calculated to reveal to us their inner nature
and its historical development. I see the essence of its significance
precisely in the proof of historical connection. I am one of those
scientists who believe in a real “natural history,” and who think as
much of an historical knowledge of the past as of an exact
investigation of the present. The incalculable value of the historical
consciousness cannot be sufficiently emphasised at a time when
historical research is ignored and neglected, and when an “exact”
school, as dogmatic as it is narrow, would substitute for it physical
experiments and mathematical formulæ. Historical knowledge cannot be
replaced by any other branch of science.


It is clear that the prejudices that stand in the way of a general
recognition of this “natural anthropogeny” are still very great;
otherwise the long struggle of philosophic systems would have ended in
favour of Monism. But we may confidently expect that a more general
acquaintance with the genetic facts will gradually destroy these
prejudices, and lead to the triumph of the natural conception of “man’s
place in nature.” When we hear it said, in face of this expectation,
that this would lead to retrogression in the intellectual and moral
development of mankind, I cannot refrain from saying that, in my
opinion, it will be just the reverse; that it will promote to an
enormous extent the advance of the human mind. All progress in our
knowledge of truth means an advance in the higher cultivation of the
human intelligence; and all progress in its application to practical
life implies a corresponding improvement of morality. The worst enemies
of the human race—ignorance and superstition—can only be vanquished by
truth and reason. In any case, I hope and desire to have convinced the
reader of these chapters that the true scientific comprehension of the
human frame can only be attained in the way that we recognise to be the
sole sound and effective one in organic science generally—namely, the
way of Evolution.



INDEX


A

Abiogenesis, 26

_Accipenser_, 234

Abortive ova, 55

Achromatin, 42

Achromin, 42

Acœla, 221

Acoustic nerve, the, 289, 290

Acquired characters, inheritance of, 349

Acrania, the, 182

Acroganglion, the, 268, 275

Adam’s apple, the, 184

Adapida, 257

Adaptation, 3, 5, 27

After-birth, the, 167

Agassiz, L., 34

Age of life, 200

Alimentary canal, evolution of the, 13, 14, 133, 308–17
— — structure of the, 169, 308–10

Allantoic circulation, the, 171

Allantois, development of the, 166

Allmann, 20

_Amblystoma,_ 243

Amitotic cleavage, 40

Ammoconida, 217

_Ammolynthus,_ 217

Amnion, the, 115
— formation of the, 134, 244

Amniotic fluid, the, 134

Amœba, the, 47–9, 210

Amphibia, the, 239

_Amphichœrus,_ 221

Amphigastrula, 80

Amphioxus, the, 105, 181–95
— circulation of the, 184
— cœlomation of the, 95
— embryology of the, 191–95
— structure of the, 183–88

Amphirhina, 230

Anamnia, the, 115

Anatomy, comparative, 208

Animalculists, 12

Animal layer, the, 16

Annelids, the, 142, 219

Annelid theory, the, 142

Anomodontia, 246

Ant, intelligence of the, 353

_Anthropithecus,_ 174, 262

Anthropogeny, 1

Anthropoid apes, the, 166, 173, 262

Anthropology, 1, 35

Anthropozoic period, 203

Antimera, 107

Anura, 243

Anus, the, 317

Anus, formation of the, 139

Aorta, the, 327
— development of the, 170

Ape and man, 157, 164, 261, 307, 351

Ape-man, the, 263

Apes, the, 257–60

_Aphanocapsa,_ 210

_Aphanostomum,_ 221

Appendicaria, 197

Appendix vermiformis, the, 32

Aquatic life, early prevalence of, 235

Ararat, Mount, 24

Archenteron, 64, 74

Archeolithic age, 203

Archicaryon, 55

Archicrania, 230

Archigastrula, 65, 193

_Archiprimas,_ 263

Arctopitheca, 261

Area, the germinative, 121

Aristotle, 9

Arm, structure of the, 306

Arrow-worm, the, 191

Arterial arches, the, 325–26
— cone, the, 324

Arteries, evolution of the, 170, 323–24

Articulates, the, 142, 219
— skeleton of the, 294

Articulation, 141–42

Aryo-Romanic languages, the, 203

Ascidia, the, 181, 188–90
— embryology of the, 196–98

Ascula, 217

Asexual reproduction, 51

Atlas, the, 247

Atrium, the, 183, 185
— (heart), the, 326

Auditory nerve, the, 289, 290

Auricles of the heart, 325

_Autolemures,_ 257

Axolotl, the, 243

B

Bacteria, 38, 210

Baer, K. E. von, 15–17

Balanoglossus, 226

Balfour, F., 21

Batrachia, 241

_Bdellostoma Stouti,_ 78

Bee, generation of the, 9

Beyschlag, W., on evolution, 50

Bilateral symmetry, 66
— — origin of, 221

Bimana, 258

Biogenetic law, the, 2, 21, 23, 179, 349

Biogeny, 2

Bionomy, 33

Bird, evolution of the, 245
— ovum of the, 44–6, 80–1

Bischoff, W., 17

Bladder, evolution of the, 244, 339

Blastæa, the, 206, 213

Blastocœl, the, 62, 74

Blastocrene, the, 99

Blastocystis, the, 62, 119, 120

Blastoderm, the, 62

Blastodermic vesicle, the, 119

Blastoporus, the, 64

Blastosphere, the, 62, 119

Blastula, the, 62, 74
— the mammal, 119

Blood, importance of the, 318
— recent experiments in mixture of, 172
— structure of the, 319

Blood-cells, the, 319

Blood-vessels, the, 318–25
— development of the, 168
— of the vertebrate, 110
— origin of the, 320–21

Boniface VIII, Bull of, 10

Bonnet, 13

Borneo nosed-ape, the, 164

Boveri, Theodor, 185

Brachytarsi, 257

Brain and mind, 278, 354–56
— evolution of the, 8, 275–80
— in the fish, 276
— in the lower animals, 275
— structure of the, 273–74

Branchial arches, evolution of the, 303
— cavity, the, 183, 189
— system, the, 110

Branchiotomes, 149

Breasts, the, 113

Bulbilla, 184

C

_Calamichthys,_ 234

_Calcolynthus,_ 217

Capillaries, the, 323

Caracoideum, the, 249

Carboniferous strata, 202

_Carcharodon,_ 234

Cardiac cavity, the, 170

Cardiocœl, the, 328

Caryobasis, 38, 54

Caryokinesis, 42

Caryolymph, 38, 54

Caryolyses, 42

Caryon, 37

Caryoplasm, 37

Catallacta, 213

Catarrhinæ, the, 173, 261

Catastrophic theory, the, 24

Caudate cells, 53

Cell, life of the, 41–3
— nature of the, 36–7
— size of the, 38

Cell theory, the, 18, 36

Cenogenesis, 4

Cenogenetic structures, 4

Cenozoic period, the, 203

Central body, the, 38, 42

Central nervous system, the, 273

Centrolecithal ova, 68

Centrosoma, the, 38, 42

Ceratodus, the, 76, 237

Cerebellum, the, 274

Cerebral vesicles, evolution of the, 276

Cerebrum, the, 273

_Cestracion Japonicus,_ 75, 79

Chætognatha, 94

Chick, importance of the, in embryology, 11, 16

Child, mind of the, 8, 355

Chimpanzee, the, 174, 262

_Chiromys,_ 257

Chiroptera, 258

_Chirotherium,_ 239

Chondylarthra, 257

Chorda, the, 17, 95, 107, 183
— evolution of the, 296

_Chordæa,_ the, 97

Chordalemma, the, 296

Chordaria, 97

Chordula, the, 3, 96, 191

Choriata, the, 166

Chorion, the, 119
— development of the, 165–6
— frondosum, 255
— læve, 255

Choroid coat, the, 286

Chorology, 33

Chromacea, 209

Chromatin, 42

Chroococcacea, 210

_Chroococcus,_ the, 210

Church, opposition of, to science in Middle Ages, 10

Chyle, 318

Chyle-vessels, 324

Cicatricula, the, 45, 81

Ciliated cells, 53, 193

Cinghalese gynecomast, 114

Circulation in the lancelet, 184

Circulatory system, evolution of the, 321–25
— — structure of the, 318

Classification, 103
— evolutionary value of, 33

Clitoris, the, 345

Cloaca, the, 249, 317

Cnidaria, 217

Coccyx, the, 295

Cochineal insect, the, 354

Cochlea, the, 289

Cœcilia, 241

Cœcum, the, 310, 317

Cœlenterata, 20, 91, 93, 104

Cœlenteria, 221

Cœloma, the, 21, 64, 91

Cœlomæa, the, 98

Cœlomaria, 21, 91, 104, 221

Cœlomation, 93–4

Cœlom-theory, the, 21, 93

Cœlomula, the, 98

Colon, the, 310, 317

Comparative anatomy, 31

Conception, nature of, 51

Conjunctiva, the, 286

_Conocyema,_ 215

_Convoluta,_ 221

Copelata, the, 197

Copulative organs, evolution of the, 344–45

Corium, the, 108, 268

Cornea, the, 286

Corpora cavernosa, the, 345, 346

Corpora quadrigemina, 274

Corpora striata, 274

Corpus callosum, the, 274

Corpus vitreum, the, 285

Corpuscles of the blood, 319

Craniology, 303

Craniota, the, 182, 229

Cranium, the, 299

Creation, 23–4

Cretaceous strata, 202

Crossopterygii, 234

Crustacea, the, 142, 219

Cryptocœla, 221

Cryptorchism, 114

Crystalline lens, the, 285
— — development of the, 287

Cutaneous glands, 268

Cuttlefish, embryology of the, 9

Cuvier, G., 17, 24

Cyanophycea, 209

Cyclostoma, the, 188, 230–32
— ova of the, 75

Cyemaria, 214

Cynopitheca, 262

_Cynthia,_ 191, 196

Cytoblastus, the, 37

Cytodes, 40

Cytoplasm, 37, 38

Cytosoma, 37

Cytula, the, 54

D

Dalton, 15

Darwin, C., 2, 5, 23, 28–9

Darwin, E., 28

Darwinism, 5, 28

Decidua, the, 167

Deciduata, 255

Deduction, nature of, 208

Degeneration theory, the, 219

Dentition of the ape and man, 259

Depula, 62

_Descent of Man,_ 30

Design in organisms, 33

Deutoplasm, 44

Devonian strata, 202

Diaphragm, the, 309
— evolution of the, 328

_Dicyema,_ 215

Dicyemida, 215

Didelphia, 248

Digonopora, 223

Dinosauria, 202

Dipneumones, 238

Dipneusta, 235–38
— ova of the, 75

Dipnoa, 236

Directive bodies, 54

Discoblastic ova, 68

Discoplacenta, 255

_Dissatyrus,_ 174

Dissection, medieval decrees against, 10

Dohrn, Anton, 219

Döllinger, 15

Dorsal furrow, the, 125
— shield, the, 123
— zone, the, 129

_Dromatherium,_ 248

Dualism, 6

Dubois, Eugen, 263

_Ductus Botalli,_ the, 350

_Ductus venosus Arantii,_ 350

Duodenum, the, 309, 317

Duration of embryonic development, 199
— of man’s history, 199

Dysteleology, 32
— proofs of, 349

E

Ear, evolution of the, 288–92
— structure of the, 288
— uselessness of the external, 32

Ear-bones, the, 289

Earth, age of the, 200–201

_Echidna hystrix,_ 249

Ectoblast, 20, 64

Ectoderm, the, 20, 64

Edentata, 250

Efficient causes, 6

Egg of the bird, 44–6, 81
— or the chick, priority of the, 211

Elasmobranchs, the, 79

Embryo, human, development of the, 158

Embryology, 2
— evolutionary value of, 34

Embryonic development, duration of, 199
— disk, the, 121–22
— spot, the, 125

Encephalon, the, 273

Endoblast, 20, 64

Endothelia, 321

Enterocœla, 93, 223

Enteropneusta, 226

Entoderm, the, 20, 64

Eocene strata, 203

Eopitheca, 259

Epiblast, 20, 64

Epidermis, the, 108, 268

Epididymis, the, 342

Epigastrula, 80

Epigenesis, 11, 13

Epiglottis, the, 309

Epiphysis, the, 108

Episoma, 129

Episomites, 130, 194

Epispadia, 346

Epithelia, 37

Epitheria, 243, 253

Epovarium, the, 342

Equilibrium, sense of, 291

Esthonychida, 257

Eustachian tube, the, 289

Eutheria, 253

Eve, 12

Evolution theory, the, 11, 208
— inductive nature of, 30

Eye, evolution of the, 285–88
— structure of the, 285

Eyelid, the third, 32

Eyelids, evolution of the, 288

F

Fabricius ab Aquapendente, 10

Face, embryonic development of the, 284

Fat glands in the skin, 269

Feathers, evolution of, 270

Fertilisation, 51
— place of, 119

Fin, evolution of the, 239, 304

Final causes, 6

Flagellate cells, 193

Floating bladder, the, 233, 241
— — evolution of the, 314

Fœtal circulation, 170–71

Food-yelk, the, 67,

Foot, evolution of the, 241, 304–6
— of the ape and man, 258–59

Fore brain, the, 278

Fore kidneys, the, 336, 337

Fossiliferous strata, list of, 201

Fossils, 180
— scarcity of, 208

Free will, 356

Friedenthal, experiments of, 172

Frog, the, 241–42
— ova of the, 71–2

Frontonia, 224

Function and structure, 7

Furcation of ova, 72

G

Gaertner’s duct, 341, 350

Ganglia, commencement of, 268

Ganglionic cell, the, 39

Ganoids, 233, 234

Gastræa, the, 3, 20, 206
— formation of the, 213

Gastræa theory, the, 20, 64, 69

Gastræads, 69, 214

Gastremaria, 214

Gastrocystis, the, 62, 119, 120

_Gastrophysema,_ 215

Gastrotricha, 224

Gastrula, the, 3, 20, 62

Gastrulation, 62

Gegenbaur, Carl, 220
— on evolution, 32
— on the skull, 300–1

Gemmation, 331

_General Morphology,_ 8, 29

_Genesis,_ 23

Genital pore, the, 335

Geological evolution, length of, 200
— periods, 201

Geology, methods of, 180
— rise of, 24

Germ-plasm, theory of, 349

Germinal disk, 46, 81
— layers, the, 14, 16
— — scheme of the, 92
— spot, the, 44
— vesicle, the, 43, 54

Germinative area, the, 121

Giant gorilla, the, 176

Gibbon, the, 173, 262

Gill-clefts and arches, 110
— formation of the, 151–52, 303

Gill-crate, the, 183, 189

Gills, disappearance of the, 244

Glœocapsa, 210

Gnathostoma, 230, 232

Goethe as an evolutionist, 27, 299

Goitre, 110

Gonads, the, 111
— formation of the, 149–50

Gonidia, 334

Gonochorism, beginning of, 322

Gonoducts, 335

Gonotomes, 146, 149

Goodsir, 189

Gorilla, the, 174, 176, 262

Graafian follicles, the, 17, 119, 347

Gregarinæ, 211

Gullet-ganglion, the, 190

Gut, evolution of the, 310–17

_Gyrini,_ 242

Gynecomastism, 114

H

Hag-fish, the, 188

Hair, evolution of the, 270
— on the human embryo and infant, 271

Hair, restriction of, by sexual selection, 271

_Haliphysema,_ 215

Halisauria, 202

Haller, Albrecht, 12

_Halosphæra viridis,_ 213

Hand, evolution of the, 250, 304–6
— of the ape and man, 258

Hapalidæ, 261

Harderian gland, the, 288

Hare-lip, 284

Harrison, Granville, 161

Hartmann, 262

Harvey, 10

Hatschek, 192

Hatteria, 243, 246

Head-cavity, the, 138

Head-plates, the, 149

Heart, development of the, 7, 10, 111, 151, 170, 322, 324–27
— of the ascidia, 190
— position of the, 327

Helmholtz, 207

Helminthes, 223

Hepatic gut, the, 109, 316

Heredity, nature of, 3, 5, 27, 56–7, 349

Hermaphrodism, 9, 23, 114, 218, 322, 346

Hertwig, 21

Hesperopitheca, 259

His, W., 19

Histogeny, 18, 19

_History of Creation,_ 6, 30

Holoblastic ova, 67, 71, 77

_Homœosaurus,_ 244, 246

Homology of the germinal layers, 20

Hoof, evolution of the, 270

Hunterian ligament, the, 344

Huxleian law, the, 171, 257, 262

Huxley, T. H., 7, 20, 29

Hydra, the, 69, 217

Hydrostatic apparatus in the fish, 315

_Hylobates,_ 173, 262

_Hylodes Martinicensis,_ 241

Hyoid bone, the, 299

Hypermastism, 113

Hyperthelism, 113

Hypoblast, 20, 64

Hypobranchial groove, the, 110, 184, 226, 316

Hypodermis, the, 268

Hypopsodina, 257

Hyposoma, the, 129

Hyposomites, 130, 194

Hypospadia, 346

I

Ichthydina, 224

_Ichthyophis glutinosa,_ 80

Ictopsida, 257

Ileum, the, 310

Immortality, Aristotle on, 10

Immortality of the soul, 58

Impregnation-rise, the, 55

Indecidua, 255

Indo-Germanic languages, 203

Induction and deduction, 31, 208

Inheritance of acquired characters, 349

Insects, intelligence of, 353

Interamniotic cavity, the, 165

Intestines, the, 309, 316–17

Invagination, 62

Iris, the, 286

J

_Jacchus,_ 261

Java, ape-man of, 263, 264

Jaws, evolution of the, 301

Jurassic strata, 202

K

Kant, dualism of, 25

Kelvin, Lord, on the origin of life, 207

Kidneys, the, 111
— formation of the, 150–51, 336–42

Klaatsch, 262

Kölliker, 21

Kowalevsky, 191

L

Labia, the, 346

Labyrinth, the, 290

Lachrymal glands, 269

Lamarck, J., 23, 25–7
— theories of, 26, 349

Lamprey, the, 230
— ova of the, 75

Lancelet, the, 60, 181–95
— description of the, 105

Languages, evolution of, 203

Lanugo of the embryo, 271

Larynx, the, 309
— evolution of the, 314

Latebra, the, 45

Lateral plates, the, 129

Laurentian strata, 201

Lecithoma, the, 117

Leg, evolution of the, 304
— structure of the, 306

Lemuravida, 257

Lemurogona, 257

Lemurs, the, 257

_Lepidosiren,_ 257

Leucocytes, 319

Life, age of, 200

Limbs, evolution of the, 152, 239, 304

Limiting furrow, the, 133

Linin, 42

Liver, the, 309, 317

Long-nosed ape, the, 164

Love, importance of in nature, 332

Lungs, the, 110
— evolution of the, 241, 314–15

Lyell, Sir C., 24

Lymphatic vessels, the, 318

Lymph-cells, the, 319

M

Macrogonidion, 331

Macrospores, 331

_Magosphæra planula,_ 213

Male womb, the, 344, 350

Mallochorion, the, 166

Mallotheria, 257

Malpighian capsules, 339, 341

Mammal, characters of the, 112
— gastrulation of the, 84

Mammals, unity of the, 247–48

Mammary glands, the, 113, 269

Man and the ape, relation of, 262, 351
— origin of, 29

_Man’s Place in Nature,_ 7, 29, 351

Mantle, the, 189

Mantle-folds, the, 185

Marsupials, the, 250–52
— ova of the, 85

Materialism, 356

Mathematical method, the, 30

Mechanical causes, 6
— embryology, 8, 19, 22

Meckel’s cartilage, 304

_Medulla capitis,_ the, 273
— _oblongata,_ the, 274
— _spinalis,_ the, 273

Medullary groove, the, 125
— tube, the, 107, 128
— — formation of the, 131, 133, 227, 267, 276

Mehnert, E., on the biogenetic law, 5

Meroblastic ova, 67, 71, 78

Merocytes, 68, 321

Mesentery, the, 98, 109, 310, 316

Mesocardium, the, 327

Mesoderm, the, 20, 64, 90, 93

Mesogastria, 215

Mesonephridia, the, 338

Mesonephros, the, 336

Mesorchium, the, 344

Mesovarium, the, 344

Mesozoic period, the, 202

Metogaster, the, 64

Metagastrula, the, 67

Metamerism, 142

Metanephridia, the, 341

Metanephros, the, 336

Metaplasm, 39

Metastoma, 64, 222

Metatheria, 248

Metazoa, 20, 62

Metovum, the, 81

Microgonidian, 331

Microspores, 331

Middle ear, the, 291

Migration, effect of, 33

Milk, secretion of the, 269

Mind, evolution of, 353–54
— in the lower animals, 353

Miocene strata, 203

Mitosis, 40, 41

Monera, 40, 206, 209

Monism, 6, 356

Monodelphia, 248

Monogonopora, 223

Monopneumones, 238

Monotremes, 118, 249
— ova of the, 84

_Monoxenia Darwinii,_ 60

Morea, the, 212

Morphology, 2, 27

Morula, the, 62, 212

Motor-germinative layer, the, 19

Mouth, development of the, 124, 139
— structure of the, 308

Mucous layer, the, 16

Müllerian duct, the, 341

Muscle-layer, the, 16

Muscles, evolution of the, 307
— of the ear, rudimentary, 292

Myotomes, 108, 146

Myxinoides, the, 188, 230

N

Nails, evolution of the, 270

Nasal pits, 284

Natural philosophy, 25
— selection, 26, 28, 349

Navel, the, 117, 134

Necrolemurs, 257

Nectocystis, the, 314

Nemertina, 224–26

Nephroduct, evolution of the, 338–39

Nephrotomes, 149, 338

Nerve-cell, the, 39

Nerves, animals without, 267

Nervous system, evolution of the, 7, 267

Neurenteric canal, the, 127

Nictitating membrane, the, 32, 286, 288

Nose, the, in man and the ape, 164
— development of the, 282–85
— structure of the, 283

Notochorda, the, 107

Nuclein, 37

Nucleolinus, 44

Nucleolus, the, 38, 44, 54

Nucleus of the cell, 37

O

Œsophagus, the, 309, 316

Oken, 5, 27, 300

Oken’s bodies, 339

Oligocene strata, 203

_Olynthus,_ 217

On the generation of animals, 9

Ontogeny, 2, 23
— defective evidence of, 208

Opaque area, the, 122

Opossum, the, 252
— ova of the, 85

Optic nerve, the, 287

Optic thalami, 274
— vesicles, 286

Orang, the, 174, 262

Ornithodelphia, 248

_Ornithorhyncus,_ 85, 249

Ornithostoma, 249

Ossicles of the ear, 289

Otoliths, 289

Ova, number of, 347
— of the lancelet, 192

Ovaries, evolution of the, 333–34

Oviduct, origin of the, 335, 342

Ovolemma, the, 44

Ovulists, 12

Ovum, discovery of the, 16
— nature of the, 40,
— size of the, 44

P

Pachylemurs, the, 257

Pacinian corpuscles, 282

Paleontology, 2
— evolutionary evidence of, 31
— incompleteness of, 208
— rise of, 24

Paleozoic age, the, 202

Palingenesis, 4

Palingenetic structures, 4

_Palæhatteria,_ 244, 246

_Panniculus carnosus,_ the, 350

Paradidymis, the, 342

Parietal zone, the, 129

Parthenogenesis, 9, 13

Pastrana, Miss Julia, 164

Pedimana, 252

Pellucid area, the, 122

Pelvic cavity, the, 138

_Pemmatodiscus gastrulaceus,_ 215

Penis-bone, the, 346

Penis, varieties of the, 345

Peramelida, 254

Periblastic ova, 68

Peribranchial cavity, the, 185, 190

Pericardial cavity, the, 328

Perichorda, the, 108, 183
— formation of the, 136

Perigastrula, 89

Permian strata, 202

Petromyzontes, the, 188, 230

Phagocytes, 49, 320

Pharyngeal ganglion, the, 275

Pharynx, the, 309

Philology, comparison with, 203

_Philosophie Zoologique,_ 25

Philosophy and evolution, 6

Phycochromacea, 209

Phylogeny, 2, 23

Physemaria, 214

Physiology, backwardness of, 7

Phytomonera, 209

Pineal eye, the, 108

Pinna, the, 291

_Pithecanthropus,_ 263, 264

Pithecometra-principle, the, 171

Placenta, the, 166, 253–54

Placentals, the, 166
— characters of the, 253
— gastrulation of the, 86

Planocytes, 49, 320

Plant-louse, parthenogenesis of the, 13

Planula, the, 89

Plasma-products, 38, 39

Plasson, 40, 59

Plastids, 36, 40, 209

Plastidules, 59

Platodaria, 221

Platodes, the, 221

Platyrrhinæ, 261

Pleuracanthida, 234

Pleural ducts, 328

Pliocene strata, 203

Polar cells, 54

Polyspermism, 58

Preformation theory, the, 11

Primary period, the, 202

Primates, the, 157, 257–60

_Primatoid,_ 263

Primitive groove, the, 69, 82, 124, 125
— gut, the, 20, 63, 214
— kidneys, the, 111, 337
— mouth, the, 20, 63
— segments, 143
— streak, the, 100, 122
— vertebræ, 144, 195, 206, 229

Primordial period, the, 201

Prochordata, 192

Prochordonia, the, 192, 218

Prochoriata, 253

Prochorion, the, 44, 119

_Proctodæum,_ the, 345

_Procytella primordialis,_ 210

Prodidelphia, 256

Progaster, the, 20, 63

Progonidia, 333

Promammalia, 247

Pronephridia, the, 151

Pronucleus femininus, 54
— masculinus, 54

Properistoma, 69

Prorenal canals of the lancelet, 186
— duct, the, 132, 139, 186
— — evolution of the, 338

Proselachii, 234

Prosimiæ, the, 257

Prospermaria, 333

_Prospondylus,_ 105, 229

Prostoma, 20, 63, 222

Protamniotes, 243–44

Protamœba, 210

Proterosaurus, the, 202, 244

Protists, 36, 38

Protonephros, 111, 336

Protophyta, 210

Protoplasm, 37, 209

_Protopterus,_ 238

Prototheria, 248

Protovertebræ, 142, 144

Protozoa, 20, 210

Provertebral cavity, the, 148
— plates, the, 136, 144

Pseudocœla, 93, 221

Pseudopodia, 48

Pseudova, 13

Psychic life, evolution of the, 8

Psychology, 8

Pterosauria, 202

Pylorus, the, 309

Q

Quadratum, the, 247

Quadrumana, 258

Quaternary period, 203

R

Rabbit, ova of the, 86–7

Radiates, the, 103

Rathke’s canals, 341

Rectum, the, 317

Regner de Graaf, 119

Renal system, evolution of the, 335–42

Reproduction, nature of, 330–31

Reptiles, 245–47

Respiratory organs, evolution of the, 314–15
— pore, the, 183, 189

Retina, the, 286

Rhabdocœla, 222

Rhodocytes, 319

_Rhopalura,_ 215

Rhyncocephala, 243

Ribs, the, 295
— number of the, 353

Rudimentary ear-muscles, 292
— organs, 32
— — list of, 349–50
— toes, 306

S

Sacculus, the, 289

_Sagitta,_ 65, 66, 191
— cœlomation of, 93

Salamander, the, 241
— ova of the, 74

Sandal-shape of embryo, 128–29

_Satyrus,_ 174, 262

Sauromammalia, 246

Sauropsida, 245

Scatulation theory, the, 12

Schizomycetes, 210

Schleiden, M., 18, 36

Schwann, T., 18, 36

Sclerotic coat, the, 286

Sclerotomes, 108, 143, 148

Scrotum, the, 344

_Scyllium,_ nose of the, 283

Sea-squirt, the, 181, 188–90

Secondary period, the, 202

Segmentation, 60, 141–42

Segmentation-cells, 54

Segmentation-sphere, the, 17

Selachii, 223
— skull of the, 301

Selection, theory of, 28

Selenka, 166, 168

Semnopitheci, 262

Sense-organs, evolution of the, 151, 280
— number of the, 281
— origin of the, 281

Sensory nerves, 279

Serocœlom, the, 165

Serous layer, the, 16

Sex-organs, early vertebrate form of the, 111
— evolution of the, 333

Sexual reproduction, simplest forms of, 331
— selection, 30, 271–72

Shark, the, 233
— nose of the, 283
— ova of the, 75
— placenta of the, 9
— skull of the, 301

Shoulder-blade, the, 306

Sickle-groove, the, 82, 121

Sieve-membrane, the, 167

Silurian strata, 202

Simiæ, the, 257–60

Siphonophoræ, embryology of the, 21

Skeleton, structure of the, 294

Skeleton-plate, the, 148

Skin, the, 151
— evolution of, 266–69
— function of the, 269

Skin-layer, the, 16

Skull, evolution of the, 149, 299–303
— structure of the, 299
— vertebral theory of the, 300

Smell, the sense of, 282

Soul, evolution of the, 353–56
— nature of the, 58, 356
— phylogeny of the, 8
— seat of the, 278

Sound, sensations of, 289–90

Sozobranchia, 242

Space, sense of, 291

Species, nature of the, 23, 34

Speech, evolution of, 264

Spermaducts, 335, 342

Spermaries, evolution of the, 333–34

Spermatozoon, the, 52–3
— discovery of the, 12, 53

Spinal cord, development of the, 8
— structure of the, 273

Spirema, the, 42

Spiritualism, 356

Spleen, the, 318

Spondyli, 142

Sponges, classification of the, 34
— ova of the, 49

Spontaneous generation, 26, 206

Stegocephala, 239

Stem-cell, the, 54

Stem-zone, the, 129

Stomach, evolution of the, 311–14, 316
— structure of the human, 309

Strata, thickness of, 200–201

Struggle for life, the, 28

Subcutis, the, 268

Sweat glands, 269

T

Tactile corpuscles, 268, 282

Tadpole, the, 242

Tail, evolution of the, 242–43
— rudimentary, in man, 159, 295, 350

Tailed men, 160–61

Taste, the sense of, 282

Teeth, evolution of the, 314
— of the ape and man, 259

Teleostei, 234

Telolecithal ova, 67, 68

Temperature, sense of, 282

Terrestrial life, beginning of, 235

Tertiary period, the, 203

_Theoria generationis,_ the, 13

Theories, value of, 181

Theromorpha, 246

Third eyelid, the, 286, 288

Thyroid gland, the, 110, 184, 315

Time-variations in ontogeny, 5

Tissues, primary and secondary, 37

Toad, the, 241

Tocosauria, 246

Toes, number of the, 240

_Tori genitales,_ the, 346

Touch, the sense of, 282

Tracheata, 142, 219

Tread, the, 45, 81

Tree-frog, the, 241

Triassic strata, 202

_Triton tæniatus,_ 74

Troglodytes, 174

Tunicates, the, 189

Turbellaria, 222

Turbinated bones, the, 283

Tympanic cavity, the, 288

U

Umbilical, cord, the, 117
— vesicle, the, 138

Unicellular ancestor of all animals, 47
— animals, 38, 47

Urachus, the, 317, 341

Urinary system, evolution of the, 335–42

Urogenital ducts, 335

_Uterus masculinus,_ the, 344, 350

Utriculus, the, 289

V

_Vasa deferentia,_ 335

Vascular layer, the, 16, 168
— system, evolution of the, 321–25
— — structure of the, 318

Vegetative layer, the, 16

Veins, evolution of the, 323–24

Ventral pedicle, the, 166

Ventricles of the heart, 325

Vermalia, 220, 223

Vermiform appendage, the, 32, 310, 317

Vertebræ, 142, 294

Vertebræa, 105

Vertebral arch, the, 148, 295
— column, the, 144
— — evolution of the, 296
— — structure of the, 294

Vertebrates, character of the, 104–10
— descent of the, 219–20

Vertebration, 142

Vesico-umbilical ligament, the, 341

_Vesicula prostatica,_ the, 344, 350

Villi of the chorion, 165

Virchow, R., 35
— on the ape-man, 303
— on the evolution of man, 264

Virgin-birth, 9, 13

Vitalism, 6

Vitelline duct, the, 138

Volvocina, 213

W

Wallace, A. R., 29

Water, organic importance of, 200

Water vessels, 336

Weismann’s theories, 349

Wolff, C. F., 13

Wolffian bodies, 339

Wolffian duct, the, 341

Womb, evolution of the, 342–43

Y

Yelk, the, 43, 45, 67

Yelk-sac, the, 117, 134

Z

Zona pellucida, the, 44

Zonoplacenta, 255

Zoomonera, 209

Zoophytes, 20, 64, 104





*** End of this LibraryBlog Digital Book "The Evolution of Man" ***

Copyright 2023 LibraryBlog. All rights reserved.