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Title: Embryology - The Beginnings of Life
Author: Leighton, Gerald R.
Language: English
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EMBRYOLOGY
THE BEGINNINGS OF LIFE
BY GERALD LEIGHTON, M.D., F.R.S.E.
AUTHOR OF "THE GREATEST LIFE," "BRITISH SERPENTS"
"HUXLEY: HIS LIFE AND WORK," ETC.
LONDON: T. C. & E. C. JACK
67 LONG ACRE, W.C., AND EDINBURGH
NEW YORK: DODGE PUBLISHING CO.
* * * * *
CONTENTS
CHAP. PAGE
I. THE CELL AND THE INDIVIDUAL 7
II. PROBLEMS OF REPRODUCTION 15
III. PROBLEMS OF REPRODUCTION (_continued_) 23
IV. THE MAKING OF A MAN 36
V. FERTILISATION AND EARLY DEVELOPMENT 47
VI. EARLY DEVELOPMENT 53
VII. THE BEGINNINGS OF THINGS 59
VIII. THE BEGINNINGS OF THINGS (_continued_) 62
IX. THE BEGINNINGS OF THINGS (_continued_) 66
X. THE BEGINNINGS OF THINGS (_continued_) 73
XI. HOW THE EMBRYO IS NOURISHED 78
XII. RECAPITULATION 84
BIBLIOGRAPHY 90
INDEX 91
* * * * *
EMBRYOLOGY
CHAPTER I
THE CELL AND THE INDIVIDUAL
What is Embryology, and what is its significance or interest to the
ordinary educated man and woman? The answer to the question is the
justification for the appearance of the following pages, and one may
regard it as a somewhat striking fact, that in the production of a
series of works of which this volume is one, those responsible for the
subjects should have deemed it advisable to include Embryology.
Embryology may be defined as that part of the science of Biology which
deals with the formation of a new individual or embryo. The definition
itself ought to be sufficient to explain the significance of the subject
for every one, because one can hardly conceive of any more profoundly
important knowledge than that which tells of the mode of origin, manner
of growth, and ultimate birth of an entirely new being. In the absence
of such accurate knowledge it is quite obvious that all one's ideas
concerning the manner in which the new individual is to be treated must
have a more or less haphazard, or at least empirical, basis. In fact
only when the science of Embryology, or the development of the
individual, becomes a part of the ordinary everyday mental equipment of
those who are responsible for bringing into the world new individuals,
and subsequently protecting and handling them, will it be reasonable to
expect that these new individuals are dealt with in the best possible
manner. In a word it is evident that education, using that term in the
very widest possible sense, can never be anything more than a blind
groping in the dark until those into whose hands it is entrusted realise
and know at least the most important fundamental facts concerning
development. It is lack of this kind of knowledge which has been
responsible for so much of the mistaken systems of the past in dealing
with the young, and it is the spread of this knowledge which alone is
the hope of better things in the future. Wherever knowledge is absent
superstition is rife, and in no sphere of life is this more painfully
obvious than in connection with the subject which we are about to study.
It would have been entirely impossible for many of the stupid and even
cruel methods of mental and physical treatment which have been meted out
to the young children in the past to have been tolerated for a moment
had this knowledge been available and sufficiently widespread.
Possessing it, a flood of light is thrown upon the fascinating and
otherwise obscure problems of heredity; and thus it lays open the pages
of the past for those who care to read them. Possessing it also it
throws upon the mental screen pictures of possibilities in the future
for all those who have eyes to see. So the study of Embryology links up
the past with the present and joins the present with the future. Is it
not, therefore, obvious that the study of such a subject means dealing
with problems the importance of which it is impossible to exaggerate;
problems which the parent, the teacher, the social reformer, the
politician, and the philanthropist will grapple with in vain unless they
call in science to their aid? Such is the meaning and significance of
the subject of our study.
In the widest sense of the word Embryology, therefore, deals with all
manner of living things, be they plant or animal. But since our purpose
here is to state, as far as possible in the space at our disposal, the
facts which are of particular importance in relation to the human
subject, we shall only glance at the rest of living creatures. A brief
look at them, however, is quite necessary in order to appreciate what
follows. Let us be quite clear of what we are in search. We want to know
as far as possible what it is that goes to the making of a man. What is
the origin of the new individual? Where does the embryo come from? What
elements are concerned in its formation? Where do these elements come
from? How are they subsequently built up into the type of the species to
which they belong? From what source do they gain their nourishment? What
influences of a degenerative nature are likely to affect them? These are
the questions which it is the business of the Embryologist to answer,
and these are the questions the answers to which afford the explanation
of man in the making. Surely they merely require to be stated that their
significance may be appreciated.
We may now glance very briefly at the simplest facts which bear upon the
subject, and which must precede our detailed study. The necessity for
reproduction and development is involved in the universal fact of
death. In all except the very simplest forms of life--those consisting
of one simple mass of protoplasm--the individual sooner or later
perishes, and if it were not that there were some methods by means of
which the individuals could give rise to new individuals obviously the
species would come to an end. No matter to what great age an individual
animal may live, and there are some such as the tortoises which do live
for centuries, sooner or later death overtakes them, and in all,
investigation of their structure shows that nature has made provision
for the carrying on of the race by means of new individuals.
Every living creature, be that creature simple or complicated, animal or
vegetable, man or a jellyfish, starts life as one single cell. The very
simplest living individuals never consist of anything else but one
single cell, and it is in these primitive forms of life alone that what
we call death can not be said to occur. Such a simple cell, after living
for a certain period, simply divides itself into two halves, each of
which gradually assumes the size and shape of what we may term the
parent cell. The first individual has simply become two separate
individuals. These two in their turn after another period of independent
existence, again each divide, thus giving rise to four, and so on. Now
here, although the original parent cell no longer exists _as a cell_,
the actual material of which it was composed still exists in the cells
which came into existence as the result of this division. The original
cell, therefore, may be literally said to have been deathless, or
immortal, though not everlasting. This is a profound thought, and one
which must be grasped at the very commencement of our study of
development, because it is one to which we shall have to recur again and
again when we come to study the cells which give rise to human beings,
in whom, too, there is a deathless continuity of cell protoplasm, or
germ-plasm as it is then called. It is upon this fact that the whole
science of Embryology depends.
The important idea to be learned from observing this process of
reproduction in the single-celled animal is this: that there is nothing
here which we may term the body of an animal as opposed to any of its
parts. The one cell is both body and organs, and everything else; in
itself it has the capacity of performing all the functions necessary for
life, including that of reproduction for the perpetuation of the
species. No part of the cell is set on one side for any special purpose
such as happens in the bodies of higher animals. There are no special
elements which go to the producing of the next generation, none of the
cells which in a mammal, for example, we call "germ-cells." The whole
individual is one cell. In fact one might almost say that there is no
individual, but only race, or if we regard the cell as an individual
then it is all germ-plasm. That is the important fact to be learned in
the reproduction of single cells.
There are some single cells, such as those of the yeast, which reproduce
in a slightly different manner, namely, by budding off a portion of
themselves and finally becoming separate, and this might be regarded as
a slightly higher stage, in so far as the original cell from which the
bud came may be still identified; but in reality the process differs
very little from that first described.
Then we may note that very low in the scale of living things there is a
process of reproduction known as conjugation, in which, although the
cells of the species appear to be all alike, yet, nevertheless, two of
them join together for purposes of reproduction. In other words we have
here a process of cell-union before we have the cell-division which
follows. It is important to note at this stage that the creatures which
we have mentioned, and even some more highly organised, such as an
am[oe]ba, which has a nucleus, go through these simple or complicated
reproductive processes in the total absence of anything which could
suggest a distinction of sex. In these cases the individuals are
obviously all of one sex, and, therefore, the distinction of sexes into
male and female is evidently something which has been added later in the
scheme of evolution, not for the purpose of reproduction itself, but for
something which is to be added to that.
Then in the slightly higher animals and plants we come to those in which
many cells go to the making of the individual, the multicellular
individuals, and amongst these we very soon see the origin of what is
termed specialisation of function. That is to say, in these higher
creatures which consist of many numbers of cells arranged so as to form
one individual, certain cells are set apart for one purpose and others
for another. Some may be for digestion, some for purposes of movement,
and others for reproduction. Here we have a new phenomenon, namely, the
setting aside of certain cells in a multicellular individual which from
the very beginning are capable of one function alone, namely,
reproducing the species. The higher one goes in the scale of life the
more striking and obvious this fact becomes, and as we shall see when
we come to the vertebrate kingdom, this setting aside of the cells which
are to produce the individuals of the next generation is the key to the
solution of the most difficult of our problems.
In these highest forms of life, however, the cell itself is becoming a
much more complicated thing than that lowly form which we first noted as
dividing into two to form two new individuals. Indeed, the cells in the
highest animals and plants are immensely complicated in their structures
and functions, and especially in connection with the changes which take
place in the nucleus of such cells. Not only the nucleus but another
small object within the cell which is neither part of the nucleus nor
part of the cell protoplasm, also is very important, and this structure
is termed the "centrosome." In fact this little body apparently begins
the whole process of cell-division by itself dividing into two parts.
Then the nucleus follows suit, and ultimately the whole cell divides.
The nucleus itself is a complicated structure, as is especially seen
during the processes of division, in which it breaks itself up into a
number of thread-like portions, and the number of these is always the
same in any given species, a fact which is of great importance in
reproduction. Why do we mention these apparently dry details? Because in
these minute and complicated nuclear movements the whole problems which
are at the bottom of development and heredity lie. The problems of life
itself can only be solved by the study of what takes place in these
minute portions of cells. It is here that the new formation of an
individual begins, and although it is no part of our purpose here to
detail all the complicated processes of nuclear division, it is
essential, in order to grasp the meaning of our subject, that we should
realize that in the changes within the cell life with its variations
begins.
The study of these wonderful cell processes, a work which demands the
most patient investigation and high technical skill, has reached such a
stage that it is a science of its own, and is called the science of
"Cytology," or the science of cells, which has been made possible only
in comparatively recent years by the invention of microscopes having
great powers of magnification, and by the application of elaborate
methods of staining to the cells themselves.
We can say no more about these processes here, but the foregoing
paragraphs may perhaps be sufficient to show us how important it is to
grasp these simple facts of cell life in their bearing upon development
itself.
CHAPTER II
PROBLEMS OF REPRODUCTION
We have seen that in the higher types of animals and plants the single
individual is made up of not one but millions and millions of cells
united together for the common purpose of the individual life, and that
in such complicated individualities some cells perform one function
while others perform others. A human individual from this point of view,
therefore, is an organised community of cells all of which, however,
sprang, in the first place, from one single cell. That original single
cell is termed, in animal Embryology, the "fertilised ovum." It is
popularly spoken of frequently as "the egg." All the other millions of
cells are the direct descendants of this fertilised ovum, or egg, even
though many of them eventually become extremely unlike the original
cell. In single-celled animals the offspring of the original cell remain
like the parent cell, but in the highly complicated creatures the
offspring split up into a great many types of cells, owing to the very
fact that all remain adherent together to form the mass of the body in
order to carry out different functions. So we find cells of one type in
glands, of another type in the brain, of another type in bones, of
another type in blood, and so forth. Nevertheless all of them sprang
from one original single cell. None of these specialised types of
cells, however, are capable of performing any other function than their
own. A bone-cell cannot receive an impression, nor originate an idea,
any more than a brain-cell can secrete bile. Each kind of cell has its
own appointed duty. The most important duty that can possibly be
allotted to any cell is obviously that of reproducing the individual for
the purpose of continuing the race or species. So we find in higher
animals that this function, like others, is relegated to a special set
of cells also derived from the original single cell, and which are
called "the germ-cells."
Leaving out of consideration the question of reproduction in lower types
of animals we may consider the nature and origin of these cells in
highest vertebrates, such as the mammals, including man. Germ-cells,
which are derived from the tissues of a female animal, are termed "ova."
Those which are derived from the tissues of a male animal are termed
"sperms." Notice that it is not these germ-cells themselves to which the
terms male and female, indicative of the two sexes, are applied, but
only to the individuals. They are male and female; the germ-cells are of
neither sex. True the germ-cells from the male, _i.e._ the sperms,
differ in appearance when seen under the microscope from those of the
female, but there is no reason to believe that there is any difference
between them in their capacity, for example, of transmitting the
characters of ancestors to succeeding generations.
At a certain stage in the life history of the animal individual and
after undergoing certain changes which need not be considered here,
these germ-cells, both sperms and ova, have reached such a stage of
maturity as to be capable of carrying on their sole function, namely,
that of reproducing the species. The actual age in human beings, for
example, at which this maturity is reached varies very much in different
races, and in different individuals of the same race.
When reproduction is about to occur a union must take place between a
germ-cell from a male body with a germ-cell from a female body; that is
to say, a union must take place between a sperm and an ovum. This union
takes place within the body of the female individual and results in the
fusion of the two cells into one single cell, which is now termed a
fertilised ovum. This fertilised ovum, in virtue of this process of
union, is now able under suitable conditions of nutrition and shelter,
such as it obtains within the female organs of reproduction, to divide
and redivide again and again, thus building up a new mass of cells as
the result of its division. The millions of cells so produced include,
as we have already seen, cells which have all the various functions
which are necessary for the continuation of the life of a human
individual; that is to say, that as the result of this division of the
fertilised ovum there are produced first of all germ-cells to secure the
still further continuance of the race, and then multitudes of all the
other kinds of cells which gradually assume the shape of an embryo or
young individual, and ultimately grow into a human being.
In all the highly complicated animals fertilisation by union of
germ-cells from male and female must precede reproduction. The result
eventually is this multicellular individual composed of a number of
different kinds of cells each set apart for its own work. But it is well
to recognise that we may regard all these cells as really of two kinds,
namely, the germ-cells and the others. That is to say, two kinds of
cells are produced as the result of fertilisation, namely, cells whose
business it ultimately will be to again take part in a similar process
of fertilisation, and so perpetuate the species, and all the other cells
which go to the forming of the various body tissues of the individual
itself. In this way we get a simple classification of the cells which
form, for example, a human being, namely, germ-cells and body-cells, the
latter often being termed "somatic." The latter are, of course, in much
greater abundance than the germ-cells. They have to form all the various
elements, organs, limbs, and structures known and described by the
anatomist. The germ-cells are a separate little group of themselves
embedded in the male and female reproductive organs for the sake of
nutrition, growth, and shelter, for many years, until they again take
part in the process of fertilisation. Note carefully that no other cells
in the body ever unite together to produce a new individual except
germ-cells.
Somatic cells reproduce by dividing directly. Germ-cells before they can
do this require to be fertilised. That is to say, the cell from the male
(the sperm) must fuse with the cell in the female (the ovum). As Dr.
Archdall Reid graphically states it, "Only the germs are marriageable;
and, as we have just seen, in the great majority of animals and plants
they observe the degrees of consanguinity very strictly, and do not
unite except with members of another cell-community, and then only to
found a new colony of cells, an offspring."
There are still some further considerations in connection with the
subject of germ-cells and germ-plasm which we must carefully consider
before leaving this part of our subject, Embryology. Everything depends
upon a perfectly clear understanding at this stage. The facts themselves
that have to be adduced in this connection are comparatively few and
simple. No fairly educated person should have any difficulty whatsoever
in grasping them. Moreover, very fortunately they are thoroughly well
established and not in dispute. But the reasoning which is based upon
these few and elementary facts, reasoning which is applied to the
methods of treatment of the individual which is produced, may be very
complicated and very debatable. Various schools of thought and opinion
exist according to the attitude taken towards the facts, some of which
we have mentioned and others of which we are about to detail. But the
facts themselves are not debatable, and we therefore see once more that
their importance at this stage cannot be exaggerated.
One or two very simple general propositions bound up with the subject of
Embryology, or individual development, may be stated in order to focus
attention upon the nature of the problem under investigation. Thus
nobody will be found to question the fundamental truth that children
resemble their parents. That is a commonplace of experience. Similarly
no one will be found to dispute another fundamental fact, namely, that
children differ from their parents. This, too, is equally a commonplace
of experience. If we examine a million human beings we find that they
all possess certain features in common, certain characteristics in
virtue of which we recognize them to be human beings. Nevertheless it is
just as true that a careful examination of the same million people
reveals the true saying that no two of them are exactly alike. Here then
are two propositions equally true within certain limits; namely, that
all human beings resemble each other, and that all human beings differ
from each other. There is resemblance; and there is variation. These two
things are universal because of the existence and characteristics of
germ-cells. We may look at this a little closer.
Every species of animal, in the process of reproduction brings forth
offspring similar to itself. This is expressed in the familiar proverb
that "like produces like." One does not expect grapes from thorns, nor
is it possible to construct a silk purse out of a sow's ear. But what is
the explanation of this proverbial fact? The answer is of great
importance, because although the fact itself is recognized as a general
principle in the reproduction of a species, it is not sufficiently
recognized in the full details of the characters of that individual. Too
many people are still apt to expect to be able to produce grapes when
the plant is a thorn, and it is unfortunately all too common to make
heroic but quite futile attempts to construct human silk purses out of
human sows' ears--so to speak--simply because of the ignorance of the
material which is being used. The most that can be done is to give such
material as is present the very best opportunity of attaining its own
utmost perfection; and this, by the way, is vastly more than has ever
been done for any considerable number of the human race.
But why this continuity of species? Why should like always produce like?
The answer has been sought by biologists ever since problems of life
attracted man's curiosity. All sorts of weird and fantastic theories
have been put forward at different times to account for this simple
fact, but it is only in comparatively recent years that the real
explanation has been forthcoming. It is perfectly obvious that in order
to secure this continuity of racial resemblance there must be something
physical or material which is actually continuous from generation to
generation to account for it. The immortal Darwin saw this very clearly,
and devoted much thought in the endeavor to find some explanation of
this very problem. The result was his theory of Pangenesis which,
ingenious as it was, was ultimately shown to have no basis on fact. In
his effort to account for the fact that children resemble their parents
even in such minute details as the shape of the nose, the colour of the
eyes, and so forth, he formulated the idea that the parents themselves
probably contributed multitudes of minute particles from their own
tissues to form the cells of their offspring. He supposed, for example,
that particles or gemmules from the eyes, nose, hair, and so forth, of
the parent, or parents, in some way or other were fused together and
gave rise to the cells which ultimately produced an embryo. Hence he
thought the explanation of the resemblance between parents and children.
This was his solution to the question of the physical continuity between
successive generations. It may be remarked in passing that it is with
something of pathos that one reads in Darwin's own works his own evident
opinion that this theory of Pangenesis was a great discovery. One
gathers almost that he himself regarded it as of greater importance than
his work on natural selection.
In the course of time, however, the real actual basis of physical
continuity was shown to be something quite different, and looking back
now upon the history of the discoveries in this connection during the
last generation one can easily imagine what speculations there must have
been in the absence of the facts which are now known to embryologists.
CHAPTER III
PROBLEMS OF REPRODUCTION (_continued_)
The one outstanding discovery which has placed the science of Embryology
on an absolutely firm basis, and which has made clear so many of the
facts, which were previously puzzling, is this: _that the germ-cells
which give rise to new individuals are themselves produced from
pre-existing germ-cells_. The entire embryo, or young infant, is derived
from one single cell which we have called the fertilised ovum, and that
in its turn was derived from the union of two germ-cells, one from the
male parent, and one from the female. These two cells in their turn were
also derived in a straight line of descent from the fertilised ovum from
which each parent sprang. In other words there has never been any
conjugation between one fertilised ovum and another in spite of the
generations of cells which have been produced between them. Put in
another way the body, or somatic cells, contribute absolutely nothing to
the original material or germ-plasm of which the germ-cells are
composed. They do not produce them in any sense of the word whatsoever,
despite the popular opinion to the contrary. This is the great discovery
of modern Embryology. Until this was known it was assumed that parents
did produce the cells from which their children sprang, and hence--it
was thought--the resemblance between them. The fact is quite otherwise.
No parent ever produces a germ-cell, and the reason why children
resemble parents and ancestors is because the germ-cells which give rise
to individuals in successive generations are produced from the
germ-cells of the previous generation. The line of descent or
inheritance, therefore, is from germ-cell to germ-cell, and not from
parents. Unless the reader makes himself absolutely familiar with the
thought expressed in these facts he will never understand the science of
Embryology.
Dr. Archdall Reid expresses this truth in the following words. "The
somatic cells of the parent, therefore, as far as we know, contribute no
living elements to the child; they merely provide temporary shelter and
nutriment. The child, therefore, does not, as is popularly supposed,
resemble his parent because his several parts are derived from similar
parts of the parent--his head from his parent's head, his hands from his
parent's hands, and so forth; he resembles him only because the
germ-plasm which directed his development was a split-off portion of the
germ-plasm which directed the development of the parent. The egg
produces the fowl, but the fowl as a whole does not produce the
egg--only one cell from the fowl, the fertilised ovum, produces it."
[Illustration: Diagram to show the origin of germ-cells and the embryo.]
Unite in the process of fertilisation to form the fertilised ovum, which
divides a given number of times and forms daughter-cells, which are
germ-cells; one of which, and one only, goes on dividing to form the
body-cells, and so produces the new individual, which as it grows
includes in itself those cells (germ-cells) previously formed.
The rest are germ-cells, which subsequently form the eggs and sperms of
the new individual, _i.e._ they are the germ-cells of the next
generation. They cannot develop independently, but when they unite with
the egg or sperm of another individual, a new fertilised ovum is formed
and the cycle begins again.
This is a startling thought, but it is one which a moment's careful
consideration will show is the only conceivable explanation of all the
facts of physical continuity. Once it is grasped a flood of light is
thrown upon the whole science of Embryology. The individual is seen to
be literally a "chip of the old block," and the "old block" means the
whole sequence of germ-cells which has preceded his formation. In the
light of this fact it is obvious why like produces like; indeed, it is
obvious that it must do so. Further, we now understand at once that
since one generation of germ-cells directly produces those of the next,
there is no reason in the world why an individual should not more nearly
resemble a remote ancestor than his own immediate parents. As a simple
matter of fact this frequently happens. He does so because the germ-cell
from which he sprang is composed of protoplasm handed down in direct
continuity by successive generations of germ-cells from time immemorial.
In fact the problem in the light of this evidence is not so much--as it
always seems to the writer--to understand why children resemble parents
and ancestors, as to understand how it is that they do not resemble them
more.
There is no difficulty now in explaining the fundamental propositions
with which we started, namely, that children resemble their parents.
There is no difficulty in the understanding why a child resembles not
only its immediate parents, but even its ancestors. There is even no
difficulty in understanding why a child should resemble its ancestors,
even though it does not resemble its parents. Given the simple truth
that the germ-plasm is continuous from one generation to another, all
these things become as clear as daylight.
But we also start with another general proposition, namely, that
children differ from their parents, and it is this question of the
variation in offspring which must now claim our attention for a moment.
By this term we mean to convey the fact that although every child has a
real resemblance to its parents or its ancestors, it inevitably and
invariably shows differences even if these be more minute than the
resemblances. In other words the offspring of a human being, though
obviously and necessarily, from the continuity of germ-plasm, it must be
another human being, is never exactly similar to any other. Now these
variations are many of them present from the very beginning, they take
their origin in the germ-plasm of the two germ-cells which form the
fertilised ovum. They are, that is to say, many of them germinal in
origin. These must be carefully distinguished from such characteristics
as are afterwards acquired by the child as the result of its adaptation
to the environment in which it passes its existence.
It would be beyond the scope of the present work to enter into all the
various theories which have been put forward to account for the fact of
variation. It will be sufficient for our purpose here if the reader
remembers that it is a universal tendency in all living protoplasm to
exhibit variations. It is just as universal as is its continuity of
likeness. Moreover, in dealing with the highest animals in which the
fertilised ovum, from which the embryo springs, is produced by the union
of germ-cells from male and female, one may readily understand that the
different lines of descent of the male and female germ-cells may well be
responsible for the differences exhibited in the offspring. Obviously
the fertilised ovum, if it has to give rise to a normal individual,
cannot retain _all the characteristics_ which were possibly existent in
both the male germ-cells and the female germ-cells. Some of them must be
suppressed or got rid of, otherwise there would be too many characters
in the resulting offspring. And, as a matter of fact, such a reduction
does actually take place in the physical tissue comprising the
fertilised cell, and it is probably at this stage that variations take
their origin.
Thus, for example, it is quite impossible that two opposing
characteristics can both be represented in the fertilised ovum. One of
them must be suppressed or thrown out or got rid of in some way or
another. For example, the union of the male element and the female
element will give rise to an embryo which may be eventually either a
male or a female individual, but cannot be both. There were
possibilities of it being either the one or the other at the beginning,
but since the two possibilities are mutually antagonistic, one or the
other must be eliminated. So again, supposing that the colour of the
eyes on the one side were brown, and on the other side blue, the
possibilities are that the fertilised ovum may give rise to an
individual having either blue eyes or brown eyes, but, again, not both.
A variation in offspring then may be regarded as a difference between
that offspring and the parents, which is due to some change in the
germ-plasm, some difference, that is, between the germ-plasm from which
the parent sprang and that from which the next generation arose. Such
differences will, of course, be introduced at the time of fertilisation.
It is important to keep clearly in mind the difference between a true
variation, in the sense that we have just used the term, and a
modification which is caused by the varying effects of influences
affecting parents and offspring. Unless these two things are kept
mentally distinct, much confusion of thought is apt to arise.
The above statement does not necessarily mean that the germ-plasm
carried in the sperms and the ova respectively, cannot be affected in
any way. Indeed one is forced to the conclusion that such germ-cells
must be influenced by the nutrient fluids supplied to them, and by the
existence of toxic or poisonous substances in the body of the parent. It
is quite conceivable, and indeed inevitable, that the individual embryo
resulting from the fusion of such poisoned germ-cells will show
modifications, but these, however, are not to be regarded in any sense
as true variations, for the simple reason that these modifications do
not take their origin in the actual germ-plasm itself, but are simply
the result of abnormal stimuli. Such modifications, no matter in what
direction they may be, are, of course, not transmissible to the next
generation, for the very obvious reason that the germ-cells which are to
be concerned with the next generation have been already produced. The
germ-plasm itself passes on unchanged in so far as its hereditary
possibilities are concerned. We see, therefore, that in order to think
clearly on this matter we must limit the meaning of the word variation
to such differences or changes in germ-plasm which indicates some real
change of an inheritable nature. The term should not be used to apply to
a mere passing environment, in which the germ-plasm happens to be,
caused by the presence of poisons, or similar factors.
Given then the fact that variations do constantly and inevitably occur
in offspring owing to new qualities arising in germ-plasm itself, it is
obvious that these variations are either what are termed "spontaneous,"
or else they must be due to the action of the surroundings on the
germ-plasm. By the term "spontaneous," in this connection, it is not
meant that these variations arise without cause or in a haphazard
manner. It is simply meant to imply that the present state of our
knowledge does not justify us in stating what does actually cause the
variation in germ-plasm, or the laws in accordance with which such
variations occur. That they must be a matter of cause and effect and law
every biologist believes, but until the law can be demonstrated, the
term "spontaneous" may well be retained to distinguish these variations
from those which arise by the obvious action of environment. For
example, the variations which occur as the results of reproduction from
two parents, do so because of the mingling of the respective germ-cells,
and such variations come under the group of "spontaneous"; whereas
changes induced on account of the food supplied, or poisonous substances
in fluids surrounding germ-cells are not spontaneous, but environmental.
A point of great interest to the embryologists is the question whether
the differences of detail which exist between children and their parents
are of the nature of spontaneous variations, taking their origin in the
germ-plasm itself, or mere modifications produced by the action of the
environment of the embryo. Further, should both these factors play a
part in producing these differences, which is of greater importance, and
in what proportion? This question is elaborated in great detail by Dr.
Archdall Reid, in his work, _The Laws of Heredity_, which ought to be
read by every intelligent citizen and parent who is interested in the
welfare of the young. In the main in this subject we follow the ideas so
ably put forward by him. He points out that the offspring of the same
parents always differ not only from the parents, but among themselves,
even if they be twins, and amongst the lower animals every member of a
litter of dogs, or pigs, or kittens, shows differences in size, colour,
activities, temperament, and characteristics. Are those differences due
to the action of environment on the embryo or do they take their origin
in the germ-cells from which the individuals came? Inasmuch as a litter
of puppies is subjected to precisely the same environment during the
whole time of development, it is perfectly obvious that such differences
as they exhibit at the time of birth must have been germinal, an
identical environment could not by any stretch of the imagination be
held responsible for producing variations. They, therefore, must be of
the spontaneous variety. Of course it may be argued that even during
development the environment of each embryo within the mother is not
identical, but it will be a gross abuse of such argument to therefore
conclude that such minute differences of surroundings could account for
one puppy being big and black, and another one small and brown in the
same litter; or that one should resemble one parent, another the other,
and a third a remote ancestor. It is, therefore, clear that _some at
least_ of the variations in offspring are germinal or spontaneous in
origin, and not in any way due to the environment of the embryo.
The question remains whether all variations are due to this cause or
whether some may be traced to environmental factors. One of the best
lines of argument and investigation on this point is that of the
bacteriologist, because microbes with which he is concerned may be
regarded as equivalent in this matter to germ-cells, all microbes being
unicellular. The problems of the germ-cell, and its heredity, therefore,
are very similar in both cases. Tried by this test we may ask whether
the changes produced in these unicellular organisms by the action of
their environment are, or are not, inherited as variations. No one
doubts for a single moment that a microbe as well as a germ-cell may be
changed, or injured, or improved, according to its own special
environment. What is in dispute is whether that change remains fixed in
the succeeding generations to which these unicellular cells give rise.
It is precisely here that the bacteriologist can offer evidence of a
most important character. He will tell us that it is quite easy to
change many of the characteristics of a microbe by altering its
environment, which is undoubtedly true, but the further statement that
they change because their germ-plasm is affected directly by the
environment is not necessarily true. These organisms and germ-cells are
composed of protoplasm whose ultimate constitution permits of their
varying spontaneously. These variations are obviously to enable them to
adapt themselves to the tissues of the animal in which they are living,
and these variations also, or modifications as they really are, are
usually lost when that environment is no longer existent. In other words
they proceed no further than to allow the microbe to exist in a new
environment. This seems to point undoubtedly to the fact that they are
caused by selection of true variations. In other words what is
ultimately produced is a condition of the germ-cell in which it becomes
very highly resistant to any influence immediately exerted upon it by
the environment, and so continues to live in successive generations
without any further modification. The conclusion, therefore, is, in Dr.
Reid's words, "that the germ-plasm is both spontaneously variable and
highly resistant to the direct action of the environment. In other words
we must believe that in any species that is not undergoing extinction
spontaneous variations greatly preponderate over those which are caused
by the direct action of the environment."
This quality of single cells, that is to say of the germ-plasm of all
species which continue to exist, in virtue of which it resists very
strongly any efforts to change it, is a very important matter to grasp.
Without it it is quite obvious that no species could maintain its
characteristic features for any length of time. Were it not for this
resistant power, germ-plasm would be easily destroyed or continually and
readily changed. The descendants from such continually changing
germ-plasm would themselves be of such infinite variety that there would
be no such thing as a definite species, so that there is no doubt
whatever that germ-plasm has become, probably by the action of natural
selection, extremely resistant to all influences of an environmental
character.
That does not mean, of course, that germ-plasm cannot be damaged, or
weakened, or changed in its tendencies. It does mean that when it is so
changed it is principally as the result of injury, which may be indeed
so severe as to destroy the germ-plasm itself. It would seem as if the
inherited tendencies of germ-cells were so intimately bound up in the
constitution of those cells as to be almost a matter of life and death
of the cells. If they be so interfered with as to be destroyed it is
hardly possible for the cell itself to continue to exist. One of the
most interesting examples of this resistance of germ-cells to their
environment is in connection with some human diseases which have existed
from time immemorial, diseases the descriptions of which are to be found
given quite accurately in the most ancient documents, but in spite of
the fact that human germ-cells have been subject to the hostile
surroundings which such human diseases involve they themselves have not
changed to any great extent. That is to say they still produce a type of
embryo and offspring practically identical with that that always was
produced.
The same truth applies to the cells which make up the body of the embryo
and the individual, as well as to the germ-cells. The body-cells, those
which make up bone, and muscle, and gland, and so forth, are constantly
exposed to all sorts of influences which must tend to damage them so far
as it is possible for the cells to be damaged and still live. These
body-cells are sometimes starved, sometimes poisoned with alcohol and
drugs, frozen by extremes of temperature, over-worked by too much
physical strain, and so on, and if it were possible for such external
influences to change the type of cells of their offspring we should
expect to see it here. But it does not occur. The internal hereditary
tendencies of these cells are so strong, and so intimately bound up with
the life of the cells themselves, that when they divide and produce
others these others are precisely similar to the parent cells, in spite
of all the unfavourable environment in which they have been. Slight
variations do, of course, occur, but these are chiefly of a germinal or
spontaneous nature, and not due to the environment.
This thought gives us some vague and imperfect idea of how immensely
complex the constitution of germ-plasm must be. This germ-plasm is very
often subjected to all sorts of unfavourable conditions, especially
those of alcohol and toxins, and such conditions have been acting upon
it more or less for an immense number of generations, and yet the
resistance to modification at the hands of these internal factors is so
great that all the processes which follow upon the fertilisation of the
ovum, all the thousand complications which thereafter ensue in the
building up of the young embryo are hardly ever interfered with. When
they are markedly interfered with such interference generally involves
the death of the embryo.
The conclusion arrived at on this subject by Dr. Archdall Reid, after a
very careful and extensive inquiry into all the evidence from many
points of view, is stated by him as follows: "Though variations may
result from the direct action of the environment, such variations are,
in effect, always injuries, and are of rare occurrence in individuals
who survive and have offspring. Adaptation (_i.e._ evolution) depends
almost exclusively on spontaneous variations. These do not imply damage
to the germ-plasm, but are products of its vital activity. Occurring in
vast abundance all round the specific and parental means, they supply
the sole material for Natural Selection."
"We conceive the germ-plasm, then, as living and active, closely
adjusted to its environment, growing, dividing, varying, capable of
being destroyed and injured, but resisting death and injury, and within
limits capable of repairing damage and returning to its original
state--as behaving exactly as a living individual does."
CHAPTER IV
THE MAKING OF A MAN
Having in this brief preliminary consideration of the fundamental facts
upon which the science of Embryology is based cleared the ground as far
as possible, we may now summarise, in a few simple statements, the point
at which we have arrived in order that we may proceed at once to the
more detailed study of the actual development of the embryo itself.
We are in search of as clear a statement as possible of the origin of
the many and varied characteristics which go to the formation of a human
embryo, and hence to the making of an individual. The variation in these
many characteristics accounts for the differences in individualities. No
two individuals are exactly similar whatever be the standard by which we
estimate them. This is true morally, ethically, and physically. In each
of these spheres there are to be found good, bad, and indifferent
individuals, but whichever they are it is quite obvious that the result
has been brought about by the influence of all the factors of heredity
and environment acting upon the capacities which were originally
implanted in the germ-plasm. An individual is the resultant of the play
upon one man's-worth of human material of all forces which have acted,
or are acting, upon that kind and amount of material. Even though two
children of the same parents be brought up under what are to all
appearances identical circumstances, they differ from the very beginning
from each other and their parents. This is true even of physical
characteristics, and even more markedly in mental features. The fact
is--and it is one which is not sufficiently recognised--that the
formation of an individual from an embryo, the making of a man, is a
biological problem fundamentally.
The following are the principal facts which we have at this stage to
bear in mind.
All living creatures are made of cells, the physical basis of which is
protoplasm. The simplest creatures consist of one such mass of
protoplasm; higher organisms consist of more than one, and often of
millions, in which case they adhere together. Cells multiply by dividing
into two, the protoplasm of the mother-cell giving rise to that of the
daughter-cells. A human embryo, therefore, which is going to give rise
to an adult individual is a community consisting of an enormous number
of cells, the whole of which have descended from one common ancestor, a
single cell known as a fertilised ovum. True, these descendants break up
into many types of cells in order that different functions may be
performed by special tissues, but none of these special cells can do
everything that is necessary for the life of the whole individual; they
can only play their own special small part. They can do nothing towards
continuing the species of the individual. This duty, like others, is
imposed upon one particular group and kind of cells, namely, the
germ-cells, which do nothing else in the animal economy but furnish the
means for the continuity of the race. Although they lie within the
tissues of the embryo, and afterwards of the adult, they take no part in
the life of that embryo or adult. They undergo certain changes in
themselves which are to fit them for their ultimate destiny and
function, but they contribute nothing to the output of energy on the
part of the individual. When these are derived from a female they are
termed "ova"; when from a male they are termed "sperms." They themselves
are neither male nor female, they are merely protected and nourished by
the general mass of cells which constitutes the male and female
individual.
When a male germ-cell or sperm unites with a female germ-cell or ovum,
within the female body, fertilisation of the ovum takes place, and this
gives rise to the fertilised germ-cell from which is to arise first the
germ-cells or direct descendants of itself, and secondly the embryo in
which these germ-cells will come to lie. This happens by the repeated
and continued division of the fertilised germ-cell, a division which
constitutes growth, and which under suitable conditions of nourishment
and protection and exercise will ultimately produce a human being. The
great mass of the cells of this individual, the body or somatic cells,
take no part whatsoever in giving rise to the germ-cells of the next
generation. These are produced from the pre-existing germ-cells, and
from no other source, and it is for this reason alone that the phenomena
of heredity are possible and that one generation is directly continuous
with its predecessors. In fact heredity may be defined as the
relationship which exists between successive generations.
We therefore see that the embryo, or the individual, is formed from one,
and one only, of the first products of the division of the fertilised
germ-cell, the rest of these products forming the other body tissues.
This idea of the continuity of the germ-plasm is the greatest
contribution of modern embryological research. It is quite fundamental,
and no clear understanding of what is involved in the making of a man is
possible without it. It teaches us that the line of ancestry and
heredity is from one generation of germ-cells to another, directly, and
never through the individual from the embryo, which, indeed, is a mere
side product in the continued chain of events. The individual is
practically the trustee of the germ-cells, but not their maker. No
embryo, and no individual, ever made germ-cells; the latter existed
first. The object of the embryo is obviously to protect and nourish the
germ-cells which have been placed within it, so that they may be
available in due time for the production of further germ-cells, and so
for the continuity of the race. Hence it is the all-pervading truth of
natural selection that the interest and survival of the individual is
almost of no account; that of the species or the race being the
paramount consideration.
Once these facts be grasped there is no longer any difficulty in
understanding why the process of reproduction in any given species
always results in the formation of embryos which resemble each other in
all the main characters of their species. It could not be otherwise,
because they come from portions of identically similar material, a
common germ-plasm. In other words, the individual inherits nothing from
its parents. He merely receives in his turn the material inheritance in
the germ-plasm which was there a generation before him.
In so far as there has been no germinal variation he and they will be
similar. Hence the common observation that the child resembles the
parent. True, so he does; but not because he gets his characters from
them, but simply because he and they obtain their characteristics from a
common source. To many this thought will be, perhaps, a new one. It is
one of the most interpreting ideas which science has given us, and in
its absence no real grasp of the origin of the physical, mental, and
other characters if there be any, of the embryo can be understood. The
present writer has elsewhere summarised this thought as follows:--
"Man is composed partly of characteristics, which are derived from
pre-existing germ-cells, and over the possession of which he has no
control whatsoever. Be they good, bad, or indifferent, these
characteristics are his from his ancestry in virtue of his inheritance.
The possession of these characteristics is to him a matter of neither
blame nor praise, but of necessity. They are inevitable."
The embryo then which is to form the individual starts its career with a
certain number of innate germinal characteristics which manifest
themselves in the form of tendencies to grow in this direction or that.
During the period of gestation a good many of these tendencies are well
developed while a good many more only manifest their exact nature in
later life. But it is upon the basis of these tendencies--and upon no
other--that the making of the individual is possible. They represent the
total assets available for the formation of character. Nothing of any
new _kind_ can be added to them.
All that can be done under the best conceivable circumstances is so to
arrange the environment and surroundings of the embryo, and the
subsequent individual, so that these tendencies are acted upon in such a
way that the best are developed, and the worst eliminated. It must be
remembered that it is under the constant action of everything that
constitutes the environment of an embryo that the mass of body-cells
gradually grows into a recognisable human personality.
The question then arises, What are the factors external to the embryo
which cause these germinal tendencies to become active and fully
developed? These factors are those of (_a_) nourishment; (_b_) use, or
exercise; and (_c_) injury. In the case of the human embryo by far the
most important of these three factors is the first. A proper supply of
nourishment and food, that is to say maternal nutrition of adequate
quantity, is sufficient up to the time of birth to cause the inborn
tendencies in all the body-cells gradually to assume the special
characteristics of muscle, bone, gland, nerve, and so forth, which make
the human embryo. After the period of embryonic life is over, the
stimulus of nutrition is still sufficient for some of these body-cells.
Thus we find that the hair, the teeth, the internal ears, and the organs
of reproduction, all grow to their full development in the absence of
any other factor or stimulus than that of nutrition. But, as we also
know, this simple stimulus is not sufficient for most of the other body
tissues to develop properly. They require the additional stimulus of
exercise which, indeed, may be said to begin even in the life of the
embryo. After that it is quite hopeless to expect a healthy embryo to
develop into a fine child unless to the stimulus of nutrition there is
added that of exercise. It is from the varying quantities and qualities
of the three factors of nourishment, exercise, and injury, that part of
the explanation is found for the variation in individuals of the same
family. Starting with a good many of the same inborn tendencies none of
them afterwards receive quite the same kind and amount of these stimuli,
under the action of which they develop. And so we reach the second
point, namely, that, in addition to innate characters certain others are
subsequently acquired by the embryo of the individual in response to
particular stimuli acting from without.
Here we are upon ground which is more or less in our own choice or
control. It is impossible to alter germ-plasm; but it is not impossible
to control the environment in which it exists. To these two groups of
characters, the germinal and those acquired under stimulus, there is to
be added the third group which we have mentioned on a previous page,
namely, those that are usually termed variations. For example, one
occasionally finds that one individual in a family, the parents of
which, and the other members of which, are quite normal, may be born
with six fingers instead of five. Similarly one of a family may have a
variation in the direction of an extraordinary capacity for the
acquisition of knowledge of certain types. Hence the genius in music,
mathematics, memory, morality, and so forth. As we have seen, these
variations are termed spontaneous, to express the fact that we are at
present ignorant of the laws in accordance with which they arise,
though, of course, it is understood that those laws must exist.
We have now surveyed the whole field of the possible origin of the
characters of an embryo, and these may be summed up in the following
tabular statement.
{ A. Inborn Characters--
{ (_a_) Inherited (growing under the stimulus of
{ nutriment).
An Embryo is { (_b_) Variations.
made up of { B. Acquired Characters, obtained
{ (_a_) By nutrition.
{ (_b_) By use.
{ (_c_) By injury.
The differences in individuals with which we are also familiar, are due
to the varying proportions of the characters in the above table, and the
characters themselves are those which constitute all the possibilities
for any given person.
Should the reader doubt this, or be sceptical as to whether the whole of
the making of a man is contained in the above simple scheme, it would
not be difficult for him to convince himself that the statement is a
true one. Let him put down this book and take a sheet of paper and a
pencil. Rule the sheet of paper into three columns, and at the top of
each column place a heading as follows: Inherited; Acquired; and
Variations. Thus:--
+----------------+---------------+-----------------+
| Inherited. | Acquired. | Variations. |
+----------------+---------------+-----------------+
| | | |
| | | |
| | | |
+----------------+---------------+-----------------+
Let him then proceed to think of as many definite characteristics of his
own as he possibly can, and then enter these characteristics in the
column which he deems appropriate. It will be found that with the great
majority of characteristics no difficulty will be presented, and it will
be quite impossible to think of anything which is a physical part of
himself, which cannot be placed in one or other of these three
categories. Even though there may be a little difficulty as to which
column should claim the entry, it will be found that this is due rather
to indecision on the part of the reader than to anything else. It is not
because he can imagine any other origin for the trait which is for the
moment puzzling, but simply because he may be uncertain as to whether it
is an inborn character, or one due to the subsequent action of
circumstances. Thus he will have no difficulty in placing in the first
column such characteristics as the possession of one nose, two eyes, the
colour of his eyes, perhaps the shape of his nose, and so forth, these
all being germinal inherited characters. Equally simple is it to see
that in the second column must be placed such parts of his individuality
as speech, writing, the _size_ of his muscles at the moment, and so
forth. These obviously have resulted from the action of circumstances on
inborn capacities. No embryo can speak or write, though it has within it
the inherited capacity to enable it to learn such things. Finally he may
find it difficult to think of anything to place under the heading of
"Variations"; but, on the other hand, should he happen to be a genius in
music or mathematics, or the possessor of six toes or a black mole on
his arm, these will indicate to him at once that they are of the nature
of "Variations." (It must be remembered, however, that they may be
transmitted to successive generations, in which case they become
germinal characters.)
In a similar way if the reader desires to follow out this analysis of
the characters which make an embryo, and which, therefore, afterwards
comprise the possibilities of an individual from the point of view of
the stimuli under which they are developed, he may easily do so. Another
sheet of paper similarly divided into three columns with the headings
"Nutrition, Use, and Injury," will enable him to see how his individual
characteristics have attained their present development as the result of
one or other of these stimuli acting upon the germinal or inherited
tendencies. Without going into detail in this matter one may simply note
that under the heading of "Injury" will come all those parts of himself
of which he has become possessed as the result of disease or accident,
whether this be physical, mental, or moral.
Now we have completed what was necessary to arrive at our conclusion of
what it is that goes to the making of an embryo, and therefore of a
human being--a personality. The conclusion is that every characteristic
which it is possible for an individual under any circumstances
whatsoever to possess is traceable ultimately to the action which takes
place between his inherited tendencies and his natural environment. This
environment, whether it be physical, mental, moral, ethical, spiritual,
or whatever other can be imagined, can only produce the whole individual
by means of acting upon what is already present. To that material
nothing can be added _except in the environment_; from that material
nothing can be taken away; the most that can be done in this direction
is to hinder its growth by suitable procedures. Hence the truth of the
phrase that "education is nothing more than the giving or withholding of
opportunity." Hence it is so entirely true that it is impossible to make
a silk purse out of a sow's ear, or to gather grapes from thorns. The
importance of thoroughly realising these simple facts of embryology
should at this stage be obvious. They constitute possibly the most
important lesson which is demanding attention at the hands of modern
teachers, parents, and sociologists.
One further word before we leave this part of our subject. It is obvious
that of our total characteristics some are acquired and some are
inherited, and the question then arises, How much is inherited in an
embryo or individual, and therefore unavoidable, and how much acquired?
It would be beyond the scope of our subject in this place to enter into
detail in this matter, but it would not be right to pass the question by
without pointing out that a careful analysis of individual
characteristics will show that under the heading of "Inherited" will be
found principally the physical traits. When the reader comes to estimate
his mental and moral characteristics, a very few moments' careful
thought will prove most conclusively to him that these must be entered
up under the heading of "Acquired." If it were not so progress in those
directions would be practically hopeless. But plain as is this truth, it
is one which is far from being realised by many well-educated people.
CHAPTER V
FERTILISATION AND EARLY DEVELOPMENT
We may now turn our attention to the consideration of some of the
phenomena connected with the early processes in the development of the
embryo. We may assume that the eggs and sperms have reached such a stage
in their life history that they are now mature. All that is necessary in
order that the development of an embryo should result is that union of
the two elements should take place. Many complicated changes have
occurred in the constitution of these eggs and sperms before this stage
is reached, but into these we need not enter. It will suffice for our
purpose to assume that they are now mature. Then as the result of a
natural instinct which suggests certain thoughts and emotions to the
male and female animals, which in turn are followed by certain definite
acts, the sperm-cell from the male and the egg or ovum-cell from the
female are brought into contact. This contact takes place in such
circumstances that the united elements are able to be protected and
nourished and so, fertilisation having thus occurred, development
begins.
The characters of these two wonderful cells, which by their union
ultimately cause the production of an embryo, are briefly as follows.
The element from the male, the sperm that is, is an extremely minute
cell which is only about 1/300 of an inch in length. As seen under a
high power of the microscope it is composed of two portions which are
spoken of as a head and the tail. The former is a flat, oval part, and
behind this is the rounded body ending in the long tail which is some
four-fifths of the total length. This long tapering tail gives to the
sperm its power of movement, for it is supposed that as the result of
the rotating or lashing movements of this tail the cell is propelled.
Indeed its rate of motion has been actually studied, and estimated to be
at about one-eighth of an inch per minute.
The cell contributed by the female, the ovum that is, has quite a
different structure and microscopical appearance. Compared with most
cells it is rather large, almost round in shape, having a diameter of
about 1/120 of an inch. Up to the time we are now considering, this
cell, along with a great many others like it, has been stored within the
female ovary, from which organ an ovum periodically escapes. Unless
fertilisation takes place by union with a sperm the discharged ovum
perishes. Should, however, the sperm-cell be available, and should it
have been able to reach a situation at which fertilisation can take
place within, the chain of events which constitute development begins.
But before fertilisation can take place the ovum has undergone what is
called the process of maturation, in which it divides twice, giving off
two small portions of itself in the process. The result of this is that
half the number of chromosomes in the ovum are lost. This process of
maturation has already taken place in the sperm before it leaves the
body of the male.
When these two cells meet, the actual fusion of their material takes
place, the head of the sperm penetrating into the substance of the ovum,
and the body of the sperm completely fusing with the nucleus of the
ovum. This gives rise to what is called the "segmentation nucleus." It
will be observed that we now have a cell in which the full number of
chromosomes for that particular species is represented once more. But
this full number has now been made up from two different sources, half
from the elements contributed from the male, and half from those of the
female. It is at this stage that the inherited tendencies, carried in
the germ-plasm on the two sides of the ancestry, become mingled, and
from thenceforward the division of the fertilised cell into many
cell-descendants goes on with extreme rapidity.
Two different lines of germ-plasm have thus been intimately mingled, and
the actual significance of this mingling has given rise to one of the
most acutely debated points in all the problems of heredity. Put into
quite plain language that problem is--What is the function of sex? It is
no part of our task here to answer that problem, but it is of interest
to point out precisely at what stage it occurs in embryology. The
obvious answer, however, may be advanced that the function of sex is to
mix the characters of the parents in such a way that some from each
source will be found in the offspring. But how these are mixed, whether
as painters mix two colours and produce a third, or as two packs of
cards are mixed having different coloured backs, is quite another
matter.
The fertilised ovum now commences to form a number of successive
generations of cells, and this it does by dividing into two, four,
eight, sixteen, thirty-two, and so forth, until a number of cells have
been produced which arrange themselves into the form of a ball. The
surface of this ball resembles that of a mulberry, each elevation
corresponding to a cell. This mass is termed by embryologists "the
morula." (See Fig. 1.)
[Illustration: FIG. 1.]
Next, within this morula some of the cells become condensed into one
particular portion, leaving a space which contains fluid. The ball is
now no longer solid, but has a portion consisting of cells, and a
portion consisting of fluid. It is now called a "blastocyst." (See Fig.
2.)
[Illustration: FIG. 2.]
The cross-section of this shows the cells projecting into a cavity. This
is the first attempt of the fertilised ovum to form itself into the
different layers, which are ultimately going to give rise to all the
different tissues of the embryo. But it is interesting to know at this
stage that the outer layer of cells, those representing a margin in the
figure, has nothing to do with the forming of the embryo at all, but
gives rise to a structure whose function afterwards is to be that of
nourishing the growing embryo.
The next obvious change is that the cells at the lower portion of the
mass which projects into the cavity appear to get flattened out--at any
rate they obviously arrange themselves in a definite and separate layer;
and this layer in its turn proceeds to go on growing by division of its
cells in such a way as to form another little closed cavity within the
larger one. This cavity is termed the "yolk sac." (See Fig. 3.) Then
another little cavity occurs, this time within the original projecting
cell-mass. This cavity is termed by embryologists the "amniotic cavity,"
and the cells which line it, and which in their turn become arranged as
a separate layer, form what is termed the "embryonic ectoderm." (See
Fig. 3.)
[Illustration: FIG. 3.]
It is in this region, and in that of the yolk sac which lies just
underneath it, that the future growth of the embryo itself occurs, and
the portion is therefore termed the "embryonic area." (See Fig. 3.)
Up to this point we have seen that two layers of cells have appeared,
one round the yolk sac, called the "entoderm," and the other lining the
amnion, called the "ectoderm." After these two germinal layers have made
their appearance, a third layer comes into existence, which, because it
begins growing from the embryonic area, and lies between the two already
mentioned, has received the name of the "mesoderm." This third germinal
layer divides into two portions before very long, and the space between
these two is that in which the body cavity itself subsequently arises.
One part of the mesoderm, situated near one end of the embryonic area,
is specially important, because in it are formed the blood-vessels which
supply the embryo, and which ultimately afterwards becomes the
"umbilical cord," which forms the connection between embryo and mother.
CHAPTER VI
EARLY DEVELOPMENT
The early development of the embryo now proceeds rapidly, and its
appearance at the stage we have just been describing is thus stated by
Dr. R. W. Johnstone:--
"If the ovum at this stage be looked at from above, the embryonic area
appears as a small shaded oval. The shading is due to an increased
growth of cells, because here the three germinal layers--embryonic
ectoderm, mesoderm, and entoderm--are in contact. At one end a patch of
darker shading indicates a still greater growth of cells. Running
forward from this is a band--the _primitive streak_--in the centre of
which lies a darker line--the _primitive groove_. At the far (anterior)
end of the primitive groove there is a dark spot--Hensen's node--from
which still another streak runs forward, the head process. Later, in
front of the primitive streak, a thickened band of ectoderm appears,
broadening out posteriorly. The edges of this band rise up to form two
folds, which meet anteriorly. The groove between them is the _medullary
groove_, and ultimately they fold over and unite to form the _neural
canal_. (See Fig. 4.)
"Along the line of the primitive streak all three germinal layers are in
contact. Superficial to it is the amnion, and below it is the yolk sac.
The embryonic area is the only part of the ovum which has to do with the
subsequent development of the embryo; the other parts of the
blastodermic vesicle become subservient as nutritive or supporting
structures.
"At this stage, and for the first three weeks of its existence, the
embryo is a 'flat disc floating on the surface of the yolk sac.'
(M'Murrich.)"
[Illustration: FIG. 4.]
This is followed by a folding of the embryo, due to the enlarging of the
amniotic cavity, the result being to form what may be termed a
"head-fold" and a "tail-fold." A further fold, however, occurs at the
sides which bend in, so that the whole embryonic mass at this stage
comes to form an incomplete tube, the incomplete portion being the lower
aspect of that tube. This remains open. In due time this lower, or
ventral portion, becomes completely closed, except just at one point.
This point is where the communication exists between the inside of the
tube, which is the embryo, and the yolk sac. A part of the yolk sac is
thus included in the embryo itself, and this has an important bearing
upon future development, because in the course of time this part comes
to be the alimentary tract of the growing embryo. The canal which joins
the yolk sac to the internal gut of the embryo (the vitelline duct)
ultimately forms, together with part of the yolk sac, the umbilical
cord. This cord, which at the time of birth is artificially severed in
order to free the fully developed embryo, is at this stage connected to
the hinder part of the body of the embryo. As the latter grows, however,
it elongates still more behind, in what we should regard as the tail
region in animals which had a well-marked tail. As a matter of fact, at
a little later stage than this there is quite a conspicuous tail in the
human embryo, which, however, comes to be embedded in the tissues later
on, and so never forms any external appendage.
So that at this stage we have the embryo representing a mass of cells
which have gradually arranged themselves, and been arranged, in the form
of a tube more or less bent, and attached near its hinder end to the
tissues which are afterwards to represent the umbilical cord.
We have neglected to describe the organs and structures which are
developed after fertilisation as a further means of protecting the
developing embryo. We have done this of set purpose, because these
structures--known as the "trophoblast"--require a considerable amount of
technical knowledge to understand. Any detailed description of them,
therefore, would be out of place here. All that is necessary for us to
say is that they are intended to serve as a means of nutrition for the
developing embryo, and take no part in the actual formation of its cells
and organs. One portion of it, however, has another function which may
be mentioned. It secretes, it is supposed, a kind of ferment which has
the power of dissolving or digesting other cells, and this is of great
importance at one stage of development--namely, when the fertilised ovum
comes to reach the womb, or uterus, in which it is to pass the rest of
its developing stage. It is believed that some of the cells in the wall
of the uterus are dissolved and digested immediately round the ovum
itself, which thus comes to lie in a cavity in the uterine wall. This
process being carried still further allows the ovum to sink deeper and
deeper into the lining membrane of the uterus. Ultimately the point of
entrance, where the cells were digested, is closed up by the formation
of a clot of blood poured out at that spot, and which thus entirely
covers in the ovum. The latter now comes to lie absolutely embedded in
the wall of the uterus in a cavity which it has itself formed. It does
not, however, occupy the whole of the cavity, but is surrounded by blood
which is escaping from the minute blood-vessels of the wall in which the
cavity has been made. This blood is, of course, the maternal blood.
"Thus we have the ovum completely embedded, lying free in a tiny cavity
in the mucous membrane lining the uterus--a cavity full of blood, in
which the ovum lies bathed, and from which it presumably absorbs
nourishment by osmosis through its trophoblast." (R. W. Johnstone.)
The uterine wall, after this embedding of the ovum within it, undergoes
a remarkable growth at this position, concerning which a word must be
said. Under normal conditions this wall is smooth, or nearly so, but
probably there are upon it some slight irregularities or projections
which are sufficient to catch the ovum when it enters the uterine
cavity. Apparently it may be arrested in this way at any part of the
wall, and at that spot it becomes embedded in the manner we have
described above. The lining membrane of the uterus under ordinary
conditions measures about one-eighth of an inch in thickness, but, after
the ovum has become embedded in it, it begins to increase until it
reaches as much as half an inch. Underneath this lining membrane lies
the muscular part of the uterine wall. The ovum itself is embedded about
the middle depth of the lining membrane, but as it continues to grow,
and increases in size and dimensions it projects more and more into the
uterine cavity, that being the direction of least resistance. Before
very long the embryo, as it now is, has reached such a size in its
growth that it entirely fills the cavity of the uterus. This stage is
reached after the third month of gestation.
Another structure, concerning which just a word must be said, is that
known as the "placenta," or more commonly as the "after-birth." We need
only say that this is first developed by means of a number of little
outgrowths by means of which the early embryo is attached to the wall of
the cavity in which it lies. These outgrowths grow into the uterine
tissue around the ovum, and they allow of blood circulating between
them. They have, as a matter of fact, two distinct functions to
perform--first, that of fixing the ovum in position, and, secondly, they
allow of the maternal blood circulating in the spaces between them, and
it is from this blood that the embryo derives its nourishment. The
blood-vessels ultimately connect with those of the umbilicus, and thence
reach the embryo. This organ, the placenta, at the time the embryo is
fully developed at birth, is a round structure about nine inches across,
and not quite an inch thick in its middle, becoming thinner towards the
edges. The surface of it next to the infant is smooth and shiny, beneath
which it is rough, that next to the maternal structures being
dark-coloured, somewhat like flesh. When the child is born, the severing
of the umbilical cord allows the placenta to remain behind in the
uterine cavity, whence it is usually expelled shortly afterwards.
Should, however, this not be done, and the embryo and the placenta be
born together, the child is said to be "born with a caul," an event
which has given rise to many superstitions.
The foregoing description of the principal events in the development of
the embryo will be sufficient for our purpose here. Further details on
the subject would necessitate a considerable knowledge of physiology and
anatomy, and those readers who desire to study the details of the
subject further may do so in any of the various works referred to in the
bibliography appended to this book.
CHAPTER VII
THE BEGINNINGS OF THINGS
We may next turn our attention to the developing embryo at a very early
stage, and note from which parts of its growing cells the different
structures are ultimately developed, remembering all the while that all
the subsequent division into specialised tissues is the result of the
inherent possibilities in one single fertilised germ-cell.
It will be remembered that, as the result of the subdivision of the
fertilised germ-cell, we had the formation of three distinct layers of
cells. These layers we saw were termed the germinal layers, and were
named respectively the "ectoderm," the "entoderm," and the
"mesoderm"--the last appearing between the two former. It is from these
three germinal layers that all the subsequent structures of the body
take their origin, and although we cannot attempt to follow out in
detail the growth of all these special tissues, it will, nevertheless,
be of interest to note, in the briefest possible way, from which portion
of the embryo they subsequently arise. Some of these we may afterwards
note in detail. The total result may be summarised by simply giving a
list of the various tissues, and the corresponding embryonic layer from
which they come. Thus:--
A. From the ectoderm arise the following structures:--
Epidermis or skin.
The hair.
Various glands.
The lens of the eye.
The whole nervous system.
The nerve parts of the sense organs.
The membrane of the mouth.
The enamel part of teeth.
The membrane of the nose.
The lower part of the bowel.
B. From the mesoderm arise the following structures:--
The connective tissues of the body.
The bones.
The teeth, except the enamel.
All the muscles.
All the blood-vessels of the circulation.
All the vessels which carry lymph.
The membranes of the heart, lungs, and bowels.
The kidneys and their ducts.
The reproductive organs.
The blood itself.
The fat and the marrow.
C. From the third layer of the embryo, the entoderm, arises:--
The lining of the alimentary tract.
The lining of the larynx.
The lining of the trachea and lungs.
The cells of the liver, the pancreas, the thyroid, and thymus.
The structure termed the notochord.
From the above very brief summary we see that the body of the
individual, with all its component tissues and parts, can be divided, as
regards its origin, into three groups according as to which embryonic
layer was concerned in its development. Moreover, if these three groups
be scrutinised a little more carefully, they will be seen to differ very
markedly from each other in the structures and tissues which are derived
from them. Thus the structures from the entoderm (see C) are practically
either in the nature of glands, or the lining of the alimentary tract.
Those tissues coming from the mesoderm (see B), on the other hand,
comprise most of what may be termed the supporting tissues of the body,
such as the bones and the muscles and ligaments, as well as the vessels
which constitute the great circulation of the blood and lymph. But
perhaps the most remarkable of all is the list of structures which take
their origin from the ectoderm of the embryo (see A). In this list will
be found the most important structures in the whole human body, as well
as some of those which are apparently of far less serious importance. It
is rather surprising to find, for example, that the whole of the nervous
system, including the brain and spinal cord, and the organs of special
sensation, should be derived from the same layer of cells as gives rise
to the very simple cells of the skin, which serve merely as a protective
covering to the other tissues. It is curious also to observe that in
addition to brain and skin, parts of the teeth also arise from this
external layer. Evidently then this ectoderm or outer layer is of the
very greatest importance in embryology, since from it arise all those
parts of the embryo itself which are the most important in its future
life.
CHAPTER VIII
THE BEGINNINGS OF THINGS (_continued_)
We have now considered, as far as is compatible with the character of a
work of this kind, the beginning and development of the embryo taken as
a whole, and for the remaining part of our study of this subject we may
devote our attention to the beginnings of some of the more important
organs and functions in the new individual. It will be impossible to
deal in detail with all the important parts which ultimately constitute
the new personality, but a selection may be made which will give some
general idea of how great results spring from very small beginnings.
What will be said here it may be hoped will be just sufficient to
stimulate the interest of those to whom the subject appeals, and who may
then turn their attention to some of the larger works which go into
greater detail in this subject, a list of which will be found in the
bibliography at the end of this volume.
It must be remembered that quite a large number of the characteristics
that we usually associate with a normal human being only come into
existence, or at any rate only become obvious, at some period longer or
shorter after birth. True, these characteristics depend for their
ultimate appearance upon the development of the corresponding structures
and organs in the growing embryo, but in the case of some of these,
those organs are not fully developed in embryonic life, and the
manifestation of the functions associated with them may be delayed
perhaps for years. This is notably the case, for example, with the
reproductive organs which, though developed during the life of the
embryo, remain functionless until the period of adolescence. The
development of the human mind and intellect too, although depending, of
course, upon the embryonic growth of the brain and the nervous system
generally, is a matter of time and the environment subsequent to birth.
It should be realised, however, in this connection, that the mind of the
new individual, and all that is involved in that term, dates back
ultimately, as regards its possibilities, to the moment at which the two
germ-cells from the male and female respectively united in
fertilisation. The adult mind develops from the mind of the infant. The
infant mind appears as the result of the possibilities and the
tendencies which were inherent in the germ-cells from which not merely
the brain but the whole embryo sprang; in other words, all that a single
human mind connotes results from the possibilities in a single cell.
Such a thought is a startling one indeed, and at first sight appears,
perhaps, somewhat incredible. But a moment's careful attention to the
problem will show at once that it is in reality no more wonderful than
the fact that this single cell produces all the millions of other cells
which in due time give rise to the skin, bone, nerve, blood, and so
forth, which make up the entire body of the embryo. The human mind,
therefore--and indeed the human soul, if that term be used in any
intelligible sense--takes its origin in the products of the
multiplications of germ-cells acted upon by their subsequent
surroundings.[1]
[1] The detailed study of this part of the subject is dealt
with in the writer's work, _The Greatest Life_ (Duckworth, London),
to which readers who are interested in this phase of the subject are
referred.
With this passing reference to the fact that some important parts of an
individual only grow to their full manifestations after embryonic life,
we may pass to the consideration of the development of some of the more
interesting parts of the embryo itself.
Amongst the most striking, and certainly the most interesting, of the
various parts of the developing embryo, those which go to form the
special senses are prominent. They are interesting not merely from their
actual mode of growth, but especially also in connection with their
evolutionary history. The study of how they have come into their present
state in the higher animals leads us back to very small
beginnings--indeed, to the time when there was no such thing as special
sense organs for sight, hearing, smell, and so forth, but where the
organism had what may be termed a diffused tactual sense over and
throughout the entire body. In the course of time this diffused general
sense became specialised, no better example of which could be quoted
than that of the sense of sight, which was referred to, as many of our
readers will doubtless remember, in Tyndall's famous Belfast address. He
was referring to Herbert Spencer's theory of the manner in which vision
was evolved. He pointed out that, as above noted, in the lowest
organisms sensation is a general thing diffused throughout the body, a
kind of general tactual sense. As the result of environment, and gradual
adaptation to external influences, certain parts of the general surface
of the organism became more responsive to these external stimuli than
other parts. These areas, being those points at which sensation was most
acutely felt, were nothing more or less than primitive sense organs.
Thus in the progress of evolution the stimulus of the eye gradually
became most pronounced in certain cells which contain pigment, these
cells being more responsive to the light stimulus than the rest of the
body. That was the beginning of an eye; a group of cells more receptive,
more easily influenced by light, than any other cells. In a slightly
higher stage of evolution we find a special overgrowth of the skin which
covers over the area in which these pigmented cells lie, obviously a
protective measure on behalf of the specialised cells referred to. Then,
still later, a lens is added, and the whole organ becomes more and more
adapted to the necessities of the case, until it reaches the
extraordinary perfection that is seen in the eye of such a bird as an
eagle. On the same general principle, the other special senses also took
their origin from this general diffused tactual sense, certain cells
becoming specially adapted for receiving the stimulus of sound, others
for taste, others for smell, and so forth.
CHAPTER IX
THE BEGINNINGS OF THINGS (_continued_)
It is not necessary to describe in detail the beginnings of all the
various structures which arise from that important layer of cells in the
embryo which is termed the ectoderm; but since it gives rise to that
part of the embryo, which eminently places man in the first place in the
world of animals, we may select it for a little further description. We
may leave out of account the beginnings of the skin and the glands, and
some other parts, and look for the moment at the origin of the nervous
system, which includes the brain, the spinal cord, and the whole nervous
mechanism of the individual. Since man's prominence depends upon the
wonderful capacities in his nervous system, it is all the more
interesting to note from what small and simple beginnings it has arisen.
As we have already seen, at a very early stage in the development of the
embryo, a folding of its cells takes place, so that the upper embryonic
area assumes the character of a groove. We may confine our attention to
this groove for the moment, leaving out of account the other two layers
of the embryo--namely, the mesoderm and the entoderm. It is this groove,
which thus early makes its appearance, which is subsequently to play
such a tremendous part in the formation of the most important
structures. It is called the "medullary groove." As growth proceeds and
the cells continue to multiply and increase in numbers, the two edges or
lips of the groove gradually approximate, and ultimately fuse together.
Obviously the effect of this is to transform what was the groove into a
closed cavity or canal, which is therefore now termed the medullary
canal. Arising in this simple manner, this equally simple structure is
destined to become the central canal of the spinal cord, and the
cavities in the brain, known as the ventricles. The walls of this canal,
be it remembered, are composed of cells of the layer of ectoderm, and it
is these cells which, as we saw, appeared very early in the development
of the embryo that are now to proceed to develop into the brain, spinal
cord, and, in fact, the whole central nervous system. At first the cells
appear all similar, but, as development goes on, they begin to
differentiate themselves into different kinds, some forming the actual
nervous cells of the brain and spinal cord, others developing into
protective structures.
The hinder or posterior part of this medullary groove and canal is
narrower than the anterior portion. This posterior narrower part is that
which gives rise to the spinal cord. It very soon changes its character
by the appearance of a number of constrictions at intervals running
along its whole length. It becomes, as it is termed, segmented. A little
later these successive segments are seen to correspond to the pairs of
spinal nerves which arise from the cord. For the first part of embryonic
life the developing spinal cord is of the same length as the canal, but
as time goes on the canal grows longer than the cord. This involves the
nerves coming from the hinder portion growing longer than others. It is
the front or anterior portion of this medullary canal which is concerned
in the development of the brain itself, and here, at an early stage, two
very obvious constrictions appear in the region of what is to be the
brain, and these constrictions divide that brain area into three
distinct parts, or vesicles. Part of the posterior vesicle ultimately
develops into the _cerebellum_, or little brain. Another part forms the
_medulla oblongata_, that important hind brain in which lie so many of
the vital centres of nervous energy. The central cavity formed by these
constrictions is of comparatively less importance, forming ultimately
what is known as the _mid-brain_. The foremost or anterior vesicle,
however, is of the very greatest importance, and its subsequent changes
are more marked than either of the other two. From it is developed the
great mass of the cerebrum itself, together with various outgrowths from
it which have most important functions. Thus two of these outgrowths
appear projecting from the lower part of the sides of the walls, and
ultimately coming to reach the outer ectoderm. These two projections, or
pouches, ultimately form the optic vesicle. Still later in development
the whole of the anterior vesicle is again constricted, thus forming two
distinct parts, the foremost of which, growing rapidly in two halves on
either side of the middle line, ultimately give rise to the two cerebral
hemispheres. These two cerebral hemispheres, therefore, arise, in the
first place, as lateral enlargements of the anterior part of one of the
primitive constrictions of the medullary canal. In their outer layers
cells continue to make their appearance with great rapidity, and thus
is formed the cerebral cortex; and the remarkable thing about this
all-important part of the brain itself is that all the cells of this
cerebral cortex appear to be produced during the life of the embryo;
there being in all probability none added after birth has occurred. That
is to say, the possibilities of the actual physical growth of brain
tissue in any given embryo are fixed from the beginning. Brain tissue,
in other words, is born, not made. It is the manner in which it is
treated afterwards upon which depends whether that given quantity of
brain-cells displays its best potentialities or not.
[Illustration: FIG. 5.--Diagram of brain at an early stage, showing the
origin of the olfactory lobe, the optic vesicle, the cerebellum, the
cerebrum, the medulla, and the spinal cord (after Martin).]
We have seen that the optic structures are concerned with this front
portion of the developing brain. The same is true of the organs which
are concerned with the special sense of smell; for about the fourth week
of the life of a human embryo there appears on the under surface of each
of the cerebral hemispheres, towards the front, a prolongation which
becomes the olfactory lobes.
It is well known that the surface of the brain of an adult human being,
or, indeed, of any of the higher vertebrates, shows upon its surface a
number of convolutions, and it is generally recognised, from a study of
the comparison of different vertebrate brains, that the more convoluted
is the surface of the adult brain the more highly developed is the
animal concerned, from the point of view of brain power. The surface of
the cerebral hemispheres, however, is quite smooth for some months of
embryonic life, and the depressions which give rise to the appearance of
the convolutions do not show themselves until about the fifth month, at
which stage the brain is relatively large.
We referred on a previous page to the origin in evolution of visual
sensation, and it may be of interest here to note a little more fully
the beginnings of the eye itself in the embryo. As has been said, the
very first appearance of these organs takes the form of a pair of
outgrowths, or processes, which are hollow, from the front part of the
anterior vesicle of the brain. These grow until they reach the ectoderm.
A remarkable change then takes place. The portion of the hollow vesicle
which reaches the outermost embryonic layer becomes folded in upon
itself so as to form a cup with a double wall; just as one might form a
cup in a blown-up paper bag by forcibly pressing one portion of it into
the other. This double-walled cup is of special interest, because from
its walls is ultimately developed that very important structure in
connection with sight, namely, the retina. As soon as this is completed
cells begin to grow from it towards the brain in the form of nerve
fibres, and these in time convert what was originally a hollow process
or growth into a solid mass of nerve tissue. This mass is the optic
nerve. Thus is completed the connection between the outer surface of the
eye and the brain itself, which is to receive the sensation. Then the
ectoderm on the surface over the cup begins to thicken, grows into the
cup itself, and ultimately forms a rounded hollow mass which we
afterwards recognise as the _lens_ of the eye. Still later this becomes
separated from the surface by another layer of cells constituting the
_cornea_, and outside that again is still another layer which makes the
_conjunctiva_.
Subsequently the contents of this cup become filled up from other
sources with a soft gelatinous tissue. Then the eyelids in time make
their appearance in the shape of folds of skin growing over the eye,
and remaining in contact until very shortly before birth occurs. And so
we see that from this wonderful layer of ectoderm there comes gradually
into existence not only the brain itself and the spinal cord, with all
the nerves, but also the special sense organs of sight and smell.
CHAPTER X
THE BEGINNINGS OF THINGS (_continued_)
Without entering into the description of the development of the whole
circulatory system, we may just mention briefly the origin of the heart
itself, which begins at a very early stage by the appearance of a small
body of cells, which come to arrange themselves in a tubular form
enclosed in the mesoderm. The two halves of this tube are at first quite
separate from each other, but gradually come together and finally unite
into a single tube with walls. The folds of these ultimately form the
heart muscle. The organ, at this stage of its development, does not lie
within the region of the chest cavity, as it afterwards does, but more
anteriorly in the region of the neck. The simple tubular arrangement,
however, is quite a passing phase, and as the tube increases in length
it becomes bent upon itself, somewhat in the form of the letter S. One
end of it now enlarges and forms a pouch on each side, these forming the
two auricles, right and left. From these auricles a partition grows
vertically, and when complete, cuts them off from each other, except
that a communication is left in the upper part (_the foramen ovale_)
which closes up after birth. This partition allows of the blood from one
side of the heart passing to the other. Another partition eventually
divides off that portion which has formed the auricles from the
remaining portion which develops into the ventricles, which in their
turn become again divided by a still further partition. In this way the
heart which, in the first place, was a simple tube, grows ultimately
into an organ with four distinct chambers, two auricles and two
ventricles, the only difference from this and the adult heart being the
communication which exists through the partition separating the two
auricles.
The development of the organ of hearing is somewhat complicated. The
first part to appear is a portion of the inner ear, which shows itself
as a round, hollowing of the ectoderm. This depression becomes deeper
and sinks further in, while its floor becomes thicker, and finally the
whole assumes the shape of a closed cavity. An outgrowth from this gives
rise to the _cochlea_. The cavity becomes divided into two portions, in
one of which the _semicircular canals_ arise. Around the whole, the
embryonic tissue has been forming into a strong protective covering,
some of which finally becomes cartilage, and some bone. The middle
portion of the ear is the remains of a cleft in the side of the embryo.
This cleft becomes changed into a canal by the closing of its edges, the
upper part ultimately forming the _tympanic cavity_, and the rest of it
remaining as the _Eustachian canal_. This canal opens into the pharynx.
In the cavity there are subsequently developed three small bones which
play an important part in the process of hearing. After the birth of the
embryo, air reaches the tympanic cavity, which then enlarges. One of the
walls of this cavity persists as the tympanic membrane or drum. Finally
the outer ear, that portion which is popularly spoken of as _the_ ear,
is formed from the upper portion of the same cleft which gave rise to
the tube of the tympanum.
We have referred in the preceding description to the origin of some
embryonic structures from a cleft in the early embryo itself. As a
matter of fact, no less than four of these clefts, or fissures, appear
in the region of the neck on each side, and are of the very greatest
interest and importance in connection with embryology. They are termed
the "branchial clefts," and are seen in the embryos of all vertebrates.
In the human embryo there are four. They are situated on each side of
the pharynx, and they correspond to the gill slits in lower vertebrates.
Amongst other structures which arise from the important layer of
ectoderm are the teeth. Of these there are during life two sets, a
temporary and a permanent. The temporary teeth, though they do not make
their appearance till after the birth of the embryo, still are partly
developed during embryonic life, lying embedded in the tissues until the
familiar process known as "cutting the teeth" takes place. This is, of
course, merely the time of their external appearance. The first stage in
the development, however, is a thickening of the epithelium of the gums
in a direction which is to correspond with the line where the teeth will
eventually pierce. This thickening is called the "dental ridge." This
grows downwards into the underlying tissue in flask-shaped growths. From
the neck of each of these flasks there is a small projection which
indicates where the permanent teeth will ultimately be. This first stage
is termed that of the _enamel germ_. This becomes surrounded by cells
which ultimately form a _dental sac_. Next, tissue from below grows
into the flask, and the further growth of this gives rise to the _enamel
organ_. Finally, enamel itself and dentine are developed, and the
embryonic tooth remains covered under the gums until it cuts them.
So far we have considered merely the mode of development of the most
important organs of the body, but we have said nothing of the most
important supporting structure, namely, the skeleton. The earliest
appearance of anything in the shape of a skeleton is the structure known
as the notochord, a structure of immense importance and interest in the
embryology of all vertebrate animals, in which it is a temporary thing
only. The first appearance of this notochord in lower animals is the
earliest indication of the vertebrate type, because it is found that in
the higher vertebrates it is the forerunner of the bony spinal column
and the skull. It appears first as a groove underneath the medullary
groove, of which we have already spoken, and its two lips unite to form
a cavity, as did those of the medullary groove. In this case, however,
the groove becomes a solid rod, then termed the notochord, and it lies
immediately under the medullary groove itself, which, as we have seen,
gives rise to the central nervous system. In the course of development,
masses of cells come to arrange themselves on each side of the
notochord, which they eventually include, and at the same time they grow
upwards and around the spinal cord which is thus enclosed. Later on
these surrounding portions become cartilage, and, still later, bone; the
notochord meanwhile gradually disappearing where the bony spinal column
appears. This primitive vertebrate structure therefore, of the
notochord, has the all-important function of coming to enclose, and
thus protect, the spinal cord and nervous system.
As regards the other bones of the body, all that need be said here is
that they are preceded by the structure which we know as cartilage, and
in the bones of the limbs at two or three different points this
cartilage begins to be transformed into bone. These points are known as
centres of ossification.
CHAPTER XI
HOW THE EMBRYO IS NOURISHED
Having noted how the embryo itself takes its origin, and then studied
something of the beginnings of some of its most important parts, we may
now very briefly refer to the subject of its own nourishment. This has
more than a mere academic interest, because obviously the proper growth
and development of all the various tissues and structures in the embryo
must depend ultimately upon the nourishment with which they are
supplied. Their own inherent characters cause them to divide and
subdivide so as to give rise to the millions of cells which are required
to make the body, but these cells, in their turn, are dependent upon
outside sources for the nourishment which enables them to keep on
growing, or to maintain their full growth when they have arrived at that
stage.
Nature has made many varied arrangements for this nutrition during
embryonic life in different classes of animals. In some a considerable
quantity of yolk is so arranged with reference to the embryo that the
latter can draw upon it for some time for its supplies. This is the
case, of course, in birds, and in some reptiles. We need here, however,
only consider the case of the human embryo.
Three sets of structures are concerned in human embryonic nourishment,
namely, the Allantois, the Villi of the Chorion, and the Placenta.
The Allantois is developed in the form of a hollow bud from the
posterior part of the primitive alimentary canal, and ultimately comes
to form the umbilical cord, and the embryonic part of the placenta. It
is this structure, the allantois, which allows at a very early date of
the embryo establishing a blood-connection with the maternal tissues,
and hence it plays a very important part in the transmission of
nourishment to the embryo. Not only does it do this, but it allows of
the removal of waste products.
The villi of the chorion are outgrowths by means of which the very early
embryo attaches itself to the walls of the cavity, which it has made for
itself in the wall of the uterus. As they grow larger, these villi push
their way into many of the small blood-vessels in the uterine wall, and
so come to lie actually in a mass of blood from which they abstract the
elements of nutrition. At first the villi themselves contain no
blood-vessels. Nourishment passes through them by a simple process of
osmosis. Later on, vessels grow into the villi themselves. The nutriment
supply is secretion, in the first place, of the uterine glands, which
these villi easily absorb. This process takes place during the first two
or three months of embryonic life. At the end of this time most of the
villi disappear, and the few that remain take part in forming the
f[oe]tal or embryonic portion of the placenta.
After the third month the embryo is nourished by the placenta itself,
which is at this stage developed. As we have seen, it arises partly
from the villi of the chorion, which is its embryonic portion. The other
part of it is maternal in origin, arising from the portion of the
uterine wall which is immediately over the embryo. The connection
between this structure, the placenta and the embryo, is constituted by
means of the umbilical cord. The function of the placenta is partly to
supply nutrition, partly to serve as an organ of respiration for the
embryo, whose lungs are, of course, not functional, and partly it acts
in the same way as the kidney does in after life, by excreting certain
products. From the placenta the embryo derives those food elements at
first provided by the secretion of the uterine glands. Afterwards these
elements are supplied by cells which lie between the f[oe]tal villi
and the blood of the mother. Its respiratory function consists in
allowing oxygen and carbonic acid gas to pass by osmosis between the
embryonic and the maternal blood. The process is exactly analogous to
that which takes place between the gills of a fish and the water in
which the fish lies. Of course, it will be easily understood that there
is as yet no great need for a large supply of oxygen, because the embryo
is merely growing, and not using its various organs.
It should be clearly understood that under ordinary conditions of
embryonic life there is no direct mixture of the blood of the mother and
that of the developing embryo. All the processes which contribute to its
growth and maintenance, including those of respiration and excretion,
take place through the intermediate structures above mentioned. This is
an extremely important point, because it means--and evidently that is
the object of the arrangement--that there may be much of an injurious
character in the blood of the mother which never reaches the embryonic
tissues at all. Doubtless the cells which form the organs of nutrition
for the embryo have a capacity for selecting the elements required for
purposes of nutrition. It is their business to look after this process.
How perfectly it is performed can at once be understood when we
recollect how very frequently the tissues of the mother herself are in
anything but perfect health, and yet the embryo is born healthy. Were it
not for this intermediary process, the embryo could hardly help being
poisoned or otherwise injured by all the varied unhealthy products and
substances which the ignorance of some mothers allows to be present in
their blood during this important period. Even with this means of
protection, the maternal blood may be so utterly deficient in nutritive
qualities, or so actively injurious from saturation with alcohol, or
from some equally toxic substance, that the fluids which reach the
embryonic cells may be very much impaired in quality. Nevertheless, it
is astonishing how much danger can be avoided in this way by Nature's
provision in the method of nourishing the embryo.
If the development and growth of the embryo in a human being runs a
perfectly normal and uninterrupted course, the following points could be
observed at various stages. At the end of the fourth week in growth, the
embryo is distinctly curved, so that the two ends--the head and the
tail--are close together, the whole being about half an inch in length.
Even at this very early stage, the canal which gives rise to the brain
and spinal cord is closed in. The vesicles of the eye and the ear have
both made their appearance, and the limbs are just beginning to show as
buds. The heart is quite obvious, and its division into its four
chambers is commencing. In another four weeks the embryo has reached the
size of one inch, and the head is beginning to take on a shape more
resembling that associated with a human being. The tail, on the other
hand, has now disappeared. The limbs have grown to the extent that both
hands and feet are starting growth, and in the region of the head both
the eyes and the ears, as well as the nose, can be distinguished. Even
at this stage, however, the sex of the embryo cannot be made out. A
month later, at the end of the twelfth week, a considerable development
has taken place. The embryo is now about three and a half inches long.
There is a general growth to be observed, and the bones are beginning to
ossify. In sixteen weeks, when the embryo measures about five inches in
length, the sex is easily distinguishable. The most characteristic thing
for the weeks succeeding this is the relatively large size of the head,
upon which hair appears at about the twenty-fourth week. In twenty-eight
weeks the embryo should weigh about 2½ lb., that is to say at the
seventh month of embryonic life. Should the child be born at this time
as the result of any of the causes which give rise to premature birth,
there is a possibility that it may live, though as a rule it does not.
Four weeks later it should weigh 3½ lb., and if born now may frequently
live, if carefully attended to. In another four weeks the embryo is
nearly eighteen inches long, and weighs about 5½ lb., and the body has a
more rounded appearance, because by this time there has been a
considerable growth in fat. If born at this stage it ought to be quite
possible to save the life. Finally, at the end of forty weeks, the
normal full embryonic period of human life, the healthy child should
weigh about 7 lb., having smooth, pink skin, and being otherwise
perfectly developed.
CHAPTER XII
RECAPITULATION
In bringing our study of Embryology to a close, we may glance briefly at
another aspect of the subject, namely, that which emphasizes the fact
that in its development the embryo recapitulates the history of its
ancestors.
It is quite obvious that the offspring of any species of animal, if they
are to live and survive in the same kind of environment as that in which
their parents live, must resemble them somewhat closely. The only way in
which Nature can secure such a sufficiently close resemblance of
offspring to parents is by insuring that they should develop along
similar lines. So it is that we find that the whole of the life history
of an individual is more or less a recapitulation, with, of course,
variations, of that of the parents and ancestors. Each successive step
from the very beginning of the fertilisation of the ovum repeats a stage
through which previous generations have passed. If from any accident a
step in this recapitulation is omitted, the embryo is to that extent
deprived of some feature possessed by a parent or ancestor; and if this
be a sufficiently important omission, it is impossible for such an
embryo to survive. That is one way in which an embryo may differ from
its parents. That is a retrogressive change. On the other hand, such an
embryo, in addition to recapitulating the stages through which its
parents passed in development, may have something new added, something
which appears for the first time. In other words a progressive variation
may appear.
Now, since the embryo follows the same developmental track as did the
parent, passing through the successive stages of germ-cells, fertilised
ovum, embryo, f[oe]tus, infant, child, youth, and adult, it follows
that should it exhibit any additional peculiarity, unnoted in the
parent, the embryo has obviously varied progressively. That is to say,
it has pursued the same line of development together with some new
addition. On the other hand, should the offspring at any of these stages
in its career be obviously without some of the characters of previous
generations, it is as certainly due to the fact that the recapitulation
of the history of development has been, in that particular, incomplete.
In all successive cases of multicellular organisms, development by a
process of repetition of what happened in the previous generation seems
to be the rule; and it would appear that only by this means could a mass
of cells which constitute an individual grow into something sufficiently
like the parents as to be recognised for their offspring. Given the fact
that a human individual starts from a single germ-cell, it could only be
by following the same steps in development trodden by the parent that
the new individual could attain a similar growth. The object of this
similarity is, of course, to provide that the offspring may live and
survive in an environment more or less similar to that of the parents.
As Dr. Archdall Reid puts it, "the embryo starting from the same point,
must follow the same road to reach the same goal. The embryo which did
not recapitulate the history of the development of a parent would be a
monstrosity."
While, however, recapitulation in development is always more or less
clear, it does not follow that it is perfectly complete, nor perfectly
identical with the development of the parent. Indeed, on the other hand,
there is always a certain amount of variation, either progressive or
retrogressive. Progressive variation means that in addition to the
development of all the parental stages, something new has been added.
Retrogressive variation means that from the total development
experienced by the parent, something has been omitted. We are here
speaking of characters of a species, and it must not be thought that we
are referring to the characters of the embryo as if they were derived
from those of their parents. This was clearly pointed out in the earlier
portion of our study. The variations in development, to which we here
refer, take their origin in the germ-plasm which tends to repeat in each
generation similar types of development. In other words the germ-plasm
from which individuals spring is of such a nature that the embryos
arising from it show in their development a recapitulation of the
evolution of their particular species. In addition they may show
variations of either a plus or a minus character. These variations are
frequently inherited, and persist throughout succeeding generations. In
course of time, if there are many of such variations, they accumulate,
and to that extent, of course, alter the life history. That is why in
watching the development of a human embryo it is impossible to trace
accurately the early ancestral development of the race from it. It
passes through the stage of a single cell, then becomes multicellular,
and gradually assumes the form of a higher and higher type of organism.
"Manifestly the additions and subtractions have been vast. It possesses,
for instance, a placenta, an organ by which it is attached to the
mother, through which it is nourished, and which at one time is larger
than the embryo itself; but which, of course, could not have been
present in its prototypes. Nevertheless the life history unfolded by the
child is just as real, just as complete, and probably more accurate than
any written chronicle that attempts to describe the whole past of a
race." (Reid.)
"There is a history in all men's lives
Figuring the nature of the times deceased."
Here we must conclude this brief sketch of some of the principal facts
in the science of Embryology, in the hope that enough has been said to
stimulate the interest of our readers in this subject to such an extent
that they may be encouraged to pursue its study still further in one of
the many textbooks that are devoted entirely, or partly, to this matter.
We would urge in conclusion that the study is well worth while, even for
those to whom it has a non-professional interest. It should be
sufficiently obvious to any earnest thinker that the problems which are
involved in the study of embryology are precisely those which are of the
very greatest importance to humanity at large. With this subject is most
intimately bound up that of heredity itself, which has been dealt with
in another volume of this series. No true understanding of what can be
done, or what should be done, in the direction of improving the lot of
generations to come, or of making the most of the generation at present
existent, can be obtained by any who are absolutely ignorant of these
topics. It is only by their study that we realise that the human embryo,
which is to become the human individual, consists, to a very large
extent, of characters and features which are unalterably settled for it
beforehand, to which nothing can be added, and from which nothing can be
taken away. In other words, the possibilities for any individual are
those which are pre-existent in the germ-plasm from which he or she
originates. These possibilities, however, depend upon the environment in
which the embryo, infant, child, and adult is subsequently placed for
their full expansion. In many directions the inherited tendencies
transmitted by the continuity of germ-plasm are unalterably and strictly
defined. In many other directions these inherited tendencies can be so
modified, drawn out, or even partially suppressed, by suitable
surroundings of a hygienic, educative, and moral nature, that if the
process be taken in hand sufficiently early wonderful successes may
result. These results are those for which the social reformer and the
philanthropist and the serious student of sociology are earnestly
striving, but it is only by a study of the sciences of Heredity and
Embryology that accurate data can be obtained from which justifiable
conclusions may be drawn.
The great fact which embryology teaches is that the past is unfolded
stage by stage, with certain omissions and additions, so that in very
truth--
"The softest dimple in a baby's smile
Springs from the whole of past eternity,
Taxed all the sum of things to bring it there."
BIBLIOGRAPHY
The following books will be found to deal in detail with some of the
general questions in the foregoing pages:--
_The Laws of Heredity_. By G. Archdall Reid, M.B., F.R.S.E. (London.
Methuen.) This great work should be read by all who are interested in
the subject of Heredity and all the problems involved in it. It deals in
an exhaustive and interesting manner with the characteristics of living
beings, the method of development, the function of sex, the present
evolution of man, and kindred topics.
_The Greatest Life_. By Gerald Leighton, M.D., F.R.S.E. (London.
Duckworth.) This work deals with the development of character (as well
as structures) from the biological point of view. The argument is that
the _whole_ of a man, mental, moral, spiritual, as well as what is
usually termed physical, develops in accordance with biological laws.
_Text-books on Physiology_. Any of the standard books will be found to
contain accounts of the early development of the embryo and its various
tissues.
INDEX
Acquired characters, 43
After-birth, 57
Alimentary tract, 60
Allantois, 79
Amnion, 53
Amniotic space, 51
Am[oe]ba, 12
Auditory, 69
Auricles, 73
Bibliography, 90
Blastocyst, 50
Blood-vessels, 60
Body cavity, 54
Bones, 60
Brain, 61, 67
Branchial clefts, 75
Caul, 58
Cell-division, 12
Centrosome, 13
Cerebellum, 68-69
Cerebrum, 68, 69
Chorion, 79
Cochlea, 74
Conjunctiva, 71
Connective tissues, 60
Cornea, 71
Cytology, 14
Dental ridge, 75
Dental sac, 75
Ear, 74
Early development, 47
Ectoderm, 51, 59, 60
Embryology, significance, 7, 8
Embryonic area, 51, 53, 66
Enamel, 75, 76
Entoderm, 51, 59, 60
Epiblast, 54
Epidermis, 60
Eustachian canal, 74
Eye, 65, 71
Fat, 60
Fertilisation, 18, 19, 47-52
Fore-brain, 69
Germ-cells, 16, 18, 19, 24
Germ-plasm, 11, 49
Glands, 60
Hair, 60
Heart, 54, 73
Hypoblast, 54
Inborn traits, 43
Inherited traits, 43, 45
Kidneys, 60
Lens of eye, 60, 71
Liver, 60
Lungs, 60
Lymph glands, 60
Marrow, 60
Maternal blood, 80
Maturation, 48
Medulla oblongata, 68
Medullary groove, 53
Mesoderm, 52, 59, 60
Mid-brain, 68
Morula, 50
Mouth, 60
Muscles, 60
Neural canal, 53
Neural groove, 54
Nervous system, 60, 61
Nose, 60
Notochord, 54, 60, 76
Nourishment, 57, 78
Optic vesicle, 69
Origin of brain, 69
Ova, ovum, 16, 17, 48, 49
Personality, 45
Placenta, 57, 79
Primitive groove, 53
Primitive streak, 53
Recapitulation, 84-88
Reproduction, 11, 17-35
Reproductive organs, 60, 63
Segmentation nucleus, 49
Sense organs, 60
Sex, 12, 49
Smell, 70
Somatic cells, 18, 24
Sperm, 17, 47, 48
Spinal cord, 53, 67
Tactual sense, 64, 65
Teeth, 60, 61
Trachea, 60
Trophoblast, 55, 56
Thymus, 60
Thyroid, 60
Umbilical cord, 55
Uterus, 56, 57
Variations, 44, 45
Weight of embryo, 83, 88
Yolk sac, 50, 53
* * * * *
Printed by BALLANTINE, HANSON & CO
Edinburgh & London
* * * * *
The table below lists all corrections applied to the original text.
p 25: the rest are -> The rest are
p 35: Natural Selection." -> closing quotes added
p 60: alimentary tract. -> period added
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