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Title: Ameboid movement
Author: Schaeffer, Asa Arthur
Language: English
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AMEBOID MOVEMENT
AMEBOID MOVEMENT
BY
ASA A. SCHAEFFER, PH.D.
PROFESSOR OF ZOOLOGY, UNIVERSITY
OF TENNESSEE
PRINCETON UNIVERSITY PRESS
PRINCETON
LONDON: HUMPHREY MILFORD
OXFORD UNIVERSITY PRESS
1920
Copyright, 1920, by
PRINCETON UNIVERSITY PRESS
Published 1920
Printed in the United States of America
[Illustration]
PREFACE
Although the subject of ameboid movement is discussed in this book
chiefly because of its intrinsic interest, yet the interests of the
student of medicine, the psychologist, the physiologist, the
evolutionist and the general biologist have constantly been kept in
mind. For the medical investigator probably finds no better means of
approach to the study of the reactions and especially the movements of
the white blood corpuscles, which play such an important part in the
economy of the human body, than the ameba; white blood corpuscles and
amebas are strikingly similar in many characteristics and in the
fundamental processes of the movement they are probably identical. The
comparative psychologist is keenly interested in the activities of the
ameba because it exhibits to him the operation of the animal mind in its
greatest simplicity. To the physiologist ameboid movement has for a long
time represented the simplest phase of muscular contraction as it is
known in the vertebrates. The philosophical evolutionist sees in the
ameba, both in its structure and in its activities, a close
approximation to the earliest ancestor of the animals. And the general
biologist, aside from his usual interest in the properties of living
matter wherever it may be found, is especially interested in discovering
how many of the activities of the ameba are common to other organisms.
But in addition to presenting an account of the main facts concerned in
the movement of the ameba from the various points of view mentioned
above, this book has a second object which is scarcely subsidiary to the
main one. This second object is to present the thesis that moving
organisms in which orienting organs are absent or not functioning,
always move in orderly paths, i. e., in helical or true spiral paths.
The movements of the ameba under controlled conditions, which, as the
following pages will show, take the form of a helical spiral projected
on a plane surface, therefore serve as an introductory study to the
movements of organisms generally. For the presumption is strong that
there is an innate tendency in all organisms that move which compels
them, when free from stimulation, to move in definite predictable paths.
This thesis is discussed at some length in Chapters XII and XIII.
In view of the fact that ameboid movement has been considered largely as
a theoretical question heretofore, I wish to state at once that my
discussion of this subject is based directly on observation and
experiment. I have no new theory of ameboid movement to offer; the list
of theories is already extensive enough. I am, on the other hand,
strongly of the opinion that this fundamental question, if it is to be
solved at all, can be solved only by persistent observation and
experiment on the ameba and related organisms themselves. “All knowledge
is vain and erroneous excepting that brought into the world by sense
perception, the mother of all certainty” (Leonardo).
CONTENTS
PAGE
CHAPTER I
Introduction 1
CHAPTER II
Historical Sketch 3
CHAPTER III
The General Features of Endoplasmic Streaming 8
CHAPTER IV
The Transformation of Endoplasm into Ectoplasm 18
CHAPTER V
Pseudopods and the Nature of the Ectoplasm 25
CHAPTER VI
The Species Question 41
CHAPTER VII
Experiments on the Surface Layer of the Ameba 48
CHAPTER VIII
On the Nature of the Surface Layer 63
CHAPTER IX
The Surface Layer and Theories of Ameboid Movement 76
CHAPTER X
Streaming, Contractility and Ameboid Movement 92
CHAPTER XI
The Surface Layer as a Locomotor Organ 105
CHAPTER XII
The Wavy Path of the Ameba 109
CHAPTER XIII
The Wavy Path of the Ameba and the Spiral Paths of
Ciliates and Other Organisms 126
CHAPTER XIV
Conclusions 142
Bibliography 146
CHAPTER I
INTRODUCTION
The manner of movement common to amebas has attracted the attention of
biologists ever since the discovery of ameba by Rösel v. Rosenhof in
1755. In his description of “Der kleine Proteus” he records the
observation that the various form changes which the ameba undergoes are
associated with the streaming of the endoplasm. This observation marks
the very beginning of the investigation of ameboid movement. And this
investigation also possesses the distinction of being the most important
single observation that has thus far been recorded in this special
field, for it is now generally understood that by ameboid movement is
meant movement due to the streaming of protoplasm.
The phenomenon of ameboid movement as discovered by v. Rosenhof, was an
isolated phenomenon. It attracted attention mainly because of its
uniqueness, for it was the only instance of the kind that was then
known. It could not be compared with any other form of movement; and the
animal itself, considered apart from the streaming of the protoplasm,
was unique also, because of its remarkable form changes which it alone,
of all the animals then known, exhibited.
But when Corti in 1774 discovered streaming protoplasm in the cells of
chara and various other plants, the ameba could no longer be said to
occupy this position of isolation. Although streaming is not accompanied
by locomotion in chara, it had been observed that movement in the ameba
was always accompanied by streaming, so it came to be generally accepted
that the really fundamental feature of ameboid movement was the
streaming of the protoplasm.
The ameba came to be of especial interest to the physiologists later on
when the finer structures of the larger animals were studied more
carefully. Thus when the normal movements of the white blood corpuscles
were discovered, no one failed to be struck with their ameboid
characteristics in almost every detail of movement, feeding habits and
gross structure. The great importance of the functions that have been
ascribed to leukocytes, and their very widespread occurrence in the
higher animals has served to give rise to the belief that ameboid
characteristics were not unique among animals, but common to many of
them. The discovery of ameboid movements among plant zoospores, among
animal ova, in the endoderm cells lining the digestive tract of a great
variety of animals, in the nuclei of some animal cells, in the wandering
cells of sponges and other animals--all these instances of ameboid
movement occurring in such widely different tissues inevitably placed it
among the most important phenomena known to occur in organisms.
Out of the discovery that ameboid movement may be exhibited in some form
or other in so many different kinds of organisms, grew the theory that
even muscular movement as known in man and the higher animals is at
bottom a specialized sort of ameboid movement; not merely
phylogenetically, but as it is now known. As we shall see however in the
following pages, this theory of muscular movement cannot be based
specifically on the streaming process per se, but it is very probable,
on the other hand, that the same process which underlies contraction of
the ectoplasm in the ameba also underlies contraction in muscular
tissue.
But this remarkable story of the development of a single unrelated
observation into a widespread biological phenomenon is not yet complete.
With its further development the following pages are concerned. It will
be shown that the movement of the surface film of the ameba is analogous
to that of some blue-green algae, diatoms and crawling euglenas, in
which organisms the surface film seems to be the vehicle of movement.
Thus the ameba finds itself related to these organisms by new ties. More
important still is the significance of the wavy path of the ameba, which
may possibly be due to the same fundamental mechanism that controls,
under suitable conditions, the direction of the path in man and many
other animals and motile plant cells. Thus the phenomenon of ameboid
movement born in nakedness and utter isolation, has become attired, in a
brief space, with the Victorian garb of a Fundamental.
CHAPTER II
HISTORICAL SKETCH
For the purpose of presenting in brief compass the main published
observations and experiments on ameboid movement, we may pass from the
observations of v. Rosenhof, mentioned in the introduction, to certain
observations which Wallich (’63) recorded. He found that a new pseudopod
is usually formed as a small break in the ectoplasm somewhere on the
ameba through which the endoplasm then flows. As the endoplasm flows out
and the new pseudopod enlarges, the breach in the ectoplasm increases in
extent, through a transformation of the ectoplasm in the immediate
vicinity of the breach, into endoplasm. But he observed also that some
of the endoplasm which flows into the new pseudopod becomes transformed
into ectoplasm. Wallich thus demonstrated that ectoplasm and endoplasm
are mutually convertible.
The conversion of ectoplasm into endoplasm and vice versa, was regarded
by Wallich, however, as a process taking place only occasionally, such
as when new pseudopods are formed. It remained for Bütschli (’80, p.
115) to point out that in a moving ameba endoplasm is continually formed
from ectoplasm at the anterior ends of all pseudopods, while the reverse
process, viz., the conversion of ectoplasm into endoplasm, takes place
continually at the posterior end of the ameba. He describes the relation
of ectoplasm to endoplasm as a “circulation”; the endoplasm, arriving at
the anterior end, becomes changed into ectoplasm, which after remaining
relatively stationary for a while on the outer side of the animal, soon
finds itself at the posterior end of the ameba, where it is slowly
changed into endoplasm. The movement of the endoplasm forward to the
anterior end of the ameba completes the cycle.
In 1898 Rhumbler, from observations on several species of amebas, came
to the conclusion that in the _change_ from ectoplasm into endoplasm,
and vice versa, must be sought the cause of ameboid movement.
Jennings (’04), however, from extended study of the physiology of the
ameba, stressing especially movement and feeding, denied that the
transformation of endoplasm into ectoplasm, and vice versa, is necessary
or even of frequent occurrence during movement. Instead of these
transformations occurring regularly, as Bütschli and Rhumbler described
them, Jennings concluded that the ectoplasm is more or less permanent,
behaving like an elastic skin, which rolls over and over as the ameba
moves along. The ectoplasm thus remains ectoplasm, and the endoplasm
retains its identity, for considerable periods of time, instead of being
continually transformed, the one into the other, as the ameba moves
along.
Although observations with regard to movement in ameba have consisted
almost wholly of the mutual relations of ectoplasm and endoplasm, it is
important to note that the existence of a _third layer_ of protoplasm,
outside of the ectoplasm, was foreshadowed by an observation of Bütschli
(’92, p. 219) while examining a pelomyxa. To his great surprise he found
that there were currents of water, as evidenced by the movement of
suspended particles, at the sides and in close contact with the
ectoplasm of the pelomyxa, which flowed slowly forwards toward the
anterior end. No details were given and no explanation offered for the
cause of the currents excepting the suggestion that there might be a
thin skin over the animal, which moves slowly forward.
Two years later Blochmann (’94) demonstrated by means of the very fine
cilia-like projections which frequently cover the outside of pelomyxas,
that the surface of the pelomyxa actually moves forward during active
locomotion. He did not state definitely whether or not he considered
this surface as a part of the ectoplasm.
This observation of Blochmann was not developed, however, until Jennings
(’04), by means of particles attached to the outer surface of amebas,
studied the forward movement of this layer. The results of Jennings’
work led him to conclude that the outer surface of amebas, which move
forward as demonstrated by attached particles of soot and other
substances, is continuous with the ectoplasm, and is really the
ectoplasm. The rate of movement of this layer was stated to be about the
same as that of the ameba as a whole. He denied the validity of
Bütschli’s suggestion that there might be a thin third layer on the
outside of amebas or pelomyxas.
But the existence of a third layer of protoplasm as distinct from the
ectoplasm, was again maintained by Schaeffer (’17) who found that in
some amebas the outer surface moves forward faster than the ameba
advances through the water. The third layer was found to be generated
over the surface of the ameba, especially in the posterior region of the
ameba, and destroyed at the anterior end.
But the purely observational aspect of the problem of ameboid movement
has not interested biologists generally as much as the ultimate cause of
the phenomenon.
The first attempt that was made to explain ameboid movement in
conformity with the demands of modern experimental science, that is, on
the basis of physical factors, was made by Berthold (’86). By means of
simple experiments with inert fluids (oils, alcohol, water, ether) which
were modeled after an experiment described by the physicist Paalzow
(’58), Berthold concluded that locomotion in ameboid organisms is due to
the physical attraction of the anterior end to the substratum. The ameba
was supposed to behave like a drop of fluid which moved towards the
point where the tension of the ameba’s surface was decreased by contact
with the substratum. The ameba did not _push out_ pseudopods according
to Berthold, but they were _pulled out_ because of a difference in
surface tension between them and the substratum. But pseudopods which
were extended into the water and out of contact with a solid substratum,
were said to be extended by a contractile effort of the posterior region
of the ameba.
Bütschli (’92, p. 187) pointed out that it was highly improbable that
pseudopods in contact with a solid substratum were projected in a
fundamentally different way from that in which free pseudopods were
extended, as explained by Berthold. Bütschli assumed that all ameboid
movement was due to the same fundamental cause. He postulated surface
tension as the active agent, as Berthold had done for the extension of
pseudopods in contact with a solid substrate; but Bütschli assumed that
the decrease in surface tension at the anterior end of the ameba was
brought about by the bursting of protoplasmic droplets of a more fluid
consistency on the surface of the ameba, the consistency of which was
less fluid, thus bringing about a decrease of surface tension and
consequent forward streaming of the endoplasm. The necessary migration
of the more fluid droplets to the surface was determined by internal
conditions. The direction in which an ameba moves was assumed to depend
therefore not upon the physical character of the substrate, as suggested
by Berthold, but upon such internal changes as control the movement of
the more liquid part of the internal protoplasm to the outer surface.
Rhumbler (’98) wrote extensively on the subject of ameboid movement,
especially from the point of view of the feeding habits of amebas. He
concluded that the flow of protoplasm, while engulfing a food object,
was a direct result of the lowering of the surface tension of the
protoplasm by contact with the food object (p. 207), thus causing its
envelopment. Numerous other writers of the time, including Quincke
(’88), Verworn (’89, ’92), Blochmann (’94), Bernstein (’00) and Jensen
(’02), agreed in a general way with Rhumbler’s position that surface
tension changes are the cause of locomotion in ameba.
In 1904 the general subject of ameban behavior was extensively studied
by Jennings, and from his observations he concluded that surface tension
cannot account for many of the reactions observed. Other factors, he
held, must be at work, such as contractility, which, acting in the
posterior region, causes the endoplasm to flow forward. But Jennings
found it impossible to explain on the same basis the extension of free
pseudopods, and the creeping of a pseudopod, or of the whole ameba, over
a solid substratum.
From further observations Rhumbler (’05, ’10) came to modify his earlier
views as stated above. The rapid advances in the study of the chemistry
of colloids doubtless suggested to Rhumbler that the change from
endoplasm to ectoplasm resembled the change from a sol to a gel state,
and that in this process of gelation lay the source of energy manifested
in ameboid movement. In thus calling attention to, and emphasizing the
colloidal nature of, the conversion of endoplasm into ectoplasm and vice
versa, the problem of ameboid movement came to be discussed from an
entirely new angle. Certain phases of Rhumbler’s theory are developed
and elaborated by Hyman (’17) who agrees in general with Rhumbler’s
conclusions.
In a series of papers on feeding and other reactions of ameba, Schaeffer
(’12, ’16, ’17) concluded that Rhumbler’s general statement, wherein he
says that changes in behavior are directly deducible from the action of
stimuli in effecting liquefaction or gelation of the ectoplasm, does not
hold in many cases of feeding, and that the mechanism controlling
locomotion and feeding is not external, as maintained by Rhumbler, but
internal.
CHAPTER III
_The General Features of Endoplasmic Streaming_
The streaming of the endoplasm is the most conspicuous feature of
ameboid movement. It is even more noticeable than the movement of the
pseudopods themselves, because of its greater speed and because it
occurs in all parts of the ameba. Its importance in movement is
essential, for no continued locomotion can be observed unless
accompanied by streaming. It may be profitable therefore to enquire into
the general features of streaming, and to observe some of the necessary
consequences streaming imposes upon such an animal as the ameba.
Let us take as an example an _Amoeba proteus_ (Pallas, ’66, emend.
Leidy, ’79, emend. Schaeffer, ’16) in characteristic movement (see
Figure 11, p. 37). The main streams of endoplasm are in the same
direction as that in which the ameba moves. In the withdrawing
pseudopods the current is, of course, toward the main mass of the ameba.
The endoplasmic stream is continuous from the posterior end to the tips
of the advancing pseudopods. The retracting pseudopods flow into the
main stream as tributaries. If, as often happens, the ameba is without
pseudopods, there is then a single stream arising in the posterior end
and flowing to the anterior end. In such a case it is readily observed
how absolutely dependent locomotion is upon endoplasmic streaming.
It often happens, such as when the ameba is receiving a strong stimulus,
that streaming is arrested and brought to a stop for a few seconds, more
or less. Presently however the endoplasm begins to flow as before. At
what point, in such a case, is the first movement of endoplasm
detectible? Is it at the free end of the pseudopod, at its middle
region, at its base, or at the posterior end of the ameba? Bütschli
(’80, p. 116) observed that in a withdrawing pseudopod the streaming
begins at the free end of the pseudopod; but his (’92, p. 201) later
explanation of ameboid movement seems to require that the endoplasm
must begin to move at the base of the withdrawing pseudopod. Jennings
(’04, p. 157) observed that in a withdrawing pseudopod the current of
endoplasm begins at the base of the pseudopod.
From numerous observations directed toward this point, it appears that
the conditions under which streaming is resumed after a pause, whether
in the same or in the reverse direction, are of great variety. The
shape, size, slenderness, and the position on the ameba of the
pseudopod, as well as the strength and character of the stimulus, are
among the factors capable of changing in whole or in part the flow of
endoplasm in a pseudopod. In an ameba that has been moving along a
homogeneous flat surface, as nearly unstimulated as may be, the
endoplasm first begins to flow out of the lower half of the retracting
pseudopod, if the pseudopod is more or less uniformly conical in shape
and rather slender. In such a case it may be said that the retracting
pseudopod was withdrawn “by the ameba,” and that it did not itself
receive an external stimulus producing retraction. If, however, the tip
of a pseudopod as described receives a strong negative stimulus, the
endoplasm frequently flows back from the tip while it is still flowing
into the pseudopod at the base. But very soon thereafter, in such an
event, the streaming becomes unified and the pseudopod is withdrawn. In
broad pseudopods about to be withdrawn, the endoplasm may begin to move
anywhere along its length. This is undoubtedly due to the continuous
local changes in the walls of the pseudopod, which are characteristic of
this species of ameba (see p. 20).
In an ameba which has been brought to a standstill, as by a sudden flash
of light, the first sign of recurring streaming is in the anterior half,
whether the original direction of streaming is resumed or reversed. If
the direction is reversed, the active pseudopods retract for a
considerable distance before a new one is projected. The endoplasmic
stream in a slender withdrawing pseudopod may not reach to the tip for
from several seconds to a minute, if the tip is slightly positively
stimulated. One may then observe ectoplasm streaming toward the tip and
toward the base, in the respective regions, at the same time, with
considerable fluctuation back and forth of the neutral zone separating
the two streams. The fate of such a pseudopod depends on its size, on
its position on the ameba, and the strength of the stimulus affecting it
and the rest of the ameba. That is, if the pseudopod is small or on the
posterior half of the ameba, or only slightly stimulated, it will be
retracted; but if it is large, or on the anterior end of the ameba, or
more strongly stimulated than the rest of the ameba, it may again become
active.
The fact that protoplasm is practically incompressible makes it clear
that if streaming can be observed to begin after a pause at some point
after it begins at others, the ectoplasmic walls of the ameba must give
way in the region where streaming begins. Since it has been established
by observation that the ectoplasm may give way at any point, it follows
that one of the principal factors affecting streaming is the elasticity
and liquefiability of the ectoplasm.
The streaming in an ameba is coordinated. The direction in which the
endoplasm flows in the several pseudopods, when there are no stimuli
received externally that produce visible changes in behavior, gives one
the impression that there is a “centre” controlling movement. The
several pseudopods do not act at all capriciously. The ameba seems to
move the pseudopods, not the pseudopods the ameba. If this impression of
coordination is correct, it is of the first importance in a study of
ameboid movement. Further on, this point will be taken up at length in
connection with the character of the path an externally unstimulated
ameba describes (p. 109); but there are certain observations which aid
in the analysis of the problem of coördination from the point of view of
the pseudopod, instead of that of the ameba as a whole, and to these
observations we may now direct our attention.
The mass of endoplasm within a pseudopod moves practically always in one
direction. In any cross-section of a pseudopod that is more or less
cylindrical in shape, the endoplasm in the center moves most rapidly,
that near to it less rapidly, while that near the ectoplasm moves very
slowly. One never observes a forward stream on one side of the pseudopod
and a backward stream on the other. Nor does one observe parallel
streams of endoplasm flowing in opposite directions within the same
[Illustration: Figure 1. Illustrating the various directions of
endoplasmic streaming in growing and retracting pseudopods. _a_, two
oppositely directed streams in a pseudopod, one directed toward the base
and the other toward the tip of the pseudopod, with a neutral zone
between. _b_, two streams flowing toward each other. Cases _c_ to _r_
are self explanatory. _s_, rotational currents observed occasionally in
various species of amebas. _t_, “fountain currents,” sometimes observed
in _Amoeba blattae_, and rarely in other forms. _u_ and _v_ represent
cases of streaming which have not been observed and which probably do
not occur. _w_, similar to _v_, but with a wide neutral zone between the
streams, represents an actual observed case. _m_ and _r_ probably occur
only very rarely; no such cases have been seen, but there seems to be no
reason why they do not sometimes occur. Excepting _m_, _u_, _r_ and _v_,
all these figures were drawn from observed cases of streaming.]
ectoplasmic tube, in an ameba of several pseudopods, excepting where
there is a wide zone of stationary endoplasm between the streams (Figure
1, _v_, _w_). But in “fountain currents,” such as Rhumbler (’98, p. 190)
described and figured for _Amoeba blattae_ Bütschli, and which may
readily be observed in most species of amebas if immersed in a solution
of gelatin thick enough to keep the amebas from sinking, there is a
central stream of endoplasm flowing forward, and a peripheral stream of
ectoplasm flowing backward, with a thin neutral zone between (Figure 29,
_d_). As we shall see further on, however, these fountain currents are
in principle the same as the currents observed in ordinary locomotion,
the apparent difference being due to the fact that there is no
locomotion. It is true, then, that within the same pseudopod at any
cross section the endoplasm always streams in one direction, and the
streaming is unified.
When new pseudopods are formed, or when old ones are retracted, and
especially when both these phenomena occur at the same time and close
together on a part of an older pseudopod, some of the details of
coordination in streaming are readily made out. In Figure 1 are shown a
number of observed cases of pseudopod formation and retraction, with the
direction of endoplasmic streams indicated at a given instant. For the
purpose of illustration, several (presumably) possible but unobserved
cases, _m_ and _r_, are sketched, and also two cases, _u_ and _v_, which
have not been observed and which probably do not occur. The general
conclusion to be drawn from these observations is that, while the
endoplasm in the body of an ameba as a whole may be streaming in several
different directions at any given instant, that is almost never the case
with an individual pseudopod, especially if the pseudopod is of small or
medium size and not too flat or otherwise irregular in shape. The
pseudopod is therefore the unit of coordinated protoplasmic streaming.
Another general observation which undoubtedly is connected in some way
with the problem of coordinated streaming is the following. In
externally unstimulated amebas, the new pseudopods are almost without
exception directed 60° or less from the direction in which the parent
pseudopods are moving.
It is a matter of common observation that an ameba may throw out a
pseudopod in any direction whatsoever when stimulated. The ameba may
reverse its direction of movement completely, or it may move in scores
of different directions at one time for awhile, if properly stimulated.
There is no restraint or limit imposed upon the ameba insofar as the
direction of movement is concerned. Why then should a great majority of
new pseudopods in an unstimulated ameba be projected at an angle of
approximately 60° to the parent pseudopod? It might seem at first sight
as if the merely physical aspect of the streaming would be a sufficient
explanation, in that less resistance would be met with in sending a
stream off at a small angle than at a large. But it is probable that
inertia plays no part in maintaining the direction of streaming (see p.
123, footnote, for further discussion). It requires perhaps more energy
for a pseudopod to flow off from the main stream at an angle of 120°
than at an angle of 30°. But it is plain that as many pseudopods are
withdrawn as are thrown out, and they are withdrawn at an angle against
the main stream of endoplasm in the ameba that is the complement of the
angle at which they were projected. Whatever energy might be saved
therefore in the projection of a new pseudopod at a small angle with the
main stream is lost in withdrawing the pseudopod against the stream at a
correspondingly large angle. It is clear therefore that the physics of
moving viscous fluids cannot solve the problem. It is probable that the
mechanism which controls the direction of locomotion as exemplified in
the wavy path of the ameba (see p. 109) is also involved in the
direction in which pseudopods are projected.
Some very interesting special cases of endoplasmic streaming are
observed during the process of feeding. As is well known, amebas capture
their food by the protoplasm flowing around it and engulfing it. If the
object is large the protoplasm may flow around it, in contact with it,
so that the shape of the object determines the direction in which the
enveloping protoplasm flows. If the object is small, particularly if it
is a live organism, the behavior of the ameba is quite different (Kepner
and Taliaferro, ’13, Schaeffer, ’16). To capture such a food object a
cup of protoplasm is gradually formed over it so as to imprison it
(Figure 2). If the food organism lies against some flat object, the food
cup is brought down to the surface of the object all around, thus making
escape impossible, before the protoplasm comes into contact with the
food organism. Schaeffer (’16, ’18) by experimental methods has shown
that the stimulus calling forth the formation of food cups as just
described, is the mechanical vibration of the water. At least the same
response was produced on the part
[Illustration: Figure 2. Endoplasmic streaming involved in the formation
of a typical food cup. _a_, the ameba is shown moving toward a live food
organism that is resting quietly on the bottom. _b_, the main pseudopod
forks, being the first indication that the feeding process has set in.
At _c_ the pseudopods have half-way surrounded the prey, but without
having come into contact with it. At _d_ the upper sheet of protoplasm,
_f_, (stippled), is flowing dome-like over the prey, while the
pseudopods continue to surround it. At _e_ the pseudopods have met and
fused with each other and the upper sheet of protoplasm has completely
covered the space encircled by the pseudopods, and has fused with the
pseudopods. _g_, sheets of protoplasm which are thrown out along the
_lower_ surface _under_ the prey, to form a floor to the food cup. Up to
stage _e_ the ameba has not come into physical contact with the prey,
but is just about to do so. With the completion of the floor of the food
cup, the process of feeding is completed.]
of the ameba when the ameba was carefully stimulated by means of very
fine clean glass needles. The conclusion is unavoidable therefore that
the shape of the food cup and the method of its formation is a racial
characteristic and is hereditary. The streaming endoplasm therefore,
upon suitable stimulation, takes on a definite form, that of a food cup.
This indicates again that the endoplasm is something more than the
ordinary fluids of physics, for out of an apparently structureless
fluid, organization is effected.
The fact that food cups are formed by amebas implies of course that
stimuli are received whose effect cannot be explained as a direct
physical reaction. Rhumbler (’10) has attempted to explain the formation
of food cups as the direct physical result of the stimulation by the
food body; but in recent experiments Schaeffer (’16) has shown that food
cups are formed over diffusing solutions of tyrosin, where the solutions
were quite as concentrated outside as inside the cup. These results
prove convincingly that the shape and size of the food cup are not
determined by direct action of the stimulating agent, but by hereditary
factors within the protoplasm of the ameba.
Other stimuli also affect streaming characteristically, though not so
strikingly perhaps as food stimuli. One of the most widely observed
effects on streaming is the momentary pause following stimulation of
many sorts. If an ameba that is moving along unstimulated externally,
suddenly comes near a food object, it frequently stops forward streaming
for about a second, and then begins again, usually at increased speed.
The ameba behaves as if it were startled. A similar reaction is observed
if a small perpendicular beam of light is flashed near the anterior end
of the ameba. Here also streaming is resumed with accelerated speed
toward the beam of light. Harrington and Leaming (’00) showed that if
strong light, especially at the blue end of the spectrum, is suddenly
thrown on the ameba, movement is arrested for a short time. Miss Hyman
(’17) has shown recently that if an ameba is strongly stimulated with a
glass needle, streaming is arrested momentarily, but the direction of
streaming when resumed subsequently, depends partly upon the former
direction of streaming and partly upon the location of the stimulus. All
of these cases of temporarily arrested movement are strikingly similar
to what is observed in the higher animals under similar conditions.
The ingestion of a large food mass produces usually a marked change in
streaming. A more or less spherical form is assumed, and if the food
mass be a live organism such as a large ciliate, the ameba frequently
remains quiet for a considerable interval. If a large amount of food is
eaten, as for example a dozen or two colpidia, the ameba may suspend
concerted streaming for an hour or more. During this time small
pseudopods are projected here and there, but there is no locomotion. But
if an ameba eats large masses of carmine, there is usually no pause
following ingestion, and the same thing is true when the ameba is
induced to eat bits of glass and other indigestible substances. It
follows therefore that the interrupted streaming of the endoplasm due to
feeding is not caused by the act of ingestion as such, but rather by the
onset and continuance of the normal digestive processes on a large
scale. These reactions are again strikingly similar to what is observed
in many vertebrates, in which a more or less definite body sense, whose
sense organs are in the splanchnic region, is supposed to be involved;
but what the explanation of similar behavior in ameba is, is not at all
clear.
Another factor of great importance in endoplasmic streaming is the
nucleus. It was observed by Hofer (’90) that amebas lacking nuclei did
not move in a coordinated manner. Štolc (’10) however records a number
of observations in which characteristic movement was observed in
enucleate amebas ten or more days after the enucleate ameba had been cut
off from a normal ameba. Hofer’s amebas died after nine or ten days,
while Štolc’s remained alive, some of them for over thirty days.
Recently Willis (’16) confirmed Hofer’s findings, but does not discuss
Štolc’s results.
The cutting of an ameba into two pieces, one with and the other without
a nucleus, is a very simple operation. It is also very easy to observe
that within an hour or so the enucleate ameba does not move normally,
and that there is no concerted endoplasmic streaming while the nucleate
ameba seems to behave normally. But Štolc’s contention that enucleate
amebas move characteristically (l. c., p. 159, 160, 167) is not
necessarily contradicted by these observations, for Štolc’s observations
refer to amebas that lived much longer than the enucleate amebas of
Hofer and of Willis. Even if an enucleate ameba is able to recover,
after many days, its power of concerted movement, there can be no doubt
that enucleate amebas do not move characteristically for a short time
after the operation, and that this effect is due to the lack of a
nucleus.
Very likely the action of the nucleus on the locomotory processes is
neither direct nor specific. The metabolic balance must be disturbed by
so fundamental an operation as the removal of the nucleus, and all
fundamental activities must in consequence be affected. That food
organisms (chilomonas and coleps) may be eaten and digested as Štolc
(’10) states, indicates however that the metabolic balance may after a
time be regained in some degree, for feeding undoubtedly calls for
concerted streaming, and digestion for the formation and transfer of
enzymes. Until this point receives further attention therefore, it
remains unknown in what way the removal of the nucleus disturbs
streaming for some time after the operation; but of the fact that
streaming is disorganized for some time, there can be no doubt.
CHAPTER IV
THE TRANSFORMATION OF ENDOPLASM INTO ECTOPLASM
Perhaps none of the factors influencing the streaming of the endoplasm
mentioned above exercises as profound and constant an influence as its
capacity to form ectoplasm. As has been intimated earlier (p. 3-9)
streaming as observed during locomotion is not supposed to be possible
at all unless accompanied by the formation of ectoplasm at the forward
ends of pseudopods, and its transformation into endoplasm at the
posterior end of the ameba. We may therefore next consider the rôle
ectoplasm plays in locomotion, and in some other fundamental activities
of the ameba.
In the first place it is necessary to define the word ectoplasm, for two
entirely different meanings are sometimes given to it. It is used often
to designate the clear non-granular layer of protoplasm which thinly
covers some of the commoner amebas, and is especially prominent in some
of the small species, where the larger part of the anterior end often
consists of protoplasm quite free from granules. The other use to which
the word is put is to designate the layer of protoplasm on or near the
outside of the ameba which is more or less rigid and motionless,
resembling the gel state of a colloid. It is the latter meaning that is
given the word as used in this discussion, while I shall follow Jennings
(’04) and other, earlier, writers in using the word hyaloplasm in
speaking of the outer clear layer. It may be necessary to add that
neither of these two words is strictly definable, for in some cases, at
least, hyaloplasm is not more rigid than the endoplasm, while in other
cases it is. Strictness of definition can, of course, come only as
investigation proceeds; and these words as well as the word endoplasm,
should not be taken as defining the properties of the substances to
which they refer, but only as labels.
The demonstration of the most conspicuous and important property of
ectoplasm in _Amoeba proteus_ is easily made. With the high power of
the microscope one focusses on the upper surface of an active pseudopod,
paying especial attention to the small crystals imbedded in the
protoplasm. These crystals, although they dance about slightly (Brownian
movement) and otherwise change position to a slight extent, nevertheless
appear to be held in place by a very viscous medium. Such movement as is
observed in these crystals appears more or less erratic; it is not
coordinated and it is only by chance in the direction of locomotion of
the ameba. While observing the practically stationary crystals of the
ectoplasm one can at the same time, though indistinctly, see the forward
sweep of the crystals and other granules in the endoplasm below. But
observation fails to detect a definite line of separation between the
stationary ectoplasm and the mobile endoplasm; the one grades off
insensibly into the other.
The formation of ectoplasm in _proteus_ is a much more complicated
process than in almost any other ameba, excepting the large species
_Amoeba carolinensis_[1] discovered by Wilson (’00). We shall have
occasion however to refer at length to the method of ectoplasm formation
in _proteus_ later on, so we may consider _proteus_ first from this
point of view, and then take up a few other species in which the process
is simpler.
It is a fact more or less familiar to observers of amebas that
_proteus_, as distinguished from the other amebas, has a number of large
irregular, roughly longitudinal folds or ridges on its pseudopods and on
its main body (Figure 3). Under normal conditions these are never
absent. They are not found at the free ends of advancing pseudopods, but
they take their origin at some little distance from the ends. It is this
characteristic of ridge formation that complicates the process of the
transformation of endoplasm into ectoplasm; for instead of having to
deal with ectoplasm formation at the anterior ends of pseudopods only,
we find this process taking place irregularly all over the surface of
the ameba.
These folds or ridges were first observed by Leidy (’79) and it is an
eloquent tribute to the keen observation of this
[Illustration: Figure 3. Formation of longitudinal ridges and grooves in
the ectoplasm of _Amoeba proteus_. _A_, _B,_ _C_, _D_, showing stages in
the development of a single pseudopod. _a_, _b_, _c_, _d_, _d_^{1},
cross sections of pseudopods at the levels indicated. The arrows show
the direction of endoplasmic streaming with special reference to the
formation of ridges. The numerals 1 to 7 indicate the order in which the
ridges were formed. Note the tongues of ectoplasm which extend into the
endoplasm, in the cross sections.]
sympathetically-minded naturalist, that of the large number of
subsequent writers on ameboid movement only one (Penard, ’02, p. 63)
seems to have noticed these folds. Leidy says that “ ... the main trunk
and larger pseudopods of the same ameba (_proteus_) assumed more or less
the appearance of being longitudinally folded. The endosarc axially
flowed as if in the interior of thick walled canals, of which the walls
appeared to be composed of finer granular matter with scattered imbedded
crystals. In the flow, all the contents did not move with the same
rapidity, and usually the smaller particles were swept quickly by the
larger ones. Other matter, including some of the largest elements
appeared to stick to the inner surface of the extemporaneous tubes, but
successively became detached to be carried along with the rest of the
contents (p. 46).” “The endosarc appeared to flow within thick walls of
ectosarc which often seemed to be longitudinally folded (p. 326).”
Penard (’02) confirms Leidy’s observation as to the existence of these
folds: “The current (of endoplasm) indeed is not unified, but there
exist many currents at the same time because of the fact that the
endosarc is divided into a certain number of longitudinal canals or
grooves by dense walls, which are of a temporary nature, being broken
down and built up from time to time. It is easy to distinguish one canal
from the other in this species, the currents being at first more or less
parallel, but terminating at the forward end, by their coalescence, as a
single mass of liquid (p. 63).” But Penard questions Leidy’s conclusion
that the walls are of ectoplasm: “Moreover Leidy deceives himself
without any doubt in considering these partitions as folds of the
_ectosarc_. The latter, in the rhizopods, is not a special substance, it
is a plasma of surface, specialized for the functions which it has to
perform, capable of modification as to its intimate structure, but only
so temporarily (p. 63).”
Although it is a very simple matter to prove to one’s satisfaction the
mere existence of these folds--a few minutes’ observation under the high
power of the microscope will do that--it is a much more difficult matter
to observe how these folds originate, because of the incessant changes
going on, as recorded by Leidy.
Very young or small pseudopods in _proteus_ have the same general
appearance as the pseudopods of other large species (_dubia_,
_laureata_, _discoides_, _annulata_, etc.); that is, there is a central
axial stream of endoplasm surrounded by a layer of ectoplasm. But there
is one difference even here, and that is the greater thickness of the
ectoplasmic walls in _proteus_ in proportion to the diameter of the
pseudopod. The ectoplasmic tube however is not solid throughout, but is
more or less honeycombed, somewhat like a network, with the spaces
filled by endoplasm.
If the ectoplasm is actually endoplasm that has passed into the gel
state, then the honeycomb condition just described resembles an
intermediate stage where only a part of the endoplasm has been
transformed. This network of endoplasm is strong enough however to
impede the flow of the main stream of endoplasm along the sides of the
pseudopod; but when large objects, such as the nucleus or food masses,
too large to be readily carried in the endoplasmic stream, impinge
against the imperfectly solidified sides of the tube of ectoplasm, the
innermost strands of the spongy network of ectoplasm snap, usually with
readiness, allowing the large object to pass by.
The surface of a young pseudopod is smooth, a cross section being oval
in shape (Figure 3, _a_); but as the pseudopod increases in size, large
folds or ridges begin to make their appearance. Usually the first ridges
to appear are lateral. They begin as small waves of hyaloplasm which
flow out along the sides of the pseudopod for a short distance and then
continue to move forward. The endoplasm then flows in a number of small
parallel streams amid numerous obstructions through the ectoplasmic tube
of the pseudopod into the wave of ectoplasm. After the ridge is well
begun, there is frequently observed a slow forward-moving stream of
endoplasm within it, but the ridge is never closed from the main
endoplasmic stream, as is readily proved by the numerous small streams
of endoplasm which continually filter through the ectoplasm into the
ridge.
In addition to the lateral ridges, which, as stated, are usually formed
first, there appear ridges on the upper side of the pseudopod as well,
and presumably also on the under side. So far as could be determined
these ridges are all formed in much the same way; that is, by the
projection of a small wave of protoplasm from some part of the surface
of the pseudopod. The ridges do not always grow by extension at the
anterior end as described above. Not infrequently a ridge ten to twenty
times as long as wide is pushed out along its whole length at once. This
is especially likely to happen in a slender pseudopod that suddenly
becomes the main pseudopod. The width of a ridge, especially on the
upper surface, does not change much after formation. One can frequently
find two or three ridges of about the same width, which run the whole
length of the ameba with the exception of a short distance at the
anterior end, where, as before stated, there are no ridges.
As the figure indicates, new ridges may be formed from previous ones,
either by lateral or endwise extension. In such case the walls of the
ridge send out thin waves of hyaloplasm followed by streams of
endoplasm, as described above in the formation of the first ridge on a
pseudopod. When a pseudopod forms a branch, the ridges on the old
pseudopod do not likewise branch, but new ridges are formed which have
no connection with old ones, but they may later coalesce with old
ridges. Such coalescence is however exceptional. Once a ridge is formed,
it retains its identity as a rule; that is, as the ameba moves forward,
the ridge in effect moves back over the ameba to lose itself in the
wrinkles at the posterior end (See Figure 11, _A_). The number of ridges
on any random selection of amebas is variable, and is moreover difficult
to state. A large ameba may have as many as six or seven side by side on
its upper surface. The number on the sides and on the lower surface are
difficult to estimate. The space between ridges is about equal to the
width of the ridges, but as one passes toward the posterior end, the
ridges become more closely crowded together.
From these observations on the formation of ridges it is evident that
they do not represent a wrinkling of the surface such as occurs in a
semi-rigid curved surface when it is made to occupy a smaller space. The
ridges are wrinkles only in appearance, not in origin. The surface of
the ridges is younger than the space between them. It appears as if the
pseudopod which has to widen as it increases in length, could not
liquify the ectoplasm uniformly all around, but only in longitudinal
strips here and there, and that through these openings the ectoplasm
then flows. There is no question about the greater readiness with which
ectoplasm is formed in this ameba as compared with many others, but
after a careful comparison of _proteus_ and _carolinensis_, where ridges
are formed, with _discoides_ (Figure 11, _B_), _dubia_ (Figure 11, _C_),
_laureata_ (Figure 4) and _annulata_, where none are formed, the only
conclusion presenting itself is that the visible physical properties of
the protoplasm of _proteus_ and _carolinensis_ give no hint as to the
cause of the presence of ridges in these species. The protoplasm of
_discoides_ and _laureata_ is about as viscous as that of _proteus_, yet
in these there is never any ridge formation.
The ridges in _proteus_ recall, of course, the ridges always observed in
_verrucosa_, _sphaeronucleosus_ (Figure 13) and their congeners,
especially while the latter are in locomotion. A _sphaeronucleosus_ is
especially favorable for study in this connection because of its greater
activity. This ameba has four or more longitudinal ridges on its upper
surface, while in locomotion, which strongly resemble those in _proteus_
and _carolinensis_. The chief difference lies in the fact that in
_sphaeronucleosus_ the ridges are extended at their anterior ends
continually, and unless the direction of locomotion is changed, the
ridges may retain their identity while the ameba moves several scores of
times the length of its body. Along the sides, however, new ridges are
continually replacing older ones. When the direction of locomotion is
changed, the old ridges usually all disappear into a jumble of ridges
and crinkles running in every conceivable direction, and with the
reestablishment of locomotion along a more or less straight path, a new
set of ridges appears. In _sphaeronucleosus_ and its congeners, the
ridges are also not wrinkles, but ridges that are formed later than the
surface contiguous to them.
It is interesting to recall also that the ectoplasm in
_sphaeronucleosus_, _verrucosa_ and the rest of this group, is much
firmer than in most other amebas.
CHAPTER V
PSEUDOPODS AND THE NATURE OF THE ECTOPLASM
In contrast with the ridge-forming amebas stand those with smooth
ectoplasm, such as the common _dubia_, _discoides_, _villosa_, and the
rarer _laureata_ and _annulata_, to mention only a few of the larger
forms. In addition to these may be mentioned all the pelomyxas and
nearly all the smaller amebas. Much the larger number of species of
amebas do not form ridges in the ectoplasm during locomotion.
[Illustration: Figure 4. _Amoeba laureata._ This ameba is multinucleate,
containing a thousand or more nuclei of the shape shown at the right.
Ameba 1000 microns long in locomotion. Nuclei 10 microns in diameter.]
Of all the amebas with smooth surfaces, the most favorable for
observation as to the formation of ectoplasm, is the giant _laureata_
(Figure 4), though it is unfortunately of infrequent occurrence. This
species is as often found in clavate form as with pseudopods. In cross
section it is circular or nearly so. It is often found with
_zoochlorella_ growing in it, upon which it seems to depend largely for
food, for it seldom has distinctive food masses in it. The nuclei are
small and very numerous and the crystals are well formed and numerous,
each in a small vacuole, and of a size about two or three times those
found in _proteus_. It will be seen therefore that there are only small
bodies in this ameba, none of which (excepting the contractile vacuole)
are large enough to change the course of the endoplasmic stream, and
streaming is thus reduced to what might be called a typical condition.
In this ameba the endoplasmic stream flows uniformly towards the
anterior end where it spreads out slightly so as to preserve the same
general diameter of the ameba, for it is a characteristic of this ameba
that the anterior end is of about the same diameter as the posterior,
when in clavate form. The ectoplasmic tube is built at the anterior end,
and remains as constructed until it is drawn in at the posterior end to
form endoplasm. It is not all the time undergoing changes such as are
observed in _proteus_. This characteristic is very well shown by
focusing with the high power of the microscope on the upper surface of
the ameba. The immobility of the ectoplasm is much more readily observed
in _laureata_ than in perhaps any other species, a condition that is due
chiefly to the large crystals whose displacement is the most convenient
criterion of ectoplasmic mobility.
The ectoplasmic tube is not as thick as in _proteus_, though it appears
to be more solid than in that species. It is thrown into folds at the
posterior end as it is liquified to form endoplasm, which indicates a
firm texture of the ectoplasm. As to the endoplasmic stream, it presents
no visible characteristics which set it apart from the fluids of
physics; it moves most rapidly in the middle, and gradually less rapidly
as the ectoplasm is approached. There is no backward movement of the
ectoplasm against the sides of the pseudopod at the anterior
end--nothing approaching a “fountain current”--which indicates that the
transformation of endoplasm into ectoplasm is rapid and complete. That
is, all the endoplasm which reaches the anterior end is turned into
ectoplasm. Typically this would result in an ameba of average size, in a
layer of ectoplasm of a thickness of about one-seventh of the diameter
of the pseudopod (for the area of the cut ectoplasmic tube would equal
the area of the endoplasmic stream). But because of friction against
the sides of the ectoplasmic tube, there is a layer of endoplasm of
appreciable thickness that is practically motionless. This layer of
endoplasm therefore makes the diameter of the endoplasmic stream appear
smaller than it actually is, and the ectoplasmic tube larger than it is.
The actual thickness of the tube of ectoplasm, as distinguished from the
flowing endoplasm, is difficult to measure, but it seems to be about
one-tenth the diameter of the pseudopod. (Kite (’13) found ameboid
ectoplasm to be from eight to twelve microns thick, but he does not
state from what part of the ameba nor from what species the ectoplasm
was taken.) This would indicate that if the transformation of endoplasm
into ectoplasm is as complete as the conditions permit, the thickness of
the friction layer would be about one-twenty-third of the diameter of
the pseudopod. These observations therefore point to the conclusion that
the tendency in _laureata_ is for all the endoplasm to be transformed
into ectoplasm at the anterior end, and for the reverse process to occur
at the posterior end.
Several of the pelomyxas also move in much the same manner as _Amoeba
laureata_, that is, in clavate form and more or less cylindrical in
shape. This is especially the case with _Pelomyxa palustris_ and _P.
belevskii_. But in these species the endoplasm is not completely
converted into ectoplasm at the anterior end, as is shown by the fact
that there is a slight backward current of endoplasm at the sides near
the anterior end (Schultze, ’75). Observation indicates also that the
ectoplasmic tube is thinner than would be the case were there complete
transformation of endoplasm into ectoplasm at the anterior end. The
origin of pseudopods in these pelomyxas is not steady and under control
as in _laureata_, but sudden and eruptive, indicating a less coherent
ectoplasm.
The nearest approach to the conditions of streaming as found in _Amoeba
laureata_ is found in _A. discoides_ (Figure 11, _B_) a species often
confounded with _proteus_. This species is frequently found in clavate
form, and the conversion of endoplasm into ectoplasm is complete at the
anterior end. In other respects of streaming and pseudopod formation,
the two species are also similar.
In another very common species of ameba, _Amoeba dubia_ (Figure 11,
_C_) the clavate stage of locomotion is comparatively rare, but when it
is found it is observed that the transformation of endoplasm into
ectoplasm at the anterior end is incomplete, and the endoplasm seems to
be of very liquid consistency. This ameba is characterized by the
possession, usually, of numerous pseudopods extending from a central
mass of protoplasm. In this stage it possesses no _main_ pseudopod as
does _proteus_, _discoides_, _laureata_ and other species, but there are
three or four pseudopods extending actively in the general direction of
locomotion. The physical characteristics of these pseudopods, in so far
as streaming is affected, are different from those of the clavate
amebas. The ectoplasmic tubes are relatively thicker, the endoplasm is
less fluid, and new pseudopods are not formed so readily. It appears
therefore that an increase of surface in the ameba serves to increase
the amount of ectoplasm that is formed during locomotion.
[Illustration: Figure 5. _Amoeba limicola_, after Penard. Figures _a_,
_b_, _e_, illustrate the “eruptive pseudopods” by means of which this
ameba moves. _f_, a variety or separate species whose ectoplasm is
somewhat firmer, and whose posterior end possesses a conspicuous uroid.
_c_, the nucleus found in _a_, _b_, _e_. _d_, the nucleus found in _f_.]
There is another group of amebas in which the endoplasm is much more
fluid than in _dubia_. To this group belong _Amoeba limicola_ (Figure 5)
and _Pelomyxa schiedti_ (Figure 6). The latter never forms pseudopods,
and the former does so very seldom. _A. limicola_ is extremely fluid,
and in locomotion the flow of the endoplasm can hardly be called
streaming, for it rushes about in the body as if it were only partially
under control. The ectoplasm does not give way steadily at the anterior
end during locomotion, allowing a steady forward flow of the endoplasm,
but it breaks away suddenly here or there, allowing the endoplasm to
rush through as if it were under considerable pressure. When the
endoplasm rushes through these breaches in the ectoplasm, it is usually
deflected back along the side of the ameba for a considerable distance,
thus leaving a part of the old ectoplasmic wall stand for a few seconds
between the reflected wave of ectoplasm and the main body of the ameba.
It is then that one can observe especially well the very thin ectoplasm
covering the ameba, the thickness of which is about one-fortieth the
diameter of the ameba. This ameba is somewhat dorso-ventrally flattened
and generally oblong in shape during locomotion.
[Illustration: Figure 6. _Pelomyxa schiedti_, after Schaeffer. _b_,
bacterial rods characteristic of the genus _Pelomyxa_. _c, v,_
contractile vacuole. _g_, glycogen bodies. _n_, nucleus. _u_, uroidal
projections. At the left is shown a series of outlines of the animal
during locomotion. Length, about 75 microns.]
_Pelomyxa schiedti_ moves in much the same way that _Amoeba limicola_
does; that is, by eruptive waves of endoplasm which are usually
deflected back along the side (Figure 6, at the left). The endoplasm is
likewise of very thin consistency. The thinness of the ectoplasm and the
ease with which it may be ruptured, is very well shown by the fact that
the large irregular glycogen bodies (Štolc, ’00) which fill it to
capacity, lie so close to the surface that it is frequently impossible
to see any protoplasm between them and the exterior. The contractile
vacuoles which are numerous, also testify in their characteristics, to
the ease with which the ectoplasm may be broken. The vacuoles never
reach but a very small size (four microns in diameter) presumably
because of the thin consistency of the endoplasm and because they can
readily break through the ectoplasm. They burst on the surface of the
ameba instantaneously, as a small air bubble might burst on pure water.
But this ameba differs from _limicola_ in that a cross section of the
body is very nearly a circle.
[Illustration: Figure 7. _Amoeba radiosa_, after Penard. _a_, the rayed
stage. _b_, the rayed stage in which some of the pseudopods are being
withdrawn. One of them is thrown into a spiral as it is being withdrawn.
_c_, the stage preceding the trophic stage shown at _d_.]
Another very interesting feature of _Pelomyxa schiedti_ is the uroid
(Figure 6, _u_), which in this species consists of a number of very thin
projections resembling pseudopods extending from the posterior end.
These projections are attached to the substratum and in some way aid in
locomotion. These uroidal projections are of considerable length, and
may persist for a considerable length of time. Thus while _schiedti_ is
unable to form pseudopods at its anterior end, it forms uroidal
projections with great ease at its posterior end. But what the
conditions are which are necessary for the formation of a uroid, a
structure which it may be added, exists in many species of amebas (and
perhaps also in Cercomonas), is quite unknown.
In contrast to the amebas thus far discussed from the point of view of
the transformation of endoplasm into ectoplasm, there are a number of
species in which two distinct methods of endoplasmic transformation
occur typically. Among these species are the small _Amoeba radiosa_
(Figure 7), _A. bigemma_ (Figure 8) and a new species which for
convenience will be referred to as _bilzi_.
[Illustration: Figure 8. _Amoeba bigemma_, after Schaeffer. _a_, usual
form in locomotion, showing the numerous pseudopods, vacuoles, nucleus
and food body. _b_, rayed stage frequently assumed when suspended in the
water. The pseudopods in this stage are clear, slender, and more rigid
than those in stage _a_. _c_, an excretion sphere attached to a
twin-crystal characteristic of this ameba. _d_, the nucleus, consisting
of a clear nuclear membrane and a mass of chromatin granules in the
center. _e_, a small sphere attached to a crystal. _f_, a twin crystal
unattached to a sphere. Length of _a_, 150 microns; of _d_, 12 microns;
of _f_, 2 microns.]
It is well known that _radiosa_ has two stages: a more or less clavate
shaped stage in which the ameba creeps along the surface of some object
(Figure 7, _d_); and a stage in which a number (eight or less) of long
and very slender tapering pseudopods are formed which usually persist
for a long time (Figure 7, _a_, _b_). These pseudopods are frequently
quite straight and regularly disposed around the central mass of
protoplasm (Penard, ’02, pp. 87, 89). In no case are any endoplasmic
granules found in these slender pseudopods; they consist entirely of
hyaloplasm. In retracting these pseudopods a curious phenomenon is
sometimes observed; the pseudopod is rolled up into several (as many as
six) turns of an almost perfect helical spiral of a diameter six to
eight times that of the pseudopod. But as the process of withdrawal
proceeds, the spiral becomes irregular, but parts of some of the turns
persist in the last vestiges preceding complete withdrawal (Figure 7,
_b_). These spirals are also observed in other species besides _radiosa_
(see p. 128 seq.)
Another species of ameba in which a trophic as well as a rayed stage is
found, is the recently described species _bigemma_. In this species the
rayed stage is only of occasional occurrence (Figure 8, _b_). The larger
the ameba is, the rarer is the rayed stage assumed. On very rare
occasions one finds a rayed stage in which the pseudopods are long,
straight, slender and tapering, and more or less regularly disposed
around the central mass of protoplasm. The trophic stage (Figure 8, _a_)
is much the more common. In this condition pseudopods are formed in
large number. They are small, conical or linear, and blunt, and they do
not determine the direction of locomotion, as they do in _proteus_,
_dubia_, or _laureata_. These pseudopods are often composed only of
hyaloplasm, though frequently the basal parts of them consist of
endoplasm. When these amebas become suspended in the water, they
frequently assume a shape that approaches the rayed condition: six or
more long conical pseudopods are run out from the central mass of
protoplasm, but the pseudopods are not straight in this case, but
irregularly curved and capable of being waved about to a slight extent.
The ameba readily passes from this stage to the trophic.
The species _Amoeba bilzi_ (Figure 9) has come under my observation on
several occasions, and its pseudopodial characters are of considerable
interest in this connection. In its usual form this ameba has the
general appearance of a _sphaeronucleosus_.
[Illustration: Figure 9. _Amoeba bilzi_. _a_, the ameba in locomotion,
showing the ectoplasmic ridges, nucleus, contractile vacuole. _b_, the
transition stage between the rayed stage (which resembles that of
_radiosa_, Figure 5, p. 30, somewhat) and the stage shown at _a_. The
whole of the ameba flows into the broad pseudopod with the arrow. Length
of _a_, 90 microns.]
In size it is about midway between the latter species and _striata_. It
always has a number of prominent longitudinal ridges on its upper
surface. Its mode of streaming is essentially like that of _striata_ or
_sphaeronucleosus_. When this ameba is disturbed and left suspended in
the water, it throws out four or five or more long slender pseudopods
composed entirely of hyaloplasm, excepting a bulbous base which consists
of granular endoplasm. The pseudopods are cylindrical with tapering
ends. They are very rigid, and once formed, persist for a considerable
length of time. When these pseudopods are about to be retracted, the
wall weakens at some point and then crinkles while the distal part of
the pseudopod bends, often at a decided angle. The crinkling of the wall
continues up and down the pseudopod while it is slowly being withdrawn.
These pseudopods, as well as those of the rayed state in _radiosa_ and
_bigemma_, are not pseudopods of locomotion but of _position_; they are
not dynamic but static structures. But there are no hard and fast
distinctions to be made between these two types of pseudopods, for at
least in _bigemma_ and _bilzi_, there are transitional forms of
pseudopods (Figure 8, _b_).
The formation of pseudopods and their character depends to some extent
upon the firmness and thickness of the ectoplasmic layer; and the
character of the ectoplasm in turn depends largely upon the consistency
of the protoplasm as a whole. In the following representative list of
amebas: _limicola_, _villosa_, _dubia_, _proteus_, _discoides_,
_laureata_, _bigemma_, _bilzi_, _radiosa_, _sphaeronucleosus_,
_verrucosa_, the given order indicates a progressively thicker and
firmer ectoplasm as one passes from _limicola_ to _verrucosa_. But from
_limicola_ to _bilzi_ the number of pseudopods directing locomotion
increases from one to an average of about twelve in _dubia_, and then
falls gradually to one in _bilzi_ and the others beyond in the list.
(See Figure 10.) Where the directive pseudopods begin to disappear, the
transitional appear, viz., in _bigemma_ and _bilzi_; but beyond these no
transitional pseudopods occur. But along with the transitional there
begin to appear also the static pseudopods, which are seen relatively
seldom in _bigemma_ and _bilzi_ while in _radiosa_ they occur at almost
all times. In _sphaeronucleosus_ and _verrucosa_ no distinctive
pseudopods of any kind occur.
If all the known species of amebas in which the necessary
characteristics have been recorded, were arranged similarly with respect
to the firmness and the thickness of the ectoplasm, the general
relations of the various kinds of pseudopods in the list would be
approximately the same as in the list given above; but there would
appear an exception here and there, indicating the operation of special
factors. Such an exception, for example, is seen in _proteus_ in the
list of species given, which because of the ridges that it forms (Figure
3) has a smaller number of pseudopods than would be the case if no
ridges were formed[2]. It may be concluded, then, that the number and
character of pseudopods depends in large part upon the ectoplasm-forming
capacity of the ameba; and that this property is intimately associated
with the degree of fluidity of the whole mass of protoplasm in the
ameba.
[Illustration: Figure 10. Graph representing the relation of firmness
and thickness of the ectoplasm with the number and character of the
pseudopods in different species of amebas. _a_, the average maximum
number of pseudopods directing locomotion in the different species of
amebas. _b_, the number of transitional pseudopods. _c_, the number of
static pseudopods. _d_, the estimated degree of firmness and thickness
of the ectoplasm of the various species of amebas, grading that of
_limicola_ as 1 and that of _verrucosa_ as 6.]
That the number and character of pseudopods formed depends _in large
part_ upon the firmness and thickness of the ectoplasm was said
advisedly. For observations indicate that there are other factors which
influence the character of pseudopods besides those which also control
the formation of ectoplasm. These other factors indicate their presence
readily in the details of structure of the pseudopods. Thus the number
of directive, transitional or static pseudopods may be the same in two
particular species, yet in their intimate structure and appearance they
are always found to differ. In _bigemma_, _bilzi_ and _radiosa_, for
example, the number of static pseudopods when formed is about the same
in the three species, but the similarity ends there. For these species
differ in the frequency with which pseudopods are formed, in their
persistence when once formed, in the ratio of length to average
diameter, in the general shape, in the frequency with which straight
pseudopods are formed, in the speed of their formation and withdrawal,
in the manner of their withdrawal, in their disposition with respect to
geometrical pattern, in the character of the bases of the pseudopods, in
the form of the free ends, and so on. Many of these characteristics are
still further analyzable into numerous other and more detailed
characters. And what is true of the static pseudopods is likewise true
of the transitional and the directive. Pseudopod formation is however
only a small part of the activity of an ameba. The formation of uroidal
projections, of vacuoles of various sorts, of crystals, and so on, are
some other general activities that are fully as subject to specific
variation as pseudopod formation. Again in behavior to food and various
other stimuli, in resistance to various factors in the environment, in
reproductive processes, and so forth, there is found similar specific
peculiarity. In fact, one looks in vain for similarity between any two
species of amebas except in their most generalized characters. From my
own experience in extended observation of several dozen species, which
included a large number of characters, as pointed out above, I have not
found two species of which I can confidently assert that any particular
character defined as accurately as possible was present in both. In
different words, my experience indicates that no two species are alike
in any respect whatsoever. Each species appears unique from every point
of view and in the smallest definable detail. The concept of specificity
therefore is much more fundamental in amebas than has been believed to
be the case hitherto (cf. Calkins, ’12). The intimate structure of
amebas is indeed similar to that of higher animals where the precipitin
reactions (Richet, ’02, ’12; Reichert and Brown, ’09; Dale, ’12; Nuttal,
’04; also Todd, ’14) have indicated that the various albumins are of
specific structure and reaction.
As an example of these specific differences, reference may be made to
the three species, _protus_, _dubia_ and _discoides_, which have been
referred to in the past, almost without exception, by the most
experienced teachers of biology, as being one species: _proteus_. Some
investigators of ameboid phenomena have likewise confused these
different amebas. Below is given a list of some of the most striking
characteristics of these three amebas. This list is of course very
sketchy. If the nuclear division phenomena, for example, were well
known, which they are not, those character differences alone would
doubtless make a list several times as long as this one. Compare with
Figure 11.
[Illustration: Figure 11. _A_, _Amoeba_ proteus in locomotion. Note
especially the longitudinal ridges. _a_^{1}, equatorial view of the
_discoid_ nucleus. _a_^{2}, a polar view of the nucleus. _a_^{3},
equatorial view of a folded or crushed nucleus frequently found in large
individuals. _a_^{4}, shape of crystals found in this species._B_,
_Amoeba discoides_ in locomotion. _b_^{1}, _b_^{2}, equatorial and polar
views of the _discoid_ nucleus. _b_^{3}, shape of the crystals found in
the ameba. _C_, _Amoeba dubia_ in locomotion. _c_^{1} and _c_^{2},
equatorial and polar views of the _ovoid_ nucleus. _c_^{3}-_c_^{10},
shapes of crystals found in _dubia_. In these drawings only such
characters as are of special interest for the purpose of this work are
emphasized. Dimensions in microns: _A_, 600; _B_, 450; _C_, 400;
_a_^{1}, 46 × 12; _b_^{1}, 40 × 18; _c_^{1}, 40 × 32; _a_^{4}, maximum,
4.5; _b_^{3}, maximum, 2.5; _c_^{3}-_c_^{10}, maxima, 10 to 30.]
This fundamental uniqueness of all the characters of the various species
of amebas naturally gives rise to the question as to what is the cause
of this condition of affairs. Why and how
| _Amoeba | _Amoeba | _Amoeba
Characteristics | discoides_ | proteus_ | dubia_
------------------+-----------------+------------------+------------------
Size in locomotion|450 microns |600 microns |400 microns
Pseudopods |cylindrical |dorso-ventrally |dorso-ventrally
| | flattened | flattened
|smooth ectoplasm |folded ectoplasm |smooth ectoplasm
|“main” pseudopod |“main” pseudopod |no “main” pseudopod
| present | present |
|cross section |cross section an |cross section
| circular | irregular oval | oval
|average number |average number |average number
| in locomotion, | in locomotion, | in locomotion,
| three | five | twelve
Crystals |very numerous |less than in |relatively few
| | _discoides_ |
|all uniform |all uniform |at least four
|truncated | truncated | varieties present;
|bi-pyramids | bi-pyramids; | few perfect
| | rarely a | crystals
| | few flat plates |
|maximum size |maximum size | maximum size
| 2.5µ 4.5µ 10µ, 12µ, 30µ
Fission |slower than |average 1 division| faster than _proteus_
| _proteus_ | in 48 hours at |
| | 20° C. |
Maximum time | | |
between divisions|20 days |8 days |6 days
Multinuclearity |binucleate |binucleate |binucleate very
| occasionally | frequent; |
| | tetranucleate |rarely
| | occasional |
Nucleus, shape |biconcave disc, |biconcave disc, |ovoidal
| never folded | frequently folded|
size |40µ × 18µ 46µ × 12µ 40µ × 32µ
General resistance| | |
to same | | |
conditions |slight |very great |greater than
| | | _discoides_
Surface of | | |
posterior end |free from debris |free from debris |carries debris
Effect of | | |
mechanical | | |
stimuli |slightly |responsive |very responsive
| responsive | |
Food cups |small |large |often enormous
Reaction to |readily eaten; |readily eaten; |eaten only occasionally;
carmine | rejected in a |rejected in a | often
| few minutes | few minutes | retained for hrs.
Distribution |sporadic, small |very common |sporadic, frequently
| numbers | | in large numbers
are the different species of amebas so absolutely different, even to the
smallest detail? Why are the apparent resemblances and similarities of
their more generalized kinetic characters, such as the formation of
pseudopods, of ectoplasm, of crystals, of contractile vacuoles, the
general character of endoplasmic streaming, the formation of
ectoplasmic ridges, and so forth, found, upon analysis, to resolve
themselves into a large number of details which differ more strikingly,
the corresponding characters of one from those of the other, than do the
generalized characters of which they are composed?
These questions apply, of course, to all other organisms as well as to
amebas. Unfortunately, however, these questions are at present
unanswerable for all organisms. But for the amebas, at least, the
problem of form can be rid of some irrelevant matter which, in numerous
instances in the past, has been assumed to be properly included.
In the first place, changing a single character of the protoplasm, such
as the degree of viscosity, cannot explain the observed diversity of
detail; neither can a variation of a number of the physical characters
of fluids produce such differences as are observed in the dynamics of
the different species of amebas. Our whole experience with the fluids of
physics speaks against such an explanation. But, on the other hand, the
invisible details of structure of a fluid may become strikingly manifest
under certain conditions, namely, those surrounding the process of
crystallization. A slight change in the physical condition may produce a
considerable variety of crystal shapes, but this variety of shape has
nevertheless very definite limits which cannot be overstepped.
Amebas like crystals are also most rigidly and definitely restricted to
a certain range of shape, which must be a direct result of the structure
of the protoplasm composing them. Amebas in fact are not any more
“shapeless” than crystals are; and it would be quite as exact to say
that the crystals of water are shapeless since a great variety of shapes
are met with in snow, hoar-frost, etc. The fact that corresponding parts
of two species of amebas resemble each other less and less closely as
they are analyzed into smaller and smaller details, is in itself
conclusive evidence that the protoplasms of the amebas are _chemically_
different; the resemblance between the gross anatomy and physiology
between two different species is due to the greater conspicuousness of
such characters as are the result of the action of physical processes.
That is to say, chemically or molecularly different masses of matter may
resemble each other in their molar aspects.
It is to be noted however that the more intimate structure of streaming
protoplasm cannot always express itself externally as it can in ameba.
As was suggested in the introduction, there is no good reason for
supposing that the causes of streaming in the various organisms in which
it is observed are fundamentally different. The problem of ameboid
movement cannot be considered apart from the streaming of protoplasm in
foraminifera, myxomycetes, plant cells, lymphocytes, desmids, diatoms
and ciliates. The streaming of endoplasm in some cells, such as in
ciliates and plant cells, does not give rise to change of shape of the
cell as it does in ameba. In these cases the character of streaming is
highly restricted; the unyielding ectoplasm or cell wall as the case may
be, prevents any but the most essential features of streaming from
occurring. Recalling the analogy of crystallization, streaming in a
plant cell or in a ciliate is analogous to crystallization occurring in
a tube or vessel too small for the crystals to form properly.
This discussion anent the fundamental chemical uniqueness of each
species of ameba is of course not complete without an examination of the
views expressed to the contrary. And it is to this side of the
discussion that we may now briefly direct our attention.
CHAPTER VI
THE SPECIES QUESTION
After the discovery of the ameba by Rösel v. Rosenhof and the
introduction of the Linnean system of nomenclature, the number of new
species of amebas that were reported increased rapidly. But in 1856
Carter suggested that what had been described as _A. radiosa_ probably
was a young stage of _A. proteus_. With the general acceptance of the
Darwinian Natural Selection Hypothesis, the ameba came to be looked upon
as standing at the bottom of the scale of organisms, and consequently
was supposed to lack generally such characters as the higher forms
possessed. The ameba became the representative of the “primordial slime”
from which by slow stages the other organisms were evolved. So of the
sixty odd species which had been described up to Leidy’s (’79) time,
Leidy, following the suggestion of Carter, was inclined to think that
the great majority of these represented only changes of shape of about
four species (not including the several species that were then known to
be parasitic). Since Leidy’s time the prevailing tendency has been to
regard most of the “new” species as mere environmental or life cycle
stages of a very few species. A very noted exception to this tendency,
however, has been Penard’s (’02) great work on the amebas and other
rhizopods of the Leman Basin, in which he describes forty-five species
of amebas (including _Gloidium_, _Protamoeba_, _Amoeba_, _Dinamoeba_,
_Pelomyxa_), paying attention mainly to the readily observed ectoplasmic
and endoplasmic characters, and the appearance of the resting nucleus.
The remarkable discoveries of Vahlkampf (’05) of the nuclear changes
during the division process turned the attention of numerous
investigators to this field, and the ectoplasmic and endoplasmic
characters thenceforward received scant attention. Thus Calkins (’04)
came to suggest as Carter had done many years before, that _A. radiosa_
was merely a young form of _A. proteus_. And Doflein (’07) intimated
that the protoplasmic characters of _vespertilio_ cannot be
distinguished from those of _verrucosa_, _radiosa_, _polypodia_, _limax_
and _guttula_. Schepotieff (’10) in a similar vein, writes: “Wir werden
demnach so bekannte und so lange Zeit als selbstständige und typische
Amöbenarten aufgefasste Formen wie _A. limax_, _A. polypodia_, und _A.
radiosa_ nur als Umwandlungsstadien andrer Arten bezeichnen dürfen.”
Gläser (’12) remarks: “The most reliable criterion for the
classification of the amebas is the division of the nucleus.” Calkins
(’12) takes the same view on this point and states that in his opinion
the ectoplasmic and endoplasmic characters of amebas conform to four
“types,” viz., _proteus_, _verrucosa_, _vespertilio_ and _limax_. The
enormous amount of work that has been done on the nuclear division
changes as compared with the small amount of work on the cytoplasmic
structure has thus naturally tended to an over-estimation of the
significance of the nuclear changes.
There are objections to making the nuclear changes the basis of the
classification of the amebas.
1. In the first place, to classify the amebas means not only labeling
the different species accurately, but also to assign to them their
proper place in the system of organisms. All organisms are classified
with this purpose in view. This is what is meant by a _natural system
of_ classification as contrasted with an _artificial system_ based on
only a part, arbitrarily selected, of each of the organisms concerned.
In the past all artificial systems have been discarded. It is perhaps
unnecessary to say that a classification based on nuclear characters
would be a highly artificial system. For in no group of organisms has it
been found possible thus far to use the nuclear changes as a basis of
classification. The great amount of labor that has been expended by
cytologists within recent years on the behavior of chromosomes, and the
immense amount of work done by the students of genetics, has failed to
show any specific relation whatever between the external characters of
organisms and the nuclear behavior.[3] In other words, the
peculiarities of mitotic processes have not been found to be correlated
with characters in the somatoplasm. It is to be remembered however that
all living organisms, with the exception of some of the bacteria, are
classified with respect to their external characters, and that in almost
all organisms the number of visible and demonstrable specific characters
becomes rapidly greater as ontogenetic development proceeds.
2. There is considerable disagreement among the investigators of the
nuclear phenomena of amebas as to the actual events occurring during the
division process. Cf. Dobell (’14) and Hartmann (’14) _in re Amoeba
lacertae_; Nägler (’09), Gläser (’12) and Wilson (’16) on the presence
of a centriole in amebas; etc. The work of Schardinger (’99), Wherry
(’13) and Wilson (’16) on the nuclear stages of amebas was done with
care, yet Wilson (’16) still remains in doubt as to whether or not these
investigators all worked on the same species.
3. Awerinzew (’04, ’06) found that the nuclear changes in _Amoeba
proteus_ are similar to those in the heliozoan _Actinosphaerium_; there
being thus greater correspondence in the nuclear changes between species
belonging to different orders than there is between species in the same
genus. Logically therefore _Actinosphaerium_ would have to be placed in
the same genus with _Amoeba proteus_.
4. There is the great practical objection that in many of the larger
species it is extremely difficult to find suitable division stages even
though thousands of individuals are at hand, and the search is continued
for days and weeks by an experienced investigator (Dobell, ’10).
Experimental work, which is usually done with one of the larger species,
would thus be greatly handicapped because of the great difficulty in
determining the nature of the organism employed.
From these considerations it appears that the attempt to classify the
amebas on the basis of the nuclear changes is highly artificial and
exceptional, and if we may judge from past attempts to classify
organisms on the basis of a single character, is foredoomed to failure.
This conclusion does not apply, however, to very minute amebas in which
no specific cytoplasmic characters have yet been established, chiefly
because of their very minuteness; such amebas could be given specific
names for reference but they could not be classified in a natural system
excepting perhaps as a group.
But the definiteness and the consistency with which the nuclear division
stages occur in any given species of ameba, lends support to the
probability that in these animals the relation existing between the
chromatin and the cytoplasm are similar to those observed in higher
animals; and that the laws governing the transmission of cytoplasmic
characters in amebas are quite as inflexible as those governing
somatoplasmic characters in the higher organisms. Among the
investigators of cytologic and genetic phenomena (among the
multicellulars) the belief is practically unanimous that the elaborate
mechanism involved in nuclear division is primarily a design for
distributing the factors concerned in heredity. Now it would be very
strange indeed if a similar and quite as complicated a mechanism in
ameba had no function to perform. For what would be the purpose of the
complicated nuclear changes in ameba if not concerned with heredity? As
has already been seen, however, there are numerous cytoplasmic
characters, in the larger amebas at least, that are inherited from one
generation to the next with as little variation as is observed in other
organisms (Schaeffer, ’16). The recent work of Jennings (’16) on
_Difflugia_ and Hegner (’18) on _Arcella_ also indicates that the
general processes of inheritance in these organisms which are closely
related to amebas, are similar to those observed in higher forms. The
conclusion seems justified therefore that the nuclear changes in amebas
mean essentially the same thing as in other organisms.
We are now therefore in a position to say that amebas are definitely and
thoroughly organized; that they are not really “shapeless”; that they
are not more subject to variation than a higher organism is; and that
each species differs from all others in probably every visible detail.
The large variety of pseudopods observed in different species are seen
not to be the result of physical or extrinsic chemical forces acting
upon ectoplasms differing in some mere physical character as viscosity.
But all these peculiarities are hereditary, and are due to a fundamental
chemical structure of the protoplasm which is specific for the species.
The highly characteristic nature of the pseudopods formed by the amebas
of any species, it is seen, is to be referred to the fundamental
structure of the protoplasm, probably its stereochemical structure. And
what is of especial importance for this discussion, the character of
streaming concerned with pseudopod formation and with movement in
general, which is specific for each species, is likewise found, to some
extent at least, to be conditioned by the specific structure of the
protoplasm.
That the specific character of the pseudopods, and the streaming which
of course lies back of it, is not wholly or perhaps even largely, due to
the specific structure of the protoplasm, is evident from a
consideration of streaming in some other organisms, without a study of
which, streaming in amebas can be only imperfectly understood.
The formation of pseudopods is not necessary to streaming. Occasionally
one sees internal currents unaccompanied by movement or ectoplasm
formation in amebas approximating spherical shape, such as in _Amoeba
blattae_ (Rhumbler, ’98) and rarely also in _proteus_ or _dubia_. But
especially well is such streaming seen in a contracted _Biomyxa_, a
naked foraminifer, and in numerous plant cells. In paramecium and other
ciliates the continuous circulation of the endoplasm,--a true streaming
process,--is an involuntary act. But in _Frontonia_, another large
ciliate, the circulation of the endoplasm is under the control of the
animal, that is to say, voluntary, and is set in motion only when
feeding, the direction of streaming being away from the mouth so as to
drag in the food (see Figure 32, p. 99). If the food particle is a long
filament of _Oscillatoria_, for example, the endoplasm circulates very
much as it does in paramecium, only more rapidly, until the whole
filament is wound up into a coil. Then streaming stops. In the second
place streaming is not necessarily accompanied by the formation of
ectoplasm as observed in ameba. In plant cells the ectoplasm is
practically stationary, while the endoplasm is in continual flux. The
transformation of endoplasm into ectoplasm and vice versa is therefore
not an essential feature of streaming, though it is of locomotion; that
is, ectoplasm is always found between endoplasm and water, though it
might be possible under certain conditions for endoplasm to come into
contact with water without stiffening. And if so, there appears to be
no reason why locomotion might not occur. It appears however under
normal conditions that a moderate tendency to ectoplasm formation
(_proteus, dubia_) leads to greater efficiency in movement than a very
weak (_limicola_) or a very strong (_verrucosa_) tendency to form
ectoplasm.
In the reticulose rhizopods, as is well known there is no ectoplasm of
the kind observed in amebas. The middle of the pseudopod, moreover, is
not the region of most rapid streaming as in ameba, but frequently
becomes congealed, on the contrary, into a rod-like structure. In
general this axial rod has the character of very stiff ectoplasm. The
character of streaming in reticulose rhizopods, however, has received
very little attention, and detailed comparisons are therefore
impossible.
Another interesting property of reticulose pseudopods, which are formed
by a streaming process, is their great power in some species, of rapid
contraction. If a diatom for example, in its movements breaks loose a
pseudopod it is often (though not necessarily) contracted very rapidly,
much more rapidly than could be the case if it were accomplished by
streaming. It frequently happens that knobs are found on a slender
pseudopod. These knobs may move back and forth with great rapidity
without visibly affecting the pseudopod (Figure 12). The process reminds
one of a block sliding on a rope. These observations indicate a very
high degree of elasticity in the formed pseudopods of such a rhizopod as
_Biomyxa_ as compared with a very low degree of elasticity in the
amebas.
It thus appears that the process of streaming is a much more fundamental
phenomenon than most of the theories accounting for ameboid movement
would lead one to suppose; for these theories concern themselves only
with streaming as observed in amebas, and many content themselves with
only two or three species. Since the general features of streaming are
similar no matter where streaming occurs, no theory is likely to gain
acceptance that explains streaming only in one group of organisms.
Streaming in rhizopods, myxomycetes, ciliates, plant cells, is most
rationally looked upon as caused by the same fundamental process; but
the detailed form it takes, especially in freely formed pseudopods, is
undoubtedly conditioned by the structure of the protoplasm, both
physically and chemically, but more especially the latter.
[Illustration: Figure 12. Illustrating the high degree of elasticity in
the pseudopods of _Biomyxa vagans_. In _a_ and _b_ are shown two stages
of a small section of the pseudopodial network, which remained unchanged
while a small lump of protoplasm (near the arrow) moved rapidly up and
down the slender pseudopod. Movement along the whole length of the
pseudopod occupied about half a second. Just exactly what the movement
was due to could not be determined, but the distance between the forks
in the pseudopod did not change, nor did the thickness of the
protoplasmic strand on which the protoplasmic lump moved change
noticeably.]
CHAPTER VII
EXPERIMENTS ON THE SURFACE LAYER OF THE AMEBA
In the preceding chapters we have discussed the streaming of the
endoplasm in various representative species of ameba, and its
transformation into ectoplasm at the anterior end. We have observed that
the details of streaming are not quite the same for any two species of
ameba, and that in consequence the character of locomotion also is
specific for every ameba. All the observations prove that movement in
ameba is always associated with streaming, and streaming (in locomotion)
with ectoplasm formation. It follows therefore that the form of movement
observed in amebas depends invariably upon the streaming of the
endoplasm accompanied by the formation of ectoplasm.
There is however another element which, although it appears to be a
consequence of ectoplasm formation, must nevertheless be included in any
account of ameboid movement because of the light it is bound to shed on
the physical processes concerned in streaming. This element is the thin
outer layer which separates the water in which the ameba lives from the
ectoplasm. It is the properties of this layer to which we may now direct
our attention.
That such a layer exists was indicated by observations of Bütschli (’92)
and Blochmann (’94), as already mentioned; but neither of these authors
stated definitely whether they considered a third layer actually to
exist or whether the ectoplasm as such moved forward. Jennings (’04), as
has been pointed out, concluded that no third layer exists and that the
particles clinging to the outsides of amebas, which are carried toward
the anterior end, are carried by the ectoplasm. Gruber (’12) concluded
however that an outer layer exists, composed of gelatinous substance,
which moves ahead at about the same rate as the ectoplasm (p. 373).
According to Gruber’s view the outer layer is a permanently
differentiated layer of material. Schaeffer (’17), on the contrary calls
it a layer of protoplasm, which moves forward faster than the forward
advance of the ameba.
It is a very simple matter to demonstrate the existence of this layer.
Although any insoluble non-toxic substance of low specific gravity such
as carmine or soot, when reduced to very small particles and mixed with
the water in which the amebas to be examined live, will cling to the
outside of the ameba so that the movement of the outer layer can be
observed; in my experience the best as well as the most convenient
substance to use is the dried flocculent colloidal sediment from ameba
cultures, rubbed to powder with the ball of the finger. This powder
swells up in water into flocculent masses which are large for their
weight and do not show such active Brownian movement as particles of
carmine or india ink, and they consequently adhere more easily to the
ameba. Moreover no foreign substances are thereby introduced into the
water.
[Illustration: Figure 13. _Amoeba sphaeronucleosus._ In locomotion. Note
the nucleus, contractile vacuole, ectoplasmic ridges. This ameba is not
known to form pseudopods. Length, 120 microns.]
Of the more common species of amebas, those with the firmer ectoplasms
are the most favorable for studying the movements of the outer layer. We
may therefore first take up several observations on _Amoeba
sphaeronucleosus_ (Figure 13). This ameba resembles the more common _A.
verrucosa_. It is about 120 microns long and is usually of an oval shape
in locomotion. It is more active and less disturbed by jars than
_verrucosa_.
Figure 14 represents a _sphaeronucleosus_ with a small particle attached
to the middle of the upper surface of the ameba. As the ameba moves
forward, shown by successive outlines, the particle likewise moves
forward, but, as will be observed, at a more rapid rate. Measuring the
distance from particle outline 1 to 4, and from ameba outline 1 to 4, it
is seen that the rate of movement of the particle compares with the rate
of movement of the ameba as 2.48 to 1.
[Illustration: Figure 14. Illustrating the movement of a particle on the
upper surface layer of an _Amoeba sphaeronucleosus_. Length of the
ameba, 120 microns.]
[Illustration: Figure 15. An _Amoeba sphaeronucleosus_ with two
particles attached to its upper surface film, one in the middle and one
at the side. _a_ moved 2.6 times as fast as the ameba while _b_, lying
nearer the side, moved only 1.9 times as fast as the ameba. Length, 100
microns.]
Particles lying near the side do not move forward as rapidly as those
lying in the middle. Figure 15 shows two particles, one of which, _a_,
lying near the middle of the ameba, moved 2.6 times as fast as the ameba
advanced in the region of the particle; while particle _b_ moved only
1.9 as fast as the ameba in front of the particle. The speed ratio of
particle _a_ to particle _b_ was as 1.26 to 1.
[Illustration: Figure 16. Illustrating more rapid movement of the
surface film in the middle of _Amoeba sphaeronucleosus_ than near the
edge. The vertical lines connecting the particle with the ameba outlines
were drawn only for convenience of reference. Length of ameba, 120
microns.]
Figure 16 shows a particle lying still more to the side than in the
preceding figure. In the first six stages the particle moved 1.85 times
as fast as the ameba. The particle then came to the edge. From stage 7
to 10 the particle moved more slowly than the ameba. At stage 11 the
particle had come to lie in the posterior half of the ameba, where the
tendency of the surface layer is to travel toward the middle of the
upper surface. In stage 12 the particle had gotten away from the edge of
the ameba and already shows a gain in speed. From stage 13 to 16 the
particle moved again about 1.83 times as fast as the ameba. But at stage
16 the edge was reached with a consequent decrease in speed of the
particle.
The direction of the path described by a particle carried on the back of
an ameba depends upon what part of the ameba is most rapidly forming
ectoplasm. That is, the particle tends to
[Illustration: Figure 17. Illustrating the different speeds with which
particles move when attached to the surface film of an _Amoeba
sphaeronucleosus_, depending upon their location. Particle _a_ moved 3.5
times as fast as the ameba and _b_ 2.7 times as fast. Length of ameba,
110 microns.]
move toward that part of the anterior edge that is advancing most
rapidly. Figures 17 and 18 illustrate this point. Figure 17 shows an
ameba with two particles on its back, and with an unequally advancing
anterior edge. Particle _a_ moved more rapidly than _b_ because: (1) it
was moving away from a more rapidly receding posterior region; (2) the
right anterior edge was advancing more rapidly than the left anterior
edge; (3) the particle was nearer the anterior edge. The rapidly
advancing right edge in stage 4 accounts for the veering of the particle
_a_ to the right. The more rapid advance of _b_ from stage 3 to 5 is due
to the remoteness of the anterior right edge, which, because of its
nearness to particle a pulls on it to a much greater extent than on
particle _b_. That is to say, when a particle lies somewhere _between_
two rapidly growing regions on the anterior edge, leading in different
directions, that particle is attracted to the edge less rapidly than a
particle lying immediately back of either advancing region. As may
readily be observed each change in speed or direction of movement of the
particle _b_ finds its explanation in the amount and location of
ectoplasm formation at the time. Large particles like _a_ do not so
readily reflect changes in the direction of pull of the surface layer.
The rapid rate of movement of particle _a_--3.5 times as fast as the
ameba--finds its explanation in an actively advancing anterior edge that
was unusually wide. Particle _b_ moved at a slower rate, 2.7 to 1. It
started from near the posterior edge where it moved comparatively slowly
for a short distance.
[Illustration: Figure 18. Illustrating the effect on the path of a
particle attached to the surface film of an _Amoeba sphaeronucleosus_
when the ameba changes its direction of movement. From stages 3 to 5 the
ameba veered to the right, also the particle. From stages 6 to 9 the
ameba turned sharply to the left, and this change of direction was
reflected in the movement of the particle. Length of the ameba, about
120 microns.]
Figure 18 shows more pronounced changes in the direction taken by a
particle attached to the back of an ameba. The change in direction at
stage 6 was caused by a wave of ectoplasm thrown out at the left side,
and cessation of movement at the anterior edge. At 7 a small wave was
thrown out at the anterior edge and a large wave on the left. At stages
8 and 9 the direction of the particle was again a response to the waves
of ectoplasm thrown out at the left anterior edge, which thus became the
anterior end.
[Illustration: Figure 19. Illustrating the rapid movement of the upper
surface of an _Amoeba sphaeronucleosus_ under the most favorable
conditions. The particle moved 3.56 times as fast as the ameba. Length
of the ameba, 130 microns.]
The movement of particles on the under side of an _Amoeba
sphaeronucleosus_ depends upon what part of the ameba is attached to the
substratum. Where the ameba is attached there is of course no movement
of the surface layer and the particles remain stationary. In an ameba
attached as shown in figure 20, _a_, there was a very slow movement of
particles forward near the middle of the attached region (x), but
whether this was related to the movement of the outer layer of the upper
surface was not determined. The movement of these particles was
considerably slower than the movement of the ameba. In another ameba
attached at the anterior and posterior ends (Figure 20, _b_) no movement
of particles on the under side could be discerned. The small particles
showing Brownian movement, with the surrounding water, are dragged along
as a mass. This movement is purely mechanical, and is what would be
expected on purely physical grounds, when a more or less cup-shaped
object is moved along in water in close contact with a flat surface.
Such particles as have become attached to the surface layer on the under
side of the ameba, because of their slower movement than that of the
ameba, eventually bring up at the sides near the posterior end, as the
ameba moves along. From here they are carried forward in the manner
already described. Thus there comes about a “rotation” of particles
adhering to an ameba as described by Jennings (’04) and Dellinger (’06),
though the explanation is different from that given by Jennings (l. c.)
as we shall see further on. No case of a similar rotation of larger
particles which had sunk into the ectoplasm, as described by Jennings
(’04, p. 142), has come under my observation.
[Illustration: Figure 20. _Amoeba sphaeronucleosus_. _a_, the under side
of the ameba. The part of the ameba attached to the substratum is
stippled. Particles attached to the surface film at _x_ moved slowly
forward. _b_, the under side of the ameba, showing the attached parts
stippled. The particles suspended in the water at _x_ moved slowly
forward with the ameba. _c_, a cross section of an ameba of shape shown
in _b_, showing the ridges on the surface. Length of the ameba, about
100 microns.]
The movement of the surface layer in _A. verrucosa_ is quite like that
of _sphaeronucleosus_. Figure 21 shows a group of three particles
carried by a _verrucosa_ while changing its direction of locomotion. The
particles changed position with regard to each other and they moved at
different speeds. Particles _a_, _b_, _c_, moved respectively 2.40,
3.26, 2.85 times as fast as the ameba advanced. Other experiments
indicate that the outer layer of _verrucosa_ moves at about the same
speed, compared with the speed of the ameba, as that of
_sphaeronucleosus_.
Amebas with so-called limax-shaped bodies do not possess surface layers
that carry particles forward with the same speed as those amebas with
broad bodies. It is only occasionally that large amebas such as
_proteus_ are found in a limax or clavate shape. One of the most
favorable of the large amebas in this respect is _discoides_. It is
frequently found in clavate shape and it possesses the further advantage
in being nearly cylindrical in cross section. It is also more in the
habit of loping along the surface in the manner described by Dellinger
(’06, p. 57) so that what is observed to take place in _discoides_ in
the clavate shape, holds likewise for free pseudopods extended into the
water out of contact with a solid support (Figure 22).
[Illustration: Figure 21. Illustrating the similarity of the movement of
the surface layer of _Amoeba verrucosa_ with that of _A.
sphaeronucleosus_. A group of three particles, connected by dotted lines
for reference, change their relative positions as the ameba
(_verrucosa_) changes its direction of movement. Length of the ameba,
150 microns.]
[Illustration: Figure 22. Illustrating the movements of an _Amoeba
proteus_, after Dellinger. At _c_ in stage 2 a pseudopod is projected
which fastens itself to the substratum as shown at _c_, 3, while _a_, 2,
is pulled loose. In 4 another pseudopod is projected which fastens
itself at _d_. The ameba is not in contact with the substratum at all
points on its under side.]
In figure 23 is shown a clavate _discoides_ with a small particle
attached to its side. The particle moved forward until it came to lie at
the anterior edge, 10. The speed of the particle from 1 to 10 was 1.36
times as fast as that of the ameba, a much slower rate than was observed
in _sphaeronucleosus_. At 6 a new pseudopod was projected for a short
distance, thus increasing the amount of new ectoplasm forming in
proportion to that of the whole ameba. This change was reflected in the
increased speed of the particle, which moved 1.64 times as fast as the
ameba from 5 to 6. At 10 the anterior end again spread out and again
the particle moved faster--twice as fast as the ameba from 9 to 10.
Stages 11, 12, 13 are added to show that the particles do not tend to go
to the under surface but remain at or very near the tip. The slight
irregularity of the waves of hyaloplasm pushed out at the anterior end
accounts for the changing position of the particle after it has reached
the anterior edge. The particle remained at the edge of the advancing
ameba for several minutes after the stage drawn at 13.
[Illustration: Figure 23. Showing the movement of a particle on the
surface layer of an _Amoeba discoides_. The particle remained on the
anterior end of the ameba for several minutes after stage 13. The ameba
was about 320 microns long.]
In another observation the effect of a narrowing of the advancing tip of
the ameba is shown very well. In figure 24 the ameba was advancing with
a broad anterior end, as shown at 1 and 2. From 2 to 4, the region where
new ectoplasm was made, narrowed down very considerably. These changes
in the width of the anterior end are reflected, as in Figure 17 by a
decrease in the relative speed of the moving particle. Thus the particle
moved 1.75 times as fast as the ameba from 1 to 2 while from 2 to 4 the
particle moved only 1.27 times as fast as the ameba.
[Illustration: Figure 24. Showing the effect of a narrow anterior end on
the rate of movement of the surface. Length of the ameba, about 320
microns.]
The movement of the third layer in _proteus_ is difficult to study owing
to the formation continually of ridges, as explained on page 20. Even in
clavate shaped amebas, waves of protoplasm are pushed out on the sides
and on the tip with consequent formation of ectoplasm, so that the ameba
grows in width slowly at the same time that it grows in length. A
typical shape of a _proteus_ in clavate form is slightly tapering toward
the anterior end. This shape is maintained by gradual extension of the
sides of the anterior half or two-thirds of the ameba as it moves along.
These conditions are just the reverse of what was seen to be the case in
_sphaeronucleosus_ and _verrucosa_, where the anterior edge was wider
than any other part of the body. But _discoides_, although free from the
ridges and grooves characteristic of _proteus_, frequently has an
anterior edge that is narrower than any part of the body, thus
necessitating extension of the sides as the ameba moves forward.
Let us now see what is the effect of ridge formation upon the movement
of the surface layer. Figure 25 shows a _proteus_ and a narrow anterior
end in _proteus_ with two pseudopods and a particle attached to the side
of the ameba at 1. Both pseudopods advanced until stage 4 was reached,
but the particle was not appreciably deflected from an approximately
straight path by the small pseudopod at the other side of the ameba.
Reference to the figure shows that the particle travelled much faster
while the pseudopod on the side was extending than after it began to
retract. The particle moved 1.43 times as fast as the ameba from 1 to 4.
But from 4 to 7 the particle moved only 1.06 times as fast as the ameba.
[Illustration: Figure 25. _Amoeba proteus._ Rate of movement of the
surface layer as compared with the rate of movement of the ameba. The
pseudopod on the right was extended to stage 5; from then on it was
retracted, as indicated by the outlines. Length of the ameba, 400
microns.]
In the earlier stages the outer layer was pulled toward the tip of both
pseudopods, in the later stages only toward one, and in this lies the
explanation for a more rapid movement of the particles in the earlier,
and a slower movement in the later stages. This effect was also observed
in _discoides_, but the fact that the particle in the later stages moved
only very little faster than the ameba is due to a narrow anterior edge
and to the formation of ectoplasm in the ridges over the surface of the
ameba. The effect of ridge formation on the movement of particles
attached to the surface film is well seen when an ameba has two forward
moving regions opposite each other. Under such conditions particles
located equidistant or nearly so between such regions, move only very
slowly or not at all, the pull upon the film being nearly or quite
equal. In a similar manner the ridges which are constantly forming on a
_proteus_ are continually competing with the anterior end in their pull
upon the surface layer, thus preventing rapid forward movement.
[Illustration: Figure 26. Showing the comparative rate of movement of
the surface film over the retracting parts of the ameba. In figures 2 to
8 only a part of the ameba is shown. Length of the ameba, 500 microns.]
Figure 26 shows that the surface layer flows away from the tip of a
retracting pseudopod that is located near the anterior end. The particle
moves slowly until the body of the ameba is reached, when movement
becomes more rapid, 8, 9. This proves that the third layer moves away
from the retracting parts of an ameba, no matter how large the total
area of these parts may be in proportion to the area of new surface that
is being made. But whether the speed of the moving third layer changes
in correspondence with a larger or a smaller ratio between building and
retracting ectoplasm has not been ascertained.
Figure 27 shows that the relative positions of particles attached to the
surface layer may readily change while the ameba deploys its psuedopods.
Three particles marked _a_, _b_, _c_ and connected
[Illustration: Figure 27. A part of an _Amoeba proteus_ illustrating
what is perhaps the most characteristic quality of the surface layer of
amebas, its fluid nature. Three particles, _a_, _b_, _c_, were moving
forward along an actively growing pseudopod. In stage 2, particles _b_
and _c_ had arrived nearly at the tip of the pseudopod. A pseudopod was
then thrown out on the right, which resulted in the movement of _a_ in
the same direction, while _b_ and _c_ remained nearly stationary. Later
on this pseudopod was retracted. _b_ and _c_ were drawn back toward the
main body of the ameba while _c_ remained behind, moving only very
slowly. Thus the relative positions of these particles was completely
changed.]
by a line for convenience of reference, were in the position indicated
at 1 when the forward end of the ameba occupied the position indicated
by outline 1. As the ameba moved forward the particle _c_ gained
slightly on _a_ and _b_ for no ascertainable reason, unless it was on
account of the projection of the large pseudopod on the opposite side.
At stage 2 a new pseudopod was started on the right, which at stage 3
had grown to large size while streaming in the original pseudopod was
arrested. At stage 3 particles _a_ and _b_ retained the same position
they had in stage 2, except for a slight turning to the right. Particle
_c_ however moved across the base of the original pseudopod and on to
the middle of the new pseudopod. At stage 4 _a_ and _b_ had again only
slightly moved to the right of the position they occupied in stages 2
and 3, while _c_ moved rapidly toward the tip of the new pseudopod. The
new pseudopod was then retracted and at stage 5 the particles had begun
to move back toward the main body of the ameba. Particles _a_ and _b_
now gained considerably on _c_ because they were located further away
from the tip of the retracting pseudopod. Particles _a_ and _b_ were
drawn to the middle of the retracting pseudopod because of the
continuous enlargement of the large pseudopod on the right, below,
through which the ameba moved on.
The most important feature of this observation is the change in the
position of the particle _c_ with respect to that of _a_ and _b_. The
latter particles retained their relative positions with very slight, if
any, change, while _c_ swung around _a_ and _b_ nearly 180°, and at the
same time changed the distance very greatly between itself and the other
particles. Moreover, _b_, at stage 5 led the procession of particles,
while at stage 1, _a_ led. No further demonstration is necessary to show
that the surface layer is distinctly fluid and dynamic, and not at all
such a static structure as an elastic permanent skin, as Jennings (’04)
and Rhumbler (’14) maintained.
CHAPTER VIII
ON THE NATURE OF THE SURFACE LAYER
The observations in the preceding chapters on the general movements of
the surface layer of amebas will afford a sufficient basis for an
inquiry into the nature of this layer. The mere demonstration of the
existence of this layer is, of course, interesting enough, for a number
of contradictory statements by various students of the amebas are
satisfactorily cleared up by these observations. But the problem of
ameboid movement affects other organisms besides amebas, and since the
movement of the surface layer is so intimately associated with ameboid
movement, it becomes of more than ordinary interest to learn something
of the nature and composition of this layer.
In the first place the property of carrying particles toward the
anterior end of amebas does not appear to be of any advantage. That is,
whatever the movements of the outer layer may be, the ameba does not
appear to be better off when particles are carried forward than when
none are carried, for such particles are very small and almost without
exception devoid of food value. The particles are masses of debris which
accidentally adhere to the ameba, and the ameba makes no visible effort
to make such particles adhere, nor to get rid of them. The ameba seems
to be quite indifferent to the presence of such particles.
On the other hand, as Schaeffer (’17) has pointed out, the capacity for
transporting particles cannot but be looked upon as a hindrance to
locomotion. As has been stated, the surface film moves in the same
direction as the ameba. Whenever the surface film comes against a solid
object, it pushes against the object, and nullifies to a certain, though
small, extent the energy expended in moving forward. And it will be seen
without further argument, of course, that the energy involved in
carrying particles forward is not only itself lost but consumes an
appreciable part of the energy available for forward movement. This
fact, together with the universal occurrence of this phenomenon among
amebas indicates beyond question that it is intimately associated with
ameboid movement as it is ordinarily understood in amebas, and that it
is almost certainly a “necessary” physical consequence of the more
fundamental physical processes involved in the movement of amebas.
That the third layer moves in the same general direction as the ameba
has already been mentioned. The direction of a moving particle is
however not necessarily parallel with the stream of endoplasm below. In
a retracting pseudopod that lies nearly parallel to and by the side of
the main advancing pseudopod, the particles on the far side and near the
base frequently move across the pseudopod at an angle (and therefore
also across the endoplasmic stream), and up the active pseudopod on the
near side. This shows conclusively that the direction of flowing
endoplasm by itself has no direct connection with the direction of flow
of the surface layer.
To say that the particles carried by the surface layer bring up at the
anterior ends of pseudopods or of the ameba when in clavate shape,
admits of further qualification. The advancing edge is not a straight
line but an arc, and the sides near the advancing edge are building at a
slower rate than the extreme tip. The most rapid formation of ectoplasm
is at that point of the ameba that is farthest ahead. At this point all
the ectoplasm to be made is still to be made, but as one passes back
along the side of the pseudopod more and more ectoplasm is encountered
and less and less remains to be made. There is therefore a gradient in
the rate and in the amount of ectoplasm formed as one passes back from
the forward end of the longitudinal axis of the pseudopod along the
side. This is especially the case with certain amebas like _Amoeba
discoides_, _A. laureata_ and others in which the pseudopods are more
nearly cylindrical. In such amebas as _A. proteus_ and _A. verrucosa_,
the factor of ridge formation complicates to some extent the
longitudinal gradient of ectoplasm formation. But in spite of these
specific differences, the general statement still holds that the rate of
ectoplasm formation at the extreme anterior end is higher than anywhere
else in the ameba, and that the rate gradually falls to zero as the
nearly straight and parallel sides of the pseudopod or ameba, as the
case may be, are approached.
Now we have seen that if a particle becomes attached to the outer layer
of such an ameba as discoides, which has nearly symmetrical pseudopods,
at some considerable distance from the tip of the pseudopod, it moves
forward until the tip of the pseudopod is reached. It does not tend to
come to rest near the tip of the pseudopod, where the rate of ectoplasm
formation is much higher than at the sides of the pseudopod, though not
as high as at the tip, but it moves on until the tip is reached. That
is, the movement of particles on the surface film is toward that small
area at the extreme anterior end where the rate of ectoplasm formation
is highest.
In such an ameba as _verrucosa_, however, the highest rate of ectoplasm
formation would be, not at a small circular area, but a very narrow
strip along the anterior edge; for the rate of ectoplasm formation over
a considerable portion of the width of the anterior end of the ameba is
practically the same, according to observation. Consequently we do not
find particles which are attached to the outer layer tending to move to
a point lying on the longitudinal axis, but their paths are found to be
straight and parallel with the longitudinal axis, if headed toward any
point over a considerable stretch of the anterior edge on either side of
the longitudinal axis.
All the evidence that is at hand therefore points to the conclusion that
the direction of movement of the surface film in a moving ameba is
toward that point where ectoplasm is formed most rapidly.
But where do the particles come from? At exactly what regions of the
ameba do they start to travel toward the anterior ends of the ameba? In
_sphaeronucleosus_ and its congeners, it is very difficult to determine
just when the particles begin to move toward the forward edge. Particles
near the posterior end on the upper surface of these amebas moved
forward slowly, much more slowly than particles near the middle.
Sometimes particles near the posterior end seem to be motionless for
some time, but the incessant though slow kneading process going on at
the posterior end makes accurate observation difficult. Only in a
general way it may be stated that particles begin their forward march at
or near the posterior end. In amebas that habitually form pseudopods
more accurate information can be obtained.
In _proteus_ or _discoides_, for example, projecting pseudopods are
often suddenly stopped and retracted, with a resultant change of an
anterior to a posterior end. Particles attached to the outer surface on
such pseudopods move toward the anterior end, of course, as long as the
pseudopod is building, in the manner described in the preceding pages.
But when the endoplasmic stream is arrested, the forward movement of the
particle likewise stops. When the endoplasm starts to flow back into the
main body of the ameba, the particle also starts moving back; but there
is a period of a few seconds after the endoplasmic stream is reversed
during which the particle remains quiet. And when it does start in to
move, it moves only slowly. Within a few seconds, however, the average
speed of movement is attained. This is true of particles located some
distance away from the tip of the pseudopod. If the particle has reached
the tip of the pseudopod before reversal of the endoplasmic stream takes
place, the particle often remains at the tip until the pseudopod is
almost completely withdrawn into the main body of the ameba (Figure 26,
p. 60). At other times such a particle becomes displaced, presumably by
irregular retraction of the tip of the pseudopod, and finds itself at
the side of the pseudopod. When this happens it moves slowly toward the
main body of the ameba, but faster than the tip of the pseudopod does.
It frequently happens, especially in _annulata_, but also in _proteus_
and other forms with many pseudopods, that when an advancing pseudopod
is about to be withdrawn, there intervenes a stage where the endoplasm
in the distal part moves away from the ameba, while that in the proximal
part moves toward the ameba, with a neutral or motionless zone between.
In such case a particle on the distal end moves slowly toward the tip
while a particle in the proximal region moves toward the base of the
pseudopod. Particles over the neutral zone are motionless. In these
cases, however, changes in the direction and speed of the ectoplasmic
stream are too frequent and the relative strengths of the distal and
proximal currents too variable, to enable one to secure very accurate
data by means of camera lucida drawings (a kinematograph is essential
for this purpose), so no figures of the speed of movement of such
particles are given. Nevertheless the general results of the
observations are as stated. It might be added that in some cases the
neutral zone for the particles attached to the surface did not coincide
exactly with the neutral zone of the endoplasm, but was located a little
further distally.
From these observations it appears that a rough index of the direction
of movement of the surface film is the direction of the streaming of the
endoplasm; and that the surface layer moves away from regions where
ectoplasm is in the process of being converted into endoplasm. Since a
particle attached to the surface may remain for some time at the tip of
a retracting pseudopod, while one that is attached to the sides of a
pseudopod moves toward its base, it appears that the speed of the moving
surface film is not directly correlated to the rate of transformation of
ectoplasm into endoplasm. The slower speed of particles near the
posterior end points also in this direction. The formation of ectoplasm
at the anterior end seems therefore to be much more intimately connected
with the movement of the surface film than the destruction of the
ectoplasm, though it is not yet clear that the liquefaction of the
ectoplasm is altogether without effect.
Now as to the speed with which the surface film moves. The foregoing
illustrations and figures show that the particles attached to a
_sphaeronucleosus_ on the upper surface move from 2.5 to 3.6 times as
fast as the ameba (Figure 19) while particles attached to a _discoides_
move only from 1.2 to 2 times as fast as the ameba moves. In _proteus_
the speed of the particles is still slower, because of the longitudinal
ridge-like waves of protoplasm which are continually being thrown out.
In this species it frequently happens that because of the numerous
ridges, the ameba moves faster than the particles attached to the outer
surface; but this is to be looked upon as a mechanical complication, not
as indicating a difference in the nature of the surface layer.
How is the difference in the speed of movement of the surface layer
between _sphaeronucleosus_ and _discoides_ to be explained? There are no
ridges to retard the movement of particles in _discoides_, while there
are ridges in _sphaeronucleosus_, where the particles move on the
average twice as fast as on _discoides_. In the first place the
advancing edge, the edge where ectoplasm is being made, is
proportionately much wider in _sphaeronucleosus_ than in _discoides_ as
compared with the amount of surface back of it. Figures 23 and 24 show
that the rate of movement of the surface film is directly proportional
to the amount of new ectoplasm forming. In the second place, the greater
part of the under surface in the forward half of _sphaeronucleosus_ is
attached to the substrate, so that the surface layer which flows toward
the anterior end is derived almost wholly from the upper surface; while
in _discoides_ the whole surface in free pseudopods, and nearly the
whole surface in attached amebas (cf. Dellinger’s observations described
on p. 56) possesses mobile surface protoplasm. Observation of moving
particles on these amebas proves this. Then again, the anterior edge of
a _sphaeronucleosus_ is not attached at the points farthest advanced,
but the point of attachment is some distance back, as indicated in
figure 20. The effect of this is to increase the amount of forming
ectoplasm in proportion to the surface of the ameba from which surface
protoplasm may be drawn. Still one other factor must be considered. As
is well known _sphaeronucleosus, verrucosa_ and their congeners possess
longitudinal ridges on the upper surface which consist of ectoplasm,
covered of course by the surface film. These ridges are formed near the
anterior edge, not by wrinkling, but by the construction of new
ectoplasm. Once formed, they remain until the ameba, so to speak, flows
out from under them. That is, the ridges undergo comparatively slight
changes until changed back into endoplasm at the posterior end of the
ameba. As the ameba flows ahead the ridges are of course continually
being added to or lengthened, by the conversion of some endoplasm into
ectoplasm. The ridges may thus retain their identity for a long time
although the substance composing them is changed every time the ameba
moves the length of its body. It is clear, therefore, that there is more
ectoplasm formed at the anterior end of a _sphaeronucleosus_ than would
be the case were the upper surface of the ameba plane; and the
conclusion therefore is obvious that the formation of ridges, occurring
as it does, chiefly at the anterior end, serves further to accelerate
the forward movement of the surface film.
If the form of _sphaeronucleosus_ were more regular than it is, the
amount of ectoplasm in the process of forming at any given moment could
be compared with a similar relation existing in _discoides_, to see
whether these respective ratios were proportional to the speed of the
moving surface films in the two amebas. As it is, the irregularity of
form of _sphaeronucleosus_ makes such computation subject to the
possibility of considerable error. In _discoides_ however the problem is
comparatively simple. I therefore did not go into this matter
extensively, but merely worked out the relations mentioned in one case,
and I mention it here to illustrate the method rather than to record the
result, which is not to be taken as very exact.
[Illustration:
Figure 28. A clavate _Amoeba discoides_, showing the amount of
ectoplasm that is constantly being made at the anterior end. Length
of the ameba, 310 microns.
]
Since the movement of the surface film is obviously a surface
phenomenon, only the surfaces of the amebas need to be taken into
account. In Figure 28 is illustrated a _discoides_ of such a shape as to
allow a fairly accurate computation of its surface. Three outlines of
the anterior end only are given; the rear portion of the ameba remained
approximately the same size and shape in the three outlines. The cross
lines at the anterior end divide the forming ectoplasm of the ameba from
the formed. As will be noticed the cross lines are drawn through the
intersections of two successive outlines. Computing the areas on both
sides of the cross lines for the two outlines and averaging them, there
is found a ratio of 1 to 10; one-eleventh of the total surface
represents forming ectoplasm, and ten-elevenths formed ectoplasm.
(One-twenty-second of the total surface was deducted for surface
attached to the substratum.) _Sphaeronucleosus_ stands in contrast with
_discoides_ for it is attached to the substratum over a much greater
area and in consequence only a slight amount of surface is drawn from
the under side. This ameba may therefore be regarded in this connection
as of only one surface, the upper. That part of outline 1 in Figure 14
cut by outline 2 indicates, as in _discoides_, the region of forming
ectoplasm, and the space between outlines 1 and 2 may be used as a basis
of computation. New ectoplasm is formed in this zone and far enough back
to include the tips of the longitudinal ridges, of which we have already
spoken (Figure 13). The zone of forming ectoplasm would therefore be
about twice as wide as the average width of the three zones between the
successive outlines in the figure, and of approximately the same shape.
On this basis, the surface occupied by forming ectoplasm is 1/5.8 of the
total surface, and the ratio of formed to forming ectoplasm is 4.8 to 1.
(For the sake of completeness, a few factors whose values cannot easily
be computed may be mentioned. 1. The anterior edge is not attached to
the substratum at its farthest point, but at some little distance back
of the edge, thus increasing the relative amount or forming ectoplasm;
but this is offset by the surface of a part of the under side at the
posterior end where the surface layer is active. 2. The ectoplasm
composing the ridges, which must be added to the formed ectoplasm, would
increase the ratio, though only slightly).
Approximately twice as much ectoplasm is therefore in the process of
formation in _sphaeronucleosus_ as in _discoides_ when compared with the
formed ectoplasm in the respective amebas, over which the surface film
is active. This ratio corresponds very well with the rate of movement of
the outer surface in these amebas, which as we have seen is about twice
as fast in _sphaeronucleosus_ as in _discoides_.
Where does the surface layer come from and what becomes of it after it
arrives at the anterior end? It moves continually forward as long as the
ameba moves forward. There would seem to be a tendency therefore for it
to accumulate at the end of a free pseudopod in such a form as
_discoides_, and even under ordinary conditions of locomotion where
there is occasional attachment to the substratum by very short
pseudopods, the surface layer is continually moving toward the anterior
end on practically all sides. Every time, therefore, that the ameba
moves a little less than its own length, there would accumulate at the
tip of the ameba, if it were not removed, an amount of surface layer
equivalent to that which covers the whole ameba. No such accumulation
can be detected however, from which we infer that it is removed as fast
as brought there. And the posterior region of the ameba, which is the
main source of the surface film, does not become poorer in this material
by reason of its continual flow forward, but new surface is made
continually to take the place of that moving forward. This process of
destruction and creation of surface is accordingly rapid during active
locomotion;--a _discoides_, moving approximately once its length at room
temperature in two minutes, destroying therefore the equivalent of its
entire coat of surface in that time; while a _sphaeronucleosus_, moving
once its length in two or three minutes, destroys all its surface every
minute.
From what has been said thus far, it must be apparent that there is
striking resemblance between the general movement of the surface layer
of the ameba, and of a surface tension layer in a drop of fluid in which
the tension is changed at some point. Let us now inquire briefly into
this resemblance.
As is well known the surface of a liquid in contact with another liquid,
solid or gas, with which it does not mix, behaves like a stretched
membrane, so that when the tension is reduced at any point the surface
layer moves away from that point. A good illustration of the effect of a
decrease of surface tension is found in a drop of clove or other oil
with which some substance that reduces the surface tension, such as
alcohol or soap, is brought into contact at one side. If previously some
dust particles have been placed on the surface of the oil drop, it will
be easy to see that the surface of the oil moves to the opposite side
from where the alcohol or soap solution touched the oil. In practice it
is a very simple matter to lower the surface tension of a drop of fluid
as described, so as to show the movement of particles on the surface.
Almost any liquid may be used for this purpose. But it is comparatively
very difficult to _increase_ the surface tension at some point of a
drop of fluid in such a way as to cause particles on the surface to move
toward that point. The principle underlying the movement of the surface
film in both cases is however exactly the same; so, although it would be
more desirable to compare the surface movements in a drop of fluid in
which the surface tension is increased at some point, because this is
what happens in an ameba during locomotion, we shall nevertheless find
it necessary to consider a drop of fluid in which the surface tension
has been lowered. The application of the illustration is readily made.
When the surface tension of a drop of fluid is lowered by bringing into
contact with it some other substance that possesses this power, the
surface rushes away at great speed in all directions from the point
where the tension is lowered, because usually the tension is reduced
very considerably. In this surface movement it is found that new surface
is made where the tension is lowered and old surface is destroyed, that
is, pulled into the interior over a large part of the surface opposite
to where the tension is lowered. The speed of the surface movement is
most rapid near the point where the tension is lowered and becomes
gradually slower as the opposite side of the drop is approached, where
there is no movement. This variation in speed of the moving surface
seems to be due largely to the small area in which the tension is
lowered as compared with the whole surface of the drop.
In the ameba the conditions are reversed. The surface layer moves
_toward_ a point with increasing speed, instead of away from a point. In
both the ameba and the drop the greatest speed is attained near the
small area where the change in surface tension occurs.
The behavior of large and heavy particles on the surface of a drop of
fluid and on an ameba are similar. A heavy particle as of sand, or a
small glass rod, laid on the ameba, is not moved by the surface layer.
It forms an island of surface matter around which the moving surface
layer flows. Precisely the same thing happens in surface layer movements
in inanimate fluids.
Again in point of thinness there is no disagreement so far as
microscopic observation goes. Neither the surface film on an ameba nor
the surface film on a fluid can be directly observed microscopically to
be different from the fluid below it. The surface layer is, as is
generally believed, of molecular dimensions, and its thickness is beyond
the limits of vision. Unless some special means is discovered therefore
for making visible the surface film, such as a process of staining, it
may be impossible to ascertain its ultimate structure directly, for it
overlies a mass of heterogeneous fluid whose composition is constantly
changing.
It seems to follow from what is observed of the surface tension layers
of the fluids of physics that such layers must be of the same
constitution as the body of the fluid over which the layer is formed,
although, as is well known, the proportion of the ingredients in the
surface layer is different from that in the body of the fluid. Now since
the resemblance between the surface layer of an ameba and a surface
layer on a drop of fluid has thus far been found to be complete, it is
pertinent at this point to discuss Gruber’s (’12) suggestion that the
movement of particles forward on an ameba is due to the forward movement
of an inert layer of mucus or gelatinous material secreted by the ameba.
To begin with, observation does not support Gruber’s suggestion. No such
layer can be seen. Such a layer, since it is shown to persist for
several minutes at least, should remain after an ameba bursts, under
experimental conditions, but no such remains can be seen. Its existence
should be demonstrable by the use of dyes, but the evidence is negative.
Indeed there is not any direct evidence that can be brought in support
of the suggestion that this surface layer is gelatinous in composition.
Moreover, as we have seen, the layer on the ameba that carries particles
forward seems to be destroyed at the anterior end, for in what other way
would particles remain at the anterior end after being brought there?
But the supposition that a gelatinous layer might be drawn into the
interior at the anterior end is also negatived by observation, for no
very small particles clinging to or imbedded in the surface substance
are ever drawn into the ameba, as would almost certainly be the case if
the substance composing the layer were gelatinous. And as to supposing
that this layer, if gelatinous, might behave essentially as a surface
tension layer and therefore be drawn in at the anterior end of the
ameba, this is contrary to the experience of physics; for the physical
nature of the ameba would make it impossible for the ameba to have a
surface layer of gelatinous matter. There do not seem to exist any
grounds therefore for supposing that the outermost layer of an ameba,
the layer that carries particles as described in the preceding pages,
can consist of an inert substance as Gruber suggests.[4]
From these considerations, then it appears that all the evidence
available, both direct and indirect, points to the conclusion that the
behavior of the surface layer on the ameba resembles in general and in
detail the behavior of a surface tension layer in an inert drop of
fluid, and that we must regard the surface layer on the ameba as a true
surface tension layer. This layer is therefore a dynamic layer,
containing free energy, and capable of performing work. It is
physiologically distinct from ectoplasm, as ectoplasm is distinct
physiologically from endoplasm. But the distinctive properties which the
surface layer possesses are functions of its position. These properties
clearly indicate that its constitution is protoplasmic, corresponding to
the fluid parts of the internal protoplasm.
The surface layer of the ameba is probably identical with what is
commonly called the _plasma membrane_ or semi-permeable membrane as
postulated by Overton (’07). The peculiar structure supposed to be
possessed by plasma membranes are held to be due chiefly to surface
forces. The fact that the surface layer of the ameba is continually
being destroyed and re-created during locomotion does not support the
view that the plasma membrane is of inert composition, as for example,
lipoidal, as has been suggested. The observations, on the contrary,
confirm Höber’s (’11) view that the plasma membranes generally are
living structures. But it may be regarded as certain that if lipoids are
present in the protoplasm of the ameba, these substances, according to
the principle of Willard Gibbs, will be found in higher concentration
in the surface film than in the body of the ameba.
Perhaps the most important question that arises in connection with the
surface layer of the ameba is: What causes it to move in the manner
described? But we can do little more than ask the question. It has been
seen that the surface film moves toward an area of increased tension
rather than from an area where the tension has been lowered. However,
since we are completely in the dark respecting the composition of the
surface layer or of the fluid parts of the ameba, it is exceedingly
hazardous to venture an explanation. If the surface layer should have
its tension lowered by a concentration of lipoids in it, we would be
faced by the necessity of explaining their removal at the anterior end.
If we turn to electrical causes we meet again with great difficulties.
An ameba moves with the electric current, when a current is passed
through the water. The surface layer under these conditions behaves
normally, as may be inferred from Jennings’ (’04) figure on page 198.
That is, the current controls the direction of the movement of the
ameba, with the current leaving the ameba at the point of highest
surface tension. This is contrary to the action of the mercuric
capillary electrometer, in which the mercury column also moves with the
current, but because of lowered surface tension where the current leaves
the mercury. The conditions surrounding these cases are so different
however, that very little can be gained by setting them in contrast to
each other.
CHAPTER IX
THE SURFACE LAYER AND THEORIES OF AMEBOID MOVEMENT
The observations recorded in the preceding two chapters, while they do
not tell us anything about the direct cause of the movement of the
surface layer, nevertheless indicate clearly enough that the area where
ectoplasm is made is the area toward which the surface film flows. There
is no question therefore of the intimate relation between the
transformation of endoplasm into ectoplasm and the movement of the
surface layer.
The apparent absence of movement in the surface film of the pseudopods
of _Difflugia_ (Schaeffer, ’17) and the definitely proved absence of
movement in the surface layer in the foraminiferan _Biomyxa_ and
myxomycete plasmodia, where no ectoplasm is formed in the manner
observed in amebas, also indicates a causal relation between movement of
the surface layer and ectoplasm formation. The relation moreover seems
to be a necessary one for the movement of the surface layer is contrary
to the processes involved in locomotion. In other words, from the
standpoint of the ameba, it is a “necessary evil,” so far as locomotion
is concerned.
The transformation of endoplasm into ectoplasm is unfortunately not
understood, though from what we know in a general way of the behavior of
colloidal solutions it seems to be a surface tension effect due to (or
accompanying) a change of phase. Something akin to gelation occurs as
Kite (’13) has shown. It is a problem in the chemistry of colloids. But
the structure or composition of the protoplasm is complex and
practically unknown, and it is quite open to criticism whether analogies
from the behavior of pure solutions of colloids, such as gelatin, afford
any real basis for an explanation.
Although a knowledge of the movements of the surface layer is
interesting enough by itself, it will achieve its true importance only
when it can be related to other processes in the ameba in a causal
manner. It does not at present give us any greater insight into the
ultimate cause of ameboid movement, although it is clear that an
important step in this direction has been taken. But no theory of
ameboid movement can be accepted that demands conditions in the ameba
that are contrary to those described in the preceding chapters, in
connection with the surface layer. From this point of view therefore the
discovery of the true nature of the outside surface of the ameba is of
importance, for it widens to a very considerable extent the
observational basis with which any theory of ameboid movement must
conform. Since the properties of the outer layer are here described in
detail for the first time, it becomes necessary to enquire to what
extent the more commonly held theories of ameboid movement conform with
the observed behavior of the surface film. Although the surface tension
theory was the first detailed theory proposed toward an explanation of
ameboid movement, I shall discuss Jennings’ (’04) observations on the
movements of the ameba first, because a great part of his work deals
with the movements of the surface film, although he did not recognize it
as distinct from the ectoplasm in its movements.
It is generally recognized that Dellinger’s (’06) work proved that
Jennings’ conception of the ectoplasm as a permanent skin in which the
ameba rolls along, is probably inadequate for such amebas as _proteus_;
though singularly enough it is still supposed that _verrucosa_ and its
congeners move in the way described by Jennings (Hyman, ’17, p. 83).
Jennings (’04) describes the movements of amebas, both _proteus_ and
_verrucosa_ “types,” as follows:
“In an advancing Amoeba substance flows forward on the upper surface,
rolls over at the anterior edge, coming into contact with the
substratum, then remaining quiet until the body of the ameba has passed
over it. It then moves upward at the posterior end, and forward again on
the upper surface, continuing in rotation as long as the ameba continues
to progress. The motion of the upper surface is congruent with that of
the endosarc, the two forming a single stream (p. 148).
“We have demonstrated above, for Amoeba at least, that the forward
movement is not confined to a thin outer layer, but extends from the
outer surface to the endosarc; in other words that the outer surface
moves in continuity with the internal substance (p. 150).
“There is no regular transformation of endosarc into ectosarc at the
anterior end. On the contrary the ectosarc here retains its continuity
unbroken, moving across the anterior end in the same manner as across
other parts of the body. In the same way the ectosarc is not regularly
transformed into endosarc in the hinder part of the body.... Such
transformation is by no means a regular accompaniment of locomotion” (p.
174).
According to Jennings, locomotion is aided by the projection of waves of
hyaloplasm at the anterior edge, “an active movement of the protoplasm
of a sort which has not been physically explained.” These waves,
attaching themselves to the substratum, enable the ameba to pull itself
along by a rolling movement as described in the quotation above.
As to the rate of movement of the outer surface as compared with that of
the endoplasm, Jennings concluded:
“The direction of movement of particles on the outer surface is the same
as that of the underlying particles of endosarc. The rate is also about
the same as for the endosarc, though often, or perhaps usually, the
outer particles move a little more slowly than those in the endosarc”
(p. 142).
In view of the observations recorded in the preceding pages it is clear
that Jennings’ statement that substance after moving forward on the
upper surface, rolls over the anterior edge is quite erroneous. The
attached particles, if heavy, may do so, but the surface film itself
does not. It is, on the contrary, taken into the interior at the
anterior edge.
The statement that the movement of the outer surface is congruent with
that of the ectoplasm can likewise not be substantiated by observation,
as has been demonstrated in the preceding pages. It is difficult to
distinguish between the ectoplasm and the surface layer in such amebas
as _sphaeronucleosus_ and _verrucosa_, for there are no large crystals
or other bodies which get caught in the ectoplasm as it is formed from
endoplasm at the anterior end. But attentive observation will
demonstrate very definitely that the ectoplasm here is stationary to the
same degree as in _proteus_. The stationary properties of the ectoplasm
are however not properly a matter for discussion; for five minutes’
observation of a _proteus_, _discoides_, _annulata_, particularly a
_laureata_, under 300 diameters magnification, will convince anyone that
the ectoplasm is stationary while the surface film, with attached
particles, moves over it. No one can possibly come to any other
conclusion. Jennings’ conclusion was due undoubtedly to an error of
observation.
Jennings’ statement that the rate of movement of the outer surface is
the same as that of the endoplasm (p. 142) when taken in connection with
his other statement that the ectoplasm is a more or less permanent skin,
presents a mechanical impossibility; for unless the outer surface moves
_twice as fast_ as the endoplasm, no rolling movement would be possible.
Several of Jennings’ figures (especially Figures 38, 39, and 41)
indicate in fact that he conceived of the outer surface as moving faster
than the ameba advances, or that the upper surface moves _over the
ameba_ as the ameba moves over the substrate. Jennings’ theory requires
that the surface layer move twice as fast as the ameba advances. Hyman
(’17) also makes a similar mistake in referring to the rate of movement
of the outer surface (p. 85).
Lest the discussion of this point be suspected of being merely
verbalistic, it should be recalled that the surface layer of _proteus_
often moves at about the _same rate_ as the ameba; that the surface
layer of _discoides_ moves about _twice as fast_ as the ameba; that the
surface layer of _verrucosa_ and _sphaeronucleosus_ moves about _three
times as fast_ as the ameba; and that the ectoplasm does not move at
all. It is of course incumbent on one to discuss what is stated; one is
not at liberty to select one of several possible interpretations.
To illustrate this point graphically so as to avoid as far as possible
future confusion Figure 29 is appended. In _a_ is shown a particle
traveling on an ameba at the same rate of speed as the ameba; at _b_ is
shown a particle that moves twice as fast as the ameba; at _c_ the
attached heavy particle does not move at all. For the sake of
completeness _d_, Figure 29, is added here. It illustrates the backward
moving ectoplasm in an ameba that is suspended in a jelly medium that
prevents the ameba from sinking to the bottom. It must be admitted that
in thus considering the rate of movement of the various tissues of the
ameba from a single standpoint, a point outside of the ameba, little
room is left for confusion.
[Illustration: Figure 29. _a_, a particle attached to an ameba and
moving at the same rate as the ameba. This condition is often observed
in _proteus_ where the surface film, owing to its destruction during the
formation of the longitudinal ridges, retards the forward movement of
this layer. _b_, a particle attached to the surface film of an ameba
moving twice as fast as the ameba. This condition is seen in
_discoides_, _verrucosa_, _sphaeronucleosus_, etc. _c_, a particle on an
ameba that does not move at all although the ameba does. This is seen
when a heavy particle is laid on an ameba, too heavy for the surface
film to move. _d_, movement of ectoplasm in an ameba suspended in a
jelly medium. The vertical lines are to be considered as stationary.]
There is comparatively little friction, if any at all, between the upper
surface and the endosarc, according to Jennings’ view, since both these
layers move at the same rate and as a single stream. On the other hand
there must be very considerable friction between the endoplasm and the
lower ectoplasm, which does not move at all. This difference in the
amount of friction must show itself in the different speeds of the
endoplasm near the upper ectoplasm and near the lower ectoplasm.
Observation indicates however that the most rapid streaming of the
endoplasm is in the middle of the ameba or pseudopod and that it
gradually becomes slower as the ectoplasm is approached on _all sides_.
We said above that if the ectoplasm were a more or less permanent skin
in which the ameba rolled as described by Jennings, the upper surface
(=ectosarc, Jennings), according to a well known mechanical principle,
would have to move ahead about twice as fast as the ameba advances. Now
the upper surface of _sphaeronucleosus_ and of _verrucosa_ in
locomotion was found to move from three to three and a half times as
fast as the ameba (Chapter VII). In discussing movement in “_verrucosa_
and its relatives” Jennings says “the essential features of the movement
seem to be (1) the advance of a wave from the upper surface at the
anterior edge; (2) the pull exercised by this wave on the remainder of
the upper surface of the body, bringing it forward. Most of the other
phenomena follow as consequences of these two” (p. 146). Thus the amount
of _stretch_ of the upper surface would exceed the amount of _pull_ on
it from 50% to 75%!
Jennings’ explanation of ameboid movement in which the important factor
is a more or less permanent ectoplasm in which the ameba rolls along,
would unquestionably produce rotary currents. Rhumbler (’98) recognized
this and after full consideration rejected the idea that the ectoplasm
is a permanent skin in which the ameba rolls along in locomotion,
because rotary currents are not observed in a moving ameba. Anyone who
doubts that rotary currents would be produced under these conditions can
convince himself by putting a quantity of glycerine and some shavings in
a large transparent rubber balloon or celloidin bag and letting it roll
slowly down an incline in front of a strong light. If not too much
glycerine is placed in the balloon, the shape of an ameba is closely
enough approximated, and the rotary currents--down at the posterior and
up at the anterior end--are well shown.
From all these considerations it is quite clear that Jennings’
explanation of ameboid movement as a rolling movement can not any longer
be maintained. His “discussion of this matter (the rolling movement
hypothesis) is an excellent example of the fact that acumen and
excellent reasoning may lead one astray in scientific matters when the
observational basis for the reasoning is not secure.” (These are
Jennings’ own words in criticism of Rhumbler on the same subject!)
The surface tension theory, with its many modifications, has had a great
many more adherents than any other theory that has been advanced to
explain ameboid movement. It represents the attempt of biologists to
explain a vital phenomenon on physical grounds. The fact that it has
been held to go further in this direction than any other, and the fact
of its greater simplicity, doubtless are responsible for its wider
acceptance. The recent criticism to which this theory has been
subjected, however, indicates clearly enough that this theory does not
really give a very adequate idea of the processes involved in ameboid
movement after all, and in so far as feeding processes are concerned,
the theory does not seem to apply at all according to Schaeffer (’16,
’17). But it could hardly have been anything more than excellent
guesswork if the surface tension theory _as advanced_ by a number of
writers had been found adequate, for the observational basis was very
narrow, as the preceding pages have shown, and as the succeeding pages
further show.
Not anything like a complete historical account of this theory with its
numerous modifications will be attempted here. It would be a large
undertaking, for nearly every biologist and biochemist has expressed
himself on the subject. It does not appear that much is gained by merely
recording the opinions, even of biologists, unless they are based on
experimental or observational data, preferably their own. Scientific
questions are not decided by ballot vote, and it is not apparent what
value such a record of opinions would have except the doubtful one of
showing whether the persons involved declared for or against the surface
tension theory. Moreover such an account would not be interesting
reading for those who want to know first of all what amebas can do. Only
the more important modifications of the surface tension theory as
applied to ameboid movements will therefore be discussed and these
modifications will be considered important in proportion to the amount
of observation or experiment on which they are based.
Attention has already been called in Chapter II to Berthold’s (’86)
theory of ameboid movement, which was the first attempt to explain this
phenomenon on physical principles. As will be remembered, Berthold
thought that the nature of the ameba’s immediate environment determined
when and in what direction it should move, the source of the energy of
movement being supposed to be a decrease in the tension of the surface
film of the ameba, brought about by some factor in the ameba’s immediate
environment.
One of the most elaborate attempts that has been made toward explaining
ameboid movement on the basis of surface tension phenomena was that of
Bütschli (’92). From his extensive knowledge of the lower organisms,
especially the protozoa, he concluded that protoplasm is an emulsion of
two fluids: a more concentrated “plasma,” insoluble in water; and a
thinner fluid, “enchylema.” Ameboid movement was brought about by
migration of enchylema droplets to the surface of the ameba at the
anterior end, where they burst and spread over the surface, lowering its
tension. The effect of this change in tension was held to be a flowing
backward of the surface of the ameba and a flowing forward of the
endoplasm. This is what happens in a drop of fluid, such as oil, on
water to one side of which is brought a soapy solution. Bütschli
described many experiments with fluids on which the surface tension was
changed by appropriate means to simulate the process of movement. After
Bütschli had developed his surface tension theory of movement, he
discovered, as has already been noted, that in a pelomyxa the surface
layer moves forward instead of backward as required by the surface
tension theory. In spite of this however he still maintained that his
theory of movement could be modified to apply to amebas generally,
although so far as I have been able to find, he did not then or
subsequently state how. From this we may infer that Bütschli himself
probably concluded that the surface tension theory of movement as he
developed it, is not of general application or is nothing more than a
step in the development of such a theory.
Rhumbler has written a number of papers on the mechanics of ameboid
movement, most of which are concerned with elaborations and
modifications of a surface tension theory very similar to Bütschli’s.
Rhumbler published a general outline of his theory in 1898. The
transformation of endoplasm into ectoplasm at the anterior end, and the
reverse process at the posterior end, was stated to be an important part
of his theory of movement, but just how this was necessary to surface
tension effects was not explained in physical terms. Feeding was assumed
to be caused by the direct action of the food body on the surface layer
(ectoplasm) of the ameba. The presence of the food body, he held,
produced a lowering of the surface tension of the ameba thus causing the
ameba to flow around it (’98, p. 207). Subsequently, however, he (’14)
came to the conclusion that many amebas cannot have fluid surfaces as
usually understood, since they do not spread as a film over water when
they come into contact with the surface. From this and other
observations Rhumbler concluded (’14, pp. 501-514) that the surfaces of
amebas are not to be compared with surface tension films on drops of
inert simple fluids; but with the surface films of emulsions which take
on the properties of a solid. Since the question of ameboid movement is
not especially discussed in this later paper, it may be assumed that in
this respect his (’98, ’10) earlier views have not been materially
modified. Rhumbler has suggested a great many physical models for the
explanation of various ameboid activities such as feeding, defecation,
movement and so forth.
In general agreement with Bütschli and Rhumbler were Verworn (’92),
Blochmann (’94), Bernstein (’00), Jensen (’01, ’02), and recently
Hirschfeld (’09) and McClendon (’12). All these authors held that
ameboid movement is a surface tension phenomenon. The application of the
surface tension theory in explaining ameboid movement demands a fluid
surface and a fluid interior and it is perhaps unnecessary to add that
Bütschli, Rhumbler and the others mentioned held that the protoplasm is
fluid. The question as to whether protoplasm is a fluid or possessed of
an internal structure was however hotly debated and we find Fleming
(’96), Heidenhain (’98), Klemensievicz (’98), Dellinger (’06) and others
opposing the group of authors just mentioned, by contending that the
streaming protoplasm must have some kind of structure. This question no
longer concerns us however, owing to our rapidly increasing knowledge of
colloidal solutions, for it is undoubtedly correct to hold that
protoplasm is colloidal.
We have already insisted (p. 46) that the problem of ameboid movement is
made more difficult by narrowing it down to the movements of ameba, and
that to see the problem in its fullest aspect requires consideration of
streaming protoplasm wherever found. Now it happens that there is in
certain respects greater diversity of streaming to be found in plant
cells than in animal cells, and it is not surprising therefore that
explanations of streaming and ameboid movement have taken a different
direction among botanists than among zoologists. It is for this reason
doubtless that Ewart (’03), while espousing the surface tension theory
as explaining streaming, does not look to the superficial surface of a
plant cell as the source of the necessary energy, but to the interior of
the protoplasm. This idea is, of course not entirely original with
Ewart, for Bütschli, as we saw, believed that protoplasm has an emulsoid
structure; but according to Bütschli’s hypothesis, the surface forces
were not brought into play in movement until the droplets of enchylema
spread over the surface and so reduced the tension. Ewart, however
points out that there is very much more surface energy present in the
interior of streaming protoplasm than is required for all the movements
known to protoplasm, including muscular contraction. According to
Ewart’s hypothesis the emulsion globules (disperse phase) have their
surface tension lowered at corresponding points by electrical currents
traversing the endoplasm, the electrical currents themselves originating
in chemical actions.
While all available evidence from the study of colloidal solutions and
from observation from protoplasm confirms Ewart’s statement that more
than sufficient energy is available in the interior of colloids for all
purposes of movement, there is little or no evidence that the proper
electrical currents are present to release or transform the surface
energy into that of movement. This step in his explanation is therefore
highly hypothetical and at present unconvincing. Moreover, this step in
the theory would not be applicable to streaming as observed in amebas,
without very considerable modification.
Recently Hyman (’17) has developed the surface tension theory of
movement in the direction indicated by Ewart. The motive power is
supposed to have its source in the contractility of the ectoplasm. The
endoplasm is held to be a passive stream, not an active stream as Ewart
supposed to be the case in plant cells. The power of contractility is
held to be due to the process of gelation of endoplasm into ectoplasm,
which is due to a change of phase, the fluid part of the endoplasm
becoming dispersed and thereby developing surface energy in proportion
as the amount of surface of the fluid is increased. This increase of
surface produces the phenomenon of contractility.
Miss Hyman is wrong however when she says (p. 90) that the withdrawal
and contraction of pseudopods are processes of gelation. This is clearly
a physical impossibility, for the ectoplasm of the withdrawing pseudopod
must become liquified into endoplasm, before it can be withdrawn. All
writers excepting Jennings and Hyman are agreed on the continual
transformation of ectoplasm into endoplasm at the posterior end while
the reverse process goes on at the anterior end; and Hyman herself
states (p. 89) that new ectoplasm is formed as the growing pseudopods
extend into the water. So there must be liquefaction of the ectoplasm in
withdrawing pseudopods, or very soon the whole ameba would be
transformed into ectoplasm. As was shown in the preceding pages,
liquefaction of the ectoplasm at the posterior end goes on at the same
rate as gelation of the endoplasm at the anterior end. But at another
place Hyman says:
“In fact according to Jennings, Dellinger, Gruber, and Schaeffer the
surface of the ectoplasm actually flows forward at about the same rate
as the forward advance, and this indicates that the advancing ectoplasm
at the tip of the pseudopodium is derived from the surface ectoplasm and
not from a transformation of endoplasm into ectoplasm at the end of the
pseudopodium as Rhumbler supposed” (p. 89).
This quotation is not strictly accurate. Jennings says: “The
pseudopodium grows chiefly from the base, so that any part of the
surface retains nearly its original distance from the tip” (p. 156).
Dellinger in a general way confirmed Jennings’ conclusions. Gruber
concluded that the outer layer was gelatinous, not protoplasmic.
Schaeffer held the third layer to be extremely thin, “too thin to be
seen easily,” so it is impossible that the ectoplasm at the tip of a
pseudopod, the thickness of which is readily seen, can be derived from
the surface film.
The main conclusion however in Miss Hyman’s paper is that there exists a
metabolic gradient in the pseudopods of advancing amebas, the highest
rate of metabolism being at the tip and the lowest at the base, for any
one pseudopod. This conclusion is bound to be of the first importance in
the explanation of ameboid movement. It will give our first real
insight into the chemistry of ameboid movement. The fact that her method
of demonstrating gradients has yielded uniform results in the hands of
Child (’15), who originated it, as well as in her own when applied to a
great many different organisms, entitles her conclusions to careful
examination.
[Illustration: Figure 30. Disintegration of an ameba in ¼ molecular KNC.
After Hyman. _a_, ameba flowing in the direction of the arrow. _b_, the
ameba has abandoned pseudopod 1 and flows into pseudopod 2, which has
become reactivated. The ameba was exposed to KNC at this stage and, as
is usual in such experiments, the posterior end at _x_ becomes active.
_c_, the youngest pseudopod, at _x_, disintegrated first. _d_, the next
youngest pseudopod, 2, disintegrated next. Pseudopod 1, the oldest,
disintegrated last.]
Of the observations there can be no doubt, for in many details earlier
observations are confirmed. Her figures show that the tips of the
pseudopods disintegrate first in the potassium cyanide solution and
later the regions further back (Figure 30). The question is, what causes
the gradient of disintegration, which Miss Hyman takes to represent also
a metabolic gradient? Where is the gradient located: in the ectoplasm or
in the endoplasm; or is the gelation process synonymous with the
metabolism that gives rise to the observed gradient? Miss Hyman does not
say; but it cannot be in the endoplasm, for it is in motion along the
whole pseudopod at about the same rate and it undergoes a demonstrable
and visible change only at the anterior end of the pseudopod. While
metabolic changes might be higher at the free end of the pseudopod,
therefore, there would not be a gradient from there on back. No recorded
observations on the endoplasm along the length of a pseudopod can be
arranged so as to form a gradient which would suggest a similar
gradient in metabolic rate; and if the endoplasm is a passively moved
fluid as Hyman’s theory seems to imply, a metabolic gradient would seem
to be precluded.
In the ectoplasm however there exists a time gradient; that at the base
of a pseudopod is older than that near the tip, and observation
generally tends to confirm the view that the older it is the firmer it
becomes. This gradient in the amount or extent of gelation corresponds
with the disintegration gradient of cyanide along a forming pseudopod.
That is, the rate of disintegration is proportional to the age of the
ectoplasm. There is however no good evidence that the age of ectoplasm
corresponds to the rate of metabolism, so that the younger the ectoplasm
is the higher will be the metabolic rate in it. The following statement
seems to bear this out: “As soon as the pseudopodium extends into the
water its surfaces gelatinizes because of contact with the water”
(Hyman, ’17, p. 89). Gelation is, according to Hyman, a passive process
and therefore not distinctively metabolic. She continues: “It is
necessary therefore for the continuous production of a pseudopodium,
that the metabolic change which is the cause of the liquefaction should
continue to occur at the pseudopodial tip. There is thus produced the
metabolic gradient along the pseudopodium which I have described....”
But if the metabolic gradient is bound up with the process of
liquefaction, it is difficult to see how there can be a _gradient_ along
the pseudopod, for liquefaction takes place only at the tip, according
to her own statement. As a matter of fact, however, _gelation_ is
constantly occurring at the tip of the pseudopod and to a less degree
back along the sides of the pseudopod. Liquefaction occurs only at the
posterior end of the ameba in orderly movement.
We must conclude therefore that while Hyman’s data are of the first
importance in contributing to the structure and behavior of the ameba,
her contention that a metabolic gradient is demonstrated in the ameba is
not convincing.
From this short account of the main theories that have been advanced to
explain ameboid movement it appears that of the modern theories the only
one that has been capable of adjusting itself to new investigations and
observations is the surface tension theory. The earlier theories under
this head were mistaken however in looking to the superficial film of
the ameba as the source of energy. But with the increase in knowledge of
the chemistry of colloids, the source of the surface energy came to be
located in the interfaces between the phases of the colloidal system. As
has already been remarked, there is more than sufficient free energy
here to account for all the movements observed in protoplasm; there
remains the problem of explaining how the surface energy is transformed
into that of movement. As Graham (’61) remarked: “The colloidal is in
fact a dynamical state of matter. The colloid possesses _energia_. It
may be looked upon as the probable primary source of the force (energy)
appearing in the phenomena of vitality.”
Now, viewing streaming wherever it occurs in the protoplasm of animals
or plant cells the surface tension theory, as far as observations
permit, applies to the various conditions of streaming as follows.
In the first place we shall begin with the assumption that is generally
held, that protoplasm is a reversible colloidal solution consisting
mainly of proteins, with some carbohydrates, lipoids, etc., on the one
hand and water on the other. Its reversibility consists of course in
being able to change from a sol to a gel state and the reverse, the
water being in the disperse phase in the gel state. The consistency of
the protoplasm therefore depends upon two factors: upon the amount of
water present, and upon the degree of its dispersion; the smaller the
droplets the more solid will be the gel because of the increase in
surface of the mass. Colloids exhibit the property of contractility in
proportion as the droplets of water are decreased in size; or, which
amounts to the same thing, in proportion as the amount of the surface of
the water is increased. It appears as if the source of energy of
contractility was the free energy in the surface films of the internal
phase of the gel.
Taking the amebas as a group and applying these principles of colloidal
solutions, we find that we can arrange the amebas in a series of four or
more grades representing differences of fluidity of the protoplasm.
Among the most fluid are _Amoeba limicola_ and _Pelomyxa schiedti_; in
the next group, with less fluid protoplasm is _Amoeba dubia_; in the
third group is _A. proteus_ and _A. discoides_; in the fourth group,
with the least fluid protoplasm, come _A. radiosa_ and _A. verrucosa_.
These groups represent a progressive increase in the amount of ectoplasm
in proportion to the endoplasm. There being less water present in the
higher groups than in the lower, which follows from a stiffer endoplasm,
it is possible for them to form endoplasm, that is, to change phase,
more readily. And as a corollary to this we may add that more pseudopods
are formed, since ectoplasm can be formed more readily. (The _verrucosa_
types possess very stiff ectoplasm, and they increase their surface by
flattening out and by forming longitudinal ridges. They cannot for some
unknown reason form pseudopods). Again with the increase in the
consistency of the protoplasm, the pseudopods become more slender (and
stiffer) and more contractile, the most slender pseudopods (_radiosa_,
_flagellipodia_) being very much more contractile than the larger ones
of _proteus_ or _discoides_, for example. An additional factor operates
here, however, for some of the slender pseudopods as of _radiosa_ and
_bilzi_ are static and for a great part of their existence practically
incapable of contraction. The high development of contractility follows,
of course, from the high degree of dispersion of the internal phase in
ectoplasm, of which these pseudopods almost wholly consist. Thus, many,
if not most, of the more generalized peculiarities of form of amebas may
be traced to the amount of water in the protoplasm.
The number of pseudopods in an ameba is an important factor in its
method of locomotion, as may readily be perceived. Since amebas
generally move with great variation in speed as one compares the
different species, whether they form very little ectoplasm or very much,
and are able to maintain themselves on their paths, it follows that
ectoplasm formation by itself does not play an important part in
originating movement. But it requires only a few minutes’ observation to
see that ectoplasm is necessary to guide the ameba, so to speak, and to
make the endoplasmic stream effective for the purpose of orderly
movement. It requires very little imagination to see what would happen
if no ectoplasm were present in a _limicola_ or any very fluid ameba.
Streaming would undoubtedly occur as before, but the currents would be
rotational and irregular and no progression could take place. The
ectoplasm furnishes just that stiff tube against which the backward
action of the endoplasm can impinge so to speak in order to enable it to
flow forward. The ectoplasm is essential for orderly movement forward,
but it is not essential for streaming.
But this does not imply that the contractile power of the ectoplasm may
not be used to aid in propelling the endoplasm in streaming. It has been
demonstrated by Miss Hyman (’17) that the ectoplasm is actually
contractile when the ameba is strongly stimulated all over its exterior
by a solution of potassium cyanide. While this proves only the
contractile powers of the ectoplasm under exceptional conditions, and
when at rest, it is not impossible that under ordinary conditions of
locomotion it may aid in streaming. There is however one observation
which may, upon further investigation, negative this possibility.
Frequently in a pseudopod about to be retracted some of the endoplasm
flows toward the tip while the rest flows toward the base (Figure 1, p.
11).
One more point needs mention in this connection, and that is the small
waves of clear protoplasm which are thrown out by many amebas at their
anterior ends during locomotion. They are especially prominent in _A.
bigemma_ (Figure 7) and in _radiosa_ (Figure 8), but they are formed in
perhaps all species. Observation does not indicate that they move in
exactly the same way as the main body of the endoplasm, even if the
larger granules could be left out of account. They behave more like the
clear pseudopods of _Difflugia_ and _Arcella_ and the foraminifera.
Although these waves are frequently not to be seen during locomotion in
_Amoeba proteus_ and other large amebas, particularly in _Pelomyxa
palustris_ and _P. belevskii_, it is possible that the wave forming
process has become indistinguishably merged with endoplasmic streaming.
It is not impossible that the projection of these waves is the purest
expression of ameboid movement. But on account of their small size and
transparency, it is very much more difficult to investigate them than
streaming of the granular endoplasm, as it is observed in amebas,
ciliates and plant cells. It seems to be true however that streaming can
occur in the entire absence of these waves, so their importance in
ameboid movement is probably secondary.
CHAPTER X
STREAMING, CONTRACTILITY AND AMEBOID MOVEMENT
The nearest relatives of the amebas are the shelled rhizopods, the
Difflugias and the Arcellas and their congeners. The movement of these
organisms is quite different from that of the amebas in that the whole
body of the endoplasm does not stream into the pseudopods, but only a
small portion of it. There is consequently no regular transformation of
ectoplasm into endoplasm at the posterior end, that is, the protoplasmic
mass within the shell. The method of movement in _Difflugia_ was
described by Dellinger (’06). A pseudopod is thrown out to a
considerable distance. It fastens itself to the substrate at the tip. It
then contracts, pulling the _Difflugia_ forwards. While this pseudopod
is contracting, another one is extended in the same direction. When it
has arrived at the maximum length, it fastens itself at the tip and then
contracts, pulling the _Difflugia_ along. Continued locomotion consists
of a repetition of this process. The pseudopods are slender and consist
nearly always of clear protoplasm. Only occasionally does one see
conspicuous endoplasmic granules flow into a pseudopod, and then only at
the base.
The transparency of the pseudopods in _Difflugia_ and the absence of
granules in the protoplasm composing them, prevents one from seeing
clearly how the pseudopods are formed, that is, whether or not there is
a regular transformation of endoplasm into ectoplasm at the anterior
end. The fact that one occasionally sees the endoplasm stream into the
base of a pseudopod in the same way as was described for ameban
pseudopods, indicates that the method of formation of pseudopods in
_Difflugia_ is in general similar to that in ameba. But the process is
not exactly the same, for the surface layer on the pseudopods of
_Difflugia_ does not move as fast as the tips of the pseudopods advance,
while in amebas the surface layer moves faster than the pseudopods. What
this difference indicates has not yet been ascertained.
The protoplasm of the pseudopods of _Difflugia_ is thick and the power
of contractility highly developed, for the pseudopods readily move about
in the water like a tentacle. The demarcation line between ectoplasm and
endoplasm is very difficult to see, consequently no definite idea can be
given as to the thickness of the ectoplasm. When a pseudopod is being
extended the whole contents seem to move at about the same rate as the
pseudopod advances, differing thus from amebas, in the pseudopods of
which the central core of the endoplasmic stream flows considerably
faster than the tip of the pseudopod advances through the water. But
when a large pseudopod is cut off from a _Difflugia_ it is able to move
after the manner of an ameba without a nucleus (Verworn, ’94).
In heliozoans protoplasmic streaming is quite different from that in
ameba or _Difflugia_. The pseudopods are usually straight, radiating
from the central body. They possess usually a central axial rod of
condensed or strongly gelatinized protoplasm around which is a layer of
thick protoplasm with the properties of ectoplasm. Heliozoans for the
most part move slowly; in fact many of them are pelagic and in these the
power of locomotion on a solid substratum is very slow. There is however
one species, _Acanthocystis ludibunda_, which, according to Penard
(’04), can move twenty times its diameter in one minute by rolling. This
illustrates a highly developed power of contractility in the pseudopods
of this organism, for since only about one-fifth of the circumference
can be in contact with the solid substratum, the pseudopods must attach
themselves, contract so as to pull the _Acanthocystis_ along, and relax
their hold, all in the space of two seconds.
Among pseudopod forming organisms, the highest development of
contractility is found in the foraminifera. As is well known, these
organisms form finely anastomosing pseudopods which frequently cover the
substratum with a network of protoplasmic strands. The terminal sections
of these strands are frequently so thin and transparent that they cannot
be seen easily with the microscope. As a rule the granular endoplasm is
observable only in the main body of the organism and in the larger
trunks of the pseudopods. Much the larger part of the pseudopods, as
measured lineally, is devoid of granular endoplasm. The great power of
contractility and the speed with which contraction may occur in
_Biomyxa_, a fresh water foraminifer, have already been mentioned
(Figure 12, p. 47). Similar observations have been recorded by other
observers, recently by Schultz (’15), who compares the contractility of
foraminiferan pseudopods to that of rubber bands. In fact as one watches
the movements of a _Biomyxa_, for example, under moderately high
magnification, one gains the impression that there seems to be no
restriction imposed upon the extent of contractility in the pseudopods.
They seem to possess perfect elasticity. As to the transformation of
endoplasm into ectoplasm, little can be said, owing to the transparency
of the protoplasm. But the whole of the pseudopod, when forming, seems
to stream forward. As in _Difflugia_, the interior streams flow at about
the same rate as the pseudopod as a whole advances. The highly developed
power of contractility however demands rapid changes in phase of the
colloidal system, and also a thick consistency. The behavior of pieces
of the pseudopodial network, when cut from a _Biomyxa_, shows clearly
that the protoplasm is actually thick, as compared with that of an
_Amoeba proteus_. When a _Biomyxa_ is contracted into a spherical mass,
the interior exhibits continual rapidly streaming movements. Some of
these are rotational but most of them are radial. All of the streams
frequently change their direction and extent. No corresponding changes
are visible in the outer peripheral layer.
Among plants, some of the algae possess ameboid protoplasts at one stage
or another of their life cycle, but the details of streaming have not
been made out. It has been reported however that the zoospores of some
parasitic fungi move to all appearances exactly like small amebas. We
likewise lack details of the streaming of the myxomycete plasmodia. From
a more or less cursory examination of a small aquatic plasmodium of
undetermined species, it appeared that the formation of pseudopods and
the process of streaming were quite different from similar processes in
the foraminifera. The pseudopods do not act independently as in
foraminifera. At almost the same moment the protoplasm begins to flow
from the pseudopods in a large section of the plasmodium and into
another section; then soon thereafter the protoplasm flows back again.
This oscillatory streaming is continued presumably as long as the
myxomycete is in the plasmodial stage. With every change in the
direction of movement of streaming, there is produced, however, a change
in the shapes of the pseudopods, so that with a number of oscillations
in streaming an appreciable degree of locomotion is effected. The
direction of locomotion can be markedly affected by changes in light
intensity and moisture distribution, as shown by the observations of
Baranetzsky (’76), Stahl (’84) and others, but just how these changes in
the direction of locomotion were produced is not recorded. There is a
definite ectoplasm and a definite endoplasm in the myxomycete plasmodia,
but the details of their transformations, the one into the other, have
not been determined; but since the surface layer is stationary, it is
probable that there is no such regular transformation of endoplasm into
ectoplasm at the anterior ends of pseudopods as there is in ameba. But
this phase of the subject needs further investigation before any
conclusions can be drawn. The power of contractility is present, but
apparently only to a slight degree. Too little is known of the streaming
process in these organisms to compare it in detail with the same
phenomenon in rhizopods.
The streaming of protoplasm in plants has received a good deal of
attention, though only comparatively little experimental work has been
done. Streaming is observed in a great many plant cells, and in some
cells such as the large internodal cells of _Chara_ and _Nitella_, the
process may be easily observed. The essential features of a plant cell
in which streaming occurs are, first, the external cell wall of
cellulose, which of course prevents any change of shape in the cell such
as is observed in naked protoplasts as, for example, ameba. Inside of
the cell wall is a layer of ectoplasm which has essentially the same
properties as the ectoplasm of amebas. In some cells such as those of
_Chara_, the ectoplasmic layer is thick and contains nearly all the
chloroplastids, while in the leaf cells of _Elodea_ the ectoplasm is
extremely thin and is practically free from chloroplastids. In the
interior of the cell are found the streaming endoplasm and one or more
large vacuoles filled with cell sap.
The streaming is of two types which are often distinguished from each
other by the names _rotational_ and _circulatory_. But the distinction
seems to be of little significance, for the same cell may at different
times show both types of streaming. When there is a single vacuole only
in the cell, it occupies the center of the cell, and the endoplasm then
rotates between it and the ectoplasm. Whenever there are strands of
endoplasm flowing across the vacuole, the peripheral streaming is no
longer rotational but it is then called circulatory. By external
stimulation of the cell, Ewart (’03) was able to change circulatory
streaming into rotational; that is, the numerous small streams
traversing the cell sap in many directions were caused to retract into a
single stream around the periphery of the cell. This change brought
about a heightened velocity in streaming, showing that the small strands
traversing the cell sap meet with some resistance. There is no essential
difference between streaming in plant cells, whether rotational or
circulatory, from the rotational streaming so commonly found in
protozoa.
[Illustration: Figure 31. Diagram of a section of a _Chara_ cell showing
rows of emulsion globules in the endoplasm, after Ewart. _a_, cell wall.
_b_, ectoplasm. _c_, endoplasm, _d_, cell sap. The arrows at the top of
the figure indicate by their lengths, the amount of movement of the
endoplasm and cell sap in streaming.]
Ewart has also observed that in the streaming of the endoplasm, there is
a variation of velocity of streaming in different parts of the stream
(Figure 31). The middle of the stream moves fastest while the layer near
the ectoplasm moves very slowly and the layer in contact with the
ectoplasm moves hardly at all. But the endoplasm in contact with the
central vacuole moves only a little more slowly than the middle of the
stream, and the effect of this is that the outer edge of the vacuole is
dragged along with the moving endoplasm. This is an important
observation and from it Ewart concludes that the energy which produces
the streaming movement must be liberated, not at the boundary between
the ectoplasm and the endoplasm, nor at that between the endoplasm and
the vacuole, but within the endoplasmic stream itself. In this
conclusion Ewart is undoubtedly correct, for as a physical phenomenon,
no other conclusion is at present possible.
Other experiments made upon the velocity of streaming in plant cells
indicate that the streaming process obeys the laws of physics. The
velocity varies with the proportion of water present in the
endoplasm,--the more water, the faster the streaming (Ewart, ’03). The
effect of temperature on streaming, noted first by Corti (’74), and
studied by Velten, (’76), Schaeffer (’98), Ewart (’03) and other
writers, is also such as would be expected if the endoplasm were a
simple physical fluid.
The rotational streaming in plant cells, such as those of _Chara_, is
very similar to the rotational streaming in paramecium and numerous
other ciliates. In these organisms it is often called cyclosis. A
paramecium differs, however, from a plant cell exhibiting rotational
streaming in that no central vacuole is present. This space in
paramecium is occupied by the gullet, the nucleus and some endoplasm
which is not in the main stream. The effect of this difference seems to
be one affecting velocity only, slowing it down, for in the _Chara_ cell
the endoplasm meets with much less friction when moving in contact with
the vacuolar wall than when moving in contact with the ectoplasm. Its
velocity is still further reduced by the large food vacuoles which are
almost always carried by the endoplasm, for these vacuoles behave like
solid bodies in the endoplasmic stream. During streaming these vacuoles
are often seen coming close to the limiting ectoplasm, when they act as
obstructions to the flow of the endoplasm. The velocity of the
endoplasmic stream in paramecium is relatively slow, ten to twenty
minutes being required for a complete revolution.
In _Frontonia leucas_, another large ciliate, rotational streaming is
under the control of the organism, and special use is made of it in
feeding. _Frontonia_ feeds mostly, if not entirely, on large particles.
It has no oral groove like paramecium has, and when swimming no ciliary
vortex is produced such as is seen in paramecium. _Frontonia_ feeds
mostly by “browsing,” that is by eating particles lying on or against
some solid support, though it is able also to feed upon particles
suspended in the water.
_Oscillatoria_ and _Lyngbia_ and other filamentous algae are the chief
food of _Frontonia_. Filaments of these algae are ingested by pulling
them into the mouth and then rolling them up into a coil in the body.
Pieces of _Oscillatoria_ six to eight times as long as the _Frontonia_
are readily eaten in this way.
As a rule the end of a filament is seized by the mouth and gradually
passed back into the body (Figure 32, _a_). As soon as the tip of the
filament is well in the mouth and in contact with the endoplasm,
streaming begins in the endoplasm in the region of the mouth and takes a
direction directly back against the aboral wall, almost, if not quite
perpendicular to the longitudinal axis. This stream of endoplasm carries
the filament back to the aboral wall, sometimes pushing out the wall a
considerable distance. Presently, however, the filament is carried
posteriorly along the aboral wall by the streaming protoplasm, which has
by this time become rotational, and after reaching the posterior end the
filament is brought up along the oral wall. The rotational streaming
continues until the entire filament is wound up, which in exceptional
cases may make four or five coils inside the animal.
The mouth has considerable grasping power. This is shown in Figure 32
where a filament of _Oscillatoria_ was bent upon itself by the mouth and
then rolled up in the body by the endoplasm in the same manner as a
single filament. The mere viscosity of the endoplasm would be
insufficient to bring about the bending of the filament. For the sake of
comparison it should be added that a similar grasping power is also
present in paramecium. The moment the food vacuole at the mouth is large
enough, the endoplasm pulls it away and moves it rapidly toward the
posterior end of the paramecium, much more rapidly than it would be
carried by the rotationally streaming endoplasm. But from the posterior
end forward the food vacuole is carried at the same rate as are the
other particles in the endoplasm. In both _Frontonia_ and paramecium
rapid endoplasmic streaming precedes for a short distance the forward
end of the ingested filament or the food vacuole (Figure 32, _a_).
[Illustration: Figure 32. Showing ingestion of alga filaments in
_Frontonia leucas_. _a_, the beginning of the ingestion of an alga
filament. Note the streaming of the endoplasm _preceding_ the end of the
filament. _b_, almost two complete coils of the filament have been
rolled up inside the _Frontonia_ by the rotary streaming endoplasm. The
endoplasm in the center of the animal is stationary. _c_, a filament, if
thin, may be grasped anywhere along its length, bent together and
swallowed in the usual manner. Diameter of _a_, 250 microns.]
If a filament of alga is too long for the _Frontonia_, or one end of it
is fast, streaming is reversed after several coils have been rolled up
and the filament is ejected. So far as could be observed, the streaming
process is reversed in all details, though the rate of ejection seemed
to be somewhat slower than the rate of ingestion. Occasionally, however,
ejection is accomplished much more quickly. If there are several coils
of a filament whose other end is fast, rolled up inside of a
_Frontonia_, the mouth sometimes stretches antero-posteriorly until the
coil as a whole without unwinding is thrown out of the body. The
viscosity of the endoplasm might lead one to expect that some of the
endoplasm would be brought out with the alga, but such is not the case.
The essential differences between rotational streaming in _Frontonia_
and in paramecium are: (1) It is under the control of the organism in
_Frontonia_ while in paramecium it is a continuous reversible process.
(2) It is much more rapid in _Frontonia_ than in paramecium. On the
other hand, the physics of streaming in both organisms is essentially
the same, so far as could be detected. In both organisms the energy of
streaming is liberated within the endoplasm. This is especially well
shown in the first stages of feeding.
Besides these organisms in which streaming occurs, either in a part of
the organism or the whole, streaming is also found to occur in a great
variety of plants other than those already mentioned; in the leukocytes
of perhaps all coelomates; in some animal egg cells, such as the
sponges, hydra and molluscs; in pigment cells, especially in batrachians
and lacertilians; in phagocytes and wandering cells of a great many
animals; in the nuclei of some animal cells; and in the intestinal
epithelial cells of perhaps all metazoans. In almost none of these cases
however do we know more than the bare fact that streaming occurs. No
details are known. Consequently in so far as the purposes of this book
are concerned it will not be apropos to discuss these cases further
except to record the thesis that there is no evidence tending to show
that these cases are not at bottom all characterized by the operation of
the same fundamental process.
In all these cases of animal and plant cells and tissues in which
ameboid movement occurs the process of streaming is easily observed in
all of them, but the phenomenon of contractility is not noticeable in
some cells except under special conditions, while in other cells it is
operating continually. This indicates that there are other factors at
work in addition to mere phase changes in the colloidal system to
produce now contractility, now streaming. A high power of contractility
and of streaming are not present in the same mass of protoplasm at the
same time, though these powers may both be present at different times
(_Biomyxa_).
Contractility can be explained in a general (though not yet in a
detailed) way as due to a change in phase, more or less complete, in the
colloidal system which is held to be the chief characteristic of the
physical aspect of protoplasm. The change of phase is of course,
associated with a change in the amount of surface energy, which is the
ultimate source of the energy of contractility.
Streaming, however, does not depend upon a _marked_ change of phase
resulting in gelation, for observation has failed to detect this process
going on to any extent whatever in streaming protoplasm. Further, an
increase in the amount of water in the protoplasm is associated with
more rapid streaming. If streaming therefore depends upon a phase change
in a colloidal system, it must be in the direction of liquefaction, that
is, changing the internal more fluid phase to the external phase. A
phase change in one direction would thus lead to contractility, while a
change in the other direction would lead to streaming.
Theories accounting for the intimate nature of the process of streaming
without special reference to ameboid movement, have been offered by many
botanists. In most plant cells in which streaming movements occur the
ectoplasmic covering does not change shape. Streaming of the endoplasm
therefore is a much less complicated process in such a case than in an
ameba where locomotion is also present. It is to be expected therefore
that a theory of streaming based upon observation of a plant cell such
as is found in _Chara_ would be different from one based upon
observation of a moving ameba. Such is found to be the case, as the
following discussion of some of the principal theories accounting for
streaming in plant cells strikingly shows.
(1) _The contractility theories_. Corti (’74), who was the first to
record observations on the process of streaming in plants thought that
the movement of the endoplasm was caused by waves of contraction passing
around the cell in a way analogous to that in which fluid may be passed
through a rubber tube by closing the finger over it and passing it along
the tube. Heidenhain (’63), Kühne (’64), Brücke (’64), Hanstein (’80),
in one form or another also have expressed their adherence to the
contractility theory. More recently Dellinger (’06, p. 356) postulated
contractile fibrillae in rhizopods similar to those postulated by Brücke
to explain protoplasmic streaming. The contractility theories are no
longer considered tenable, for no waves of contractility can be
demonstrated, as the theories of Corti, Heidenhain, et al. demand, and
contractile fibers can neither be demonstrated nor can they be conceived
to exist in endoplasm which exhibits all the essential properties of a
fluid.
(2) _The imbibition theories_. Sachs (’65), Hoffmeister (’67) and
Englemann (’79) conceived of streaming as being caused by certain
constituents of the cell imbibing water and later discharging it. Sachs
and Hoffmeister thought that waves of imbibition and extrusion of water
passing progressively along the cell was able to cause movement of the
protoplasm. Ewart (’03) has shown, however, that as much as 2000 times
its own volume of water would have to be imbibed by a cell of _Nitella_
in the course of a day to account for the amount of streaming observed,
and that no sign of the extrusion of water could be detected by
observing small suspended particles in the immediate vicinity of the
cell. Englemann’s theory involving a change of shape of his hypothetical
supra-molecular “Inotagmas,” by the imbibition of water and the
subsequent release if it, which was supposed to account for the movement
of the protoplasm while streaming, has been considered too hypothetical
and too far removed from the realm of experiment to be of real value,
either as an explanation or as a working hypothesis.
(3) _The oxidation theory of Verworn_. Verworn (’92, ’09) has postulated
a “Biogen Molecule” which exists only in living protoplasm and
dissociates when protoplasm dies into a number of chemical molecules of
albumin and other substances. Ameboid movement and streaming generally,
according to Verworn, is caused by the lowering of the superficial
surface tension in the moving mass of protoplasm followed by streaming
of the protoplasm toward the point of lowered tension. The lowering of
the surface tension is brought about by a union of the Biogen Molecule
with oxygen. With the dissociation of the biogen-oxygen compound,
presumably through a respiratory process, the surface tension rises
again. This theory does not hold for amebas, for we saw in the preceding
pages that the surface tension is higher at the anterior ends of
pseudopods than elsewhere on the ameba. And in plants, as Ewart (’03)
has shown, oxygen does not seem necessary to the streaming process, for
the endoplasm of _Chara_ cells continues to stream for many days in the
entire absence of oxygen. It is possible that there would be enough
loosely fixed oxygen in the endoplasm of _Chara_ to supply the demands
of Verworn’s theory; but the very hypothetical nature of his theory
prevents one from discussing this possibility.
(4) _The electrical theories_. These fall into three classes: (a) _The
galvanic theory_. Amici (’18) suggested that the chloroplastids floating
in the endoplasm of plant cells acted as galvanic cells, setting up
currents in the endoplasm which in some way caused the endoplasm to
move. Dutrochet and Becquerel (’38) also held to this explanation. A
fatal defect of this theory is that streaming occurs in a great variety
of cells, myxomycete plasmodia, amebas, stamen hairs of _Tradescantia_,
etc., in which no chloroplastids occur; and there is no ground for
assuming that the causes of streaming in cells with chloroplastids is
fundamentally different from that in other cells. (b) _The
electromagnetic theory_. Velten (’72, ’73) and Hörmann (’98) are chiefly
responsible for the development of the electromagnetic theory. They hold
that chloroplastids have an independent movement of their own; but the
principal postulate of this theory is that there is electric repulsion
between the ectoplasm and the endoplasm. Ewart (’03) has pointed out,
however, that this theory is contradicted by the fact that when
streaming becomes very active in _Elodea_, the ectoplasm becomes
exceedingly thin and therefore would show movement in the direction
opposite to that of the endoplasm if there were magnetic repulsion
between these layers. Moreover, the formation of threads of endoplasm
across the central vacuoles in plant cells, and the much branched
network of pseudopods in plasmodia and foraminifera would be very
difficult if not quite impossible to explain on this assumption. (c)
_The electro-chemical surface-tension theory of Ewart_. As the result of
a considerable amount of experiment and observation on endoplasmic
streaming in plants, Ewart (’03) has come to the conclusion that there
are differences in electrical potential between the protoplasm-vacuole
boundary and the protoplasm-cell wall boundary, and that as a
consequence electrical currents are passing between these points,
traversing the protoplasmic stream. If now it is assumed that the
particles in the endoplasm, which are electrically polarized, have the
surface tension of their corresponding ends decreased when electric
currents traverse the endoplasmic stream, the particles and, of
necessity, the whole stream of endoplasm would move in the direction of
lowered surface tension (Figure 31, p. 96). Continuous chemical actions
would be necessary to maintain the conditions as outlined. This theory
accords with the facts so far as it goes, but it does not explain the
streaming in threads across the vacuole in the plant cell, thus
necessitating two theories for the explanation of streaming within a
single cell at the same moment. Moreover a central vacuole of cell sap
seems always to be required to fulfill the conditions of this theory,
and this, as is readily seen, makes it impossible to apply it to
streaming in amebas, myxomycetes, foraminifera and ciliates.
The fundamental cause of streaming is therefore still to be discovered,
for neither the theories of streaming as applied to ameba, nor those
described above which refer especially to plant cells, are satisfactory.
But a significant point in these theories is that with increasing
information, they come more and more to demand a colloidal structure in
the protoplasm. It is the surface energy in the interfaces in the
colloidal system which comes to be regarded as the primary source of the
energy. But all attempts thus far to explain exactly how this energy is
utilized have been unsuccessful. Gaidukov’s (’10) observation is of some
interest, however in this connection. He found that the occasional
stopping of streaming in cells of _Vallisneria_ is accompanied by a
cessation of Brownian movement, which indicates a change from a sol to a
gel state. This proves therefore that colloidal changes are possible in
streaming protoplasm, and that the general search for an explanation of
streaming along this line is proceeding in the right direction. The
researches of Bancroft (’13, ’14) and especially of Clowes (’13, ’16) on
the nature of the change of phase in emulsions are very instructive in
this connection; and it is undoubtedly true that as rapid progress is
now being made by the investigation of colloidal solutions as by the
direct study of protoplasm, in solving the problem of streaming.
The problem of the _control_ of the streaming process, which is of
course much the most important feature of streaming, will probably be
solved, at least in part, when the mechanics of streaming is
understood.
CHAPTER XI
THE SURFACE LAYER AS A LOCOMOTOR ORGAN
The discussion of the surface film of ameba and its movements during
locomotion naturally led to a discussion of the various theories that
have been offered to explain ameboid movement and protoplasmic
streaming. Now the fact that the ameba possesses a traveling surface
film which can carry particles recalls similar behavior in Oscillatorias
and in the diatoms. No new observations have been made very recently,
but by comparison of the behavior of particles carried by an
Oscillatoria filament and by ameba, it is found that the nature of the
movement, the rate of movement, the degree of adhesion of the particles,
the sizes of the particles carried and so on, are similar in both
organisms. This indicates that there is a surface layer on Oscillatoria
threads that is similar to that which has been described in amebas, and
whose movement is probably also effected by changes in surface
extension; but just how this change is effected is not clear owing to
the spiral path the particles take as they travel along the Oscillatoria
filament. The spiral has an angle of about sixty degrees, which must be
related in some way to the finer structure of the cells of which the
filament is composed. The suggestion that movement is caused by the
rapid and forcible exudation of mucus is exceedingly improbable if not
physically impossible. It is difficult to see how the spiral direction
of the flow of mucus could be brought about, to say nothing of the
frequent change in direction of the flow. In a surface tension film,
however, the direction of movement is readily determined by the location
of the points where the tension is changed. Mucus secreting glands would
need special structural devices such as secretory tubes bent at an angle
to control the direction of flow, while no such structural devices are
necessary if the propelling force is surface energy. In short, it is
difficult to see how any movement at all could be produced by the act of
secretion of mucus, while from what we have seen in the ameba, surface
tension changes could easily produce movement in Oscillatoria. The
spiral feature of the movement has no explanation that is based on
observational data. It may be added here that the surface film in amebas
is powerful enough to enable them to move by means of it. One sometimes
sees _sphaeronucleosus_ or small individuals of _verrucosa_, that are
lying loose on the substratum, actively streaming, but moving slowly and
more or less irregularly backwards. This movement is due to the activity
of the surface film.
The suggestion that no extra-cellular protoplasmic layer has been
demonstrated in Oscillatoria is not a cogent argument against the
surface tension hypothesis, since the surface film would need to be but
a small fraction of a micron thick, too thin to be demonstrated by
histological methods now in vogue. It is also to be remembered that the
surface film in ameba can be demonstrated in no other way at present
than by its particle-carrying capacity.
The main features of the movements of diatoms are very similar to those
of Oscillatoria. Müller (’89, ’97, ’99) has shown that the gliding
movements of diatoms are not due to the ejection of water, but to the
streaming of protoplasm on the outside of the shells. Foreign particles
are carried by these shallow streams of protoplasm in quite the same
manner as by the surface film of the ameba. And there seems to be no
evidence against the assumption that these shallow streams, at least the
surface films over them, owe their movement to changes in surface
tension.
Desmids also glide about slowly, leaving a track of mucus behind. Only
one explanation for locomotion has been advanced, and that is that it is
due to the secretion of mucus (Klebs, ’85). This explanation is likely
to be as wide of the mark as the similar explanation in the case of
Oscillatoria. There is no question concerning the excretion of mucus,
but the source of the locomotive energy is probably here also surface
energy, though the observational data are too few to try to locate the
regions where the changes in tension occur.
It has been a matter of considerable surprise to me to find that the
so-called “crawling” euglenas, in addition to the diatoms, desmids,
Oscillatoria, Beggiatoa and perhaps other forms of life such as the
Gregarinidas, also possess extra-cellular films which carry particles
as do amebas and Oscillatoria, and move about through the agency of this
film. The film travels spirally around the euglena as it does in
Oscillatoria filaments. In at least two species the film moves parallel
to the spiral striations on the outer surface. In one species no spiral
striations could be detected, although the film moved spirally. The
species of euglenas in which these movements were observed, were not
identified.
The character of the movement of the euglenas is very similar to that of
the diatoms excepting that most of the diatoms do not revolve on their
longitudinal axes. The movement of particles on the surface film of
euglenas is quite like that in Oscillatoria, though it is only under
exceptional circumstances that one can see particles attached to the
surface film. The movement of the particles indicates that the surface
film moves from the anterior end toward the posterior end, but whether
the “spine” is to be included was not definitely determined. The degree
of cohesiveness of the film is high, for locomotion is rapid, even if
only a small part of the posterior end is in contact with the
substratum, as when moving over an Oscillatoria filament. To one who has
seen the movements of the surface films of amebas, diatoms and
Oscillatorial filaments, the most reasonable conclusion seems to be that
the cause of locomotion in crawling euglenas is the same as that in
Oscillatoria and diatoms.
Evidence contributing to this conclusion is found in the circumstance
that crawling euglenas, diatoms and Oscillatoria threads are much more
refractory to galvanic currents than flagellate euglenas or other
flagellates or ciliates: The electrical apparatus at my disposal was
rather crude, but I was unable to find that I could influence the
direction or character of movement of Oscillatoria filaments, diatoms or
crawling euglenas without injuring the organisms. Currents which had
produced a marked effect on ciliates or flagellates produced no effect
whatever on amebas, diatoms, Oscillatoria or crawling euglenas. Diatoms
are particularly resistant to the effect of electrical currents.
The general conclusion regarding the source of energy of the moving
surface films, whether found on amebas, diatoms, desmids, or crawling
euglenas, is that all derive their motive power from the energy in the
superficial films of these organisms; while ameboid streaming, if it is
a surface tension phenomenon as seems to be the case, depends upon the
surface energy of the interfaces of the emulsoid colloidal system in the
endoplasm. It has already been seen that those cases of locomotion due
in large measure to the power of contractility in the ectoplasm
(Difflugia, Foraminifera) are also explained as being due to a change of
phase in the colloidal system, which is in itself a surface tension
effect. It appears therefore that all the lower organisms that move,
excepting flagellated or ciliated organisms (of whose motor mechanism we
have no detailed knowledge), depend upon surface energy as the source of
the energy of movement.
CHAPTER XII
THE WAVY PATH OF THE AMEBA
In the preceding chapters we discussed the various factors which
characterize ameboid movement: the streaming of the endoplasm, the
formation of ectoplasm, and the behavior of the surface film. The
discussion has involved only momentary cross-sections of the life of an
ameba, following the method of investigation in general use for the
solving of problems connected with ameboid movement. It has been tacitly
assumed that if one could explain ameboid movement at any particular
cross-section in time, one understood the whole process of ameboid
movement no matter how long it continued, excepting, of course, the
action of various kinds of stimuli that produced changes in direction,
speed, etc., of streaming. It was not assumed that time was an element
in the practical sense in the explanation of locomotion. A few seconds’
or a few minutes’ comprehensive observation was supposed to furnish
sufficient basis for an explanation.
Sometime ago I discovered however that the path of an ameba as it moves
over a flat surface free from particles possesses character; it is not
an aimless irregular zigzagging here and there, such as has been
generally supposed, and in occasional instances asserted, to be the
case. On the contrary, the path of an ameba during the course of an hour
or two consists of a succession of gentle right and left-hand curves
alternating with each other. The general appearance of the path is that
of a flattened spiral. Having observed a part of an ameba’s path,
therefore, one can predict with considerable accuracy in what direction
the ameba will continue to move. Thus that scientific bugaboo “Random
Movement” is evicted from that strongest of his strongholds, the aimless
wanderings of the ameba.
The mechanism producing the sinuosities in the path of the amebas is
easily disturbed by external or internal stimulation of various sorts,
resembling in this respect the spiral path of a paramecium, which is
also easily changed by the presence of various solid and dissolved
substances in the culture medium. But the mechanism controlling the
direction of the path of an ameba is apparently much more delicate than
that in paramecium, for it is only occasionally that a considerable
succession of regular sinuosities are described by an ameba in moving
over a flat surface. On the other hand, a few fair curves are found in
the path of practically every ameba if carefully observed for an hour or
more under favorable conditions.
To observe the path an ameba describes in moving over a flat surface,
the following conditions must be fulfilled. One must have a small glass
dish with a flat bottom, polished preferably, but not necessarily, of
the size of a small petri dish, but square so as to fit into a
mechanical stage. The dish should be filled with culture fluid free from
solid particles. Centrifuging the culture medium, or dialyzing distilled
water in the culture medium, will yield a satisfactory medium. It is
only by experience that one can pick out an ameba that seems to be in an
optimum condition for this purpose, that is, free from strong internal
stimuli, such as those from a large mass of food, etc. Just as we speak
of “clean-limbed” athletes, meaning thereby a high degree of muscular
coördination, so one who has worked with these animals for some time
acquires the capacity to pick out “clean-limbed” amebas; though how
these differ from others is just as impossible to describe adequately as
to tell what a clean-limbed athlete is. But having selected two or three
amebas that move in a well coördinated manner and passed them through
two or three changes of water free from particles, they are placed in
the middle of a dish and allowed to remain for ten or fifteen minutes
before observations are begun. A small shade should be placed in front
of the dish if very strong light can reach it. It does not matter if
diffuse light reaches the dish. A camera lucida with its appurtenances
is absolutely essential. In addition to the ordinary precautions the
edge of the paper must be laid parallel with the side of the mechanical
stage, for a number of sheets of paper will have to be used up in the
course of an hour or two and these must be pasted together properly to
reconstruct the path. The best magnification shows the ameba two to five
cm. long on the paper. Drawings should be made quickly but carefully,
beginning and ending with the posterior end of the ameba.
One of the best examples of the sinuous path of an ameba is shown in
Figure 33. It is the path of an _Amoeba bigemma_ from a natural out-door
culture. The observations were made under the conditions outlined above.
The temperature, which is an important factor, was unfortunately not
recorded, but it was about 28° C.
Practically the whole of the path of this ameba consists of right and
left-hand curves which are nearly uniform in length, each wave being
about eight to ten times the length of the ameba. Since the drawings
were made at intervals of a minute, the waves are therefore from eight
to ten minutes long in time, measuring from crest to crest. Some of the
waves are flatter than others, for example wave No. 4, but otherwise it
is like the others. Wave 7 is a double wave due to a change of
direction. Instead of turning to the right at 9:23 the ameba changed its
direction and turned to the left. The smoothness with which this turn
was made indicates that it originated in the mechanism producing the
sinuous course itself, or that it proceeded from a very slight stimulus
external to it. At 9:49 the direction of movement was changed again, but
just enough to disturb the formation of a smooth wave. The general
direction of locomotion was not changed. It may be assumed that this
change was produced by a stimulus external to the wave-producing
mechanism. The irregularity and shortness of wave 13 was probably due to
the same stimulus that disturbed wave 11. Shortly after 9:58 the ameba
came within sensing range of a mass of debris which it pushed away and
followed, thus causing a change in the direction of movement. Although
waves begin to appear again after this, some of them very smooth, they
are not typical for they are too short, ranging from a little over three
lengths (wave 16) to a little over six lengths (wave 23). It is likely
that the disturbance caused by the mass of debris at 9:58 together with
the onset of the division crisis produced the succession of atypical
waves. An external disturbance that is sufficiently strong to change the
direction of locomotion usually persists for the duration of at least
one wave length thereafter. It will be noted that
[Illustration: Figure 33. Illustrating the path of an _Amoeba bigemma_
under controlled conditions, in ordinary diffuse light from a north
window. For convenience of reference the waves in the path are numbered
from 1 to 23. The figures were drawn with the aid of a camera lucida at
intervals of one minute. The path was recorded from 8:58 to 10:26, when
fission occurred. After fission, the path of one of the daughter cells
was followed for a short time, but the ingestion of a mass of debris
destroyed the regularity of the path. The vertical lines at the point
where fission occurred indicate that the figures _above_ the lines were
moved up the length of the lines. The first figure beyond wave 13 was
influenced in its movements by a mass of debris lying to its left.
Average length of the ameba, 150 microns.]
the approach of the division crisis did not tend to destroy the action
of the wave mechanism, but only slowed down movement and shortened the
waves. The path of one of the daughter amebas was followed for a short
time, in which there is evidence of a wavy path, but it soon came upon a
small mass of debris which it ingested and soon thereafter reversed its
direction of movement. This behavior made it unprofitable to continue
further observation of this ameba. For the gradual change in direction
to the left from wave 1 to 6 in the path of the parent ameba, no
adequate explanation has suggested itself.
That amebas react to light has been shown by Verworn (’89), Davenport
(’97), Harrington and Leaming (’00), Mast (’10) and especially Schaeffer
(’17). It appeared desirable therefore to control the rays of light, for
it was thought possible that light might be a factor in the formation of
the wavy path. Since no method has yet been devised that permits of the
observation of the path of the ameba other than a succession of camera
lucida outlines, it is impossible to omit light altogether in the
experiments. The next best procedure was therefore followed, viz., the
alternation of periods of darkness of a few minutes’ duration with
brief--ten-second--periods of light, to permit the drawing of camera
lucida outlines. The dish in which the amebas were observed was placed
in a light-tight box and all light excluded except that which passed
through “Daylite” glass with an opal surface on both sides between the
condenser and the light source. None but parallel beams, passed through
a condenser, reached the ameba. The metal parts of the objective were
also blackened. The work was done in a dark room.
Figures 34 and 35 show sections of the path of two _Amoeba discoides_
under these experimental conditions. The amebas were for the most part
in clavate shape, which is the most favorable shape for the formation of
smooth waves. In figure 34, from 2:29¼ to 2:42¼ the ameba was in
continuous light. A section of a little more than one wave is shown.
Although pseudopods were thrown out at considerable distances to the
right and left of the path, a smooth, wavy path was nevertheless
maintained. At 2:42½ the light was turned off until 2:58½ except for two
ten-second flashes at 2:47 and 2:52. During the first period of
[Illustration: Figure 34. The path of an _Amoeba discoides_ under light
controlled conditions. At 2:42¼ the light was turned off until 2:52,
except for a ten-second flash at 2:47. The smoothness of the wavy path
was thus maintained in complete darkness. Length of the ameba, about 400
microns.]
[Illustration: Figure 35. Path of an _Amoeba discoides_ showing that
continuous light is unnecessary for the maintenance of a wavy path. The
ameba moved under light controlled conditions from 3:02¼ to 3:13¼. From
then until 3:43 the light was turned off except for 10-second flashes at
3:18, 3:22½, 3:24, and so on. The ameba had probably come to rest for
some reason between 3:24 and 3:30½, for an unexpectedly small amount of
space was covered in that time. In spite of this disturbance, however,
the evidence indicates that light is without causal effect in the wavy
path of the ameba. Length of the ameba, 450 microns.]
darkness the ameba merely kept on in the direction it was going when the
light was turned off. But during the second period of darkness the ameba
changed its course in such a way as to make a smooth curve. In the third
period of darkness the ameba continued on its course completing the
wave. It is thus apparent that continuous light is not necessary to the
formation of waves nor is it detrimental to their formation. Figure 35
shows essentially the same thing as Figure 34. The light was turned on
from 3:30½ to 3:32. During this time the behavior of the ameba was
irregular, but whether this was caused by the light or not, cannot be
stated. At 3:43 the ameba came into contact with a small particle which
changed its course. The slow speed of movement of these two amebas was
due to the low temperature (20° C.), the experiments being performed in
January. The apparent connection between longer waves and darkness has
not yet been investigated.
Figure 36 shows the path of an _Amoeba proteus_ under controlled light
conditions as above described, but instead of moving over a polished
plate glass surface as in the previous experiments, the ameba in this
experiment moved over a fine ground-glass surface. It will be observed
that for the first twenty minutes the path shows smooth waves, although
at 11:53 and 11:55 there was a slight disturbance which was associated
with the formation of numerous pseudopods. From 12:07 on, however, the
path becomes irregular, the wave-like character being almost
obliterated. Associated with this irregularity is the presence of
numerous pseudopods. This is a sample of a number of records which
indicates that in _proteus_ and _discoides_ the presence of numerous
pseudopods in some way prevents the ameba from moving in a path marked
by smooth and conspicuous waves.
[Illustration: Figure 36. Showing how smooth waves in the path of an
_Amoeba proteus_ in clavate shape become disturbed by the projection of
prominent pseudopods. Although there was considerable disturbance due to
the formation of pseudopods at 11:53 the conformation of the wave was
not changed until the stage preceding the one at 12:07, when the
formation of numerous pseudopods resulted in an irregular path. Length
of the ameba, about 600 microns.]
When a wave in the path for some reason becomes unusually long, there is
likely to be a very abrupt and decided change in the direction of
movement, which is away from the _convex_ side of the wave. Figure 37 is
inserted here to illustrate this point. The ameba should have turned to
the left at 3:43 to keep the waves of typical size, and at 3:45 a
pseudopod was extended in this direction a short distance, but again the
curve toward the right persisted. But at 3:48½ the ameba broke up into
several pseudopods at right angles with the main axis, and through one
[Illustration: Figure 37. An illustration showing that sudden changes
from the expected direction of the wavy path are centrifugal, not
centripetal; that is, away from the focus of the wave, not toward it.
The tendency to break away from the first smooth wave became apparent at
3:45, as indicated by the extension of pseudopods near the anterior end.
In the stage about two minutes later, a number of small pseudopods were
thrown out in various directions. At 3:48½ several large pseudopods were
thrown out near the anterior end almost at right angles to the main
axis, instead of at an angle of about sixty degrees or less, as is the
usual case. The same thing occurred at 3:57, except that the angle of
the pseudopod was not so large.]
of these the ameba moved on with the reestablishment of the wavy path.
The tendency to wave formation evidently has to overcome resistance of
some sort.
[Illustration: Figure 38. _Amoeba dubia_ usually moves with numerous
large pseudopods, but this illustration shows that there is very good
reason for concluding that there is a tendency in this ameba to move in
wavy paths. Length of the ameba, 400 microns.]
Although amebas in clavate shapes describe the smoothest waves in their
paths, waves may also be detected in the paths of amebas that habitually
form many pseudopods. The path of an _Amoeba dubia_ is shown in Figure
38. The ameba moved on an opal surface under light-controlled
conditions. If we had not already seen how _proteus_, _discoides_, and
especially _bigemma_ formed smooth waves in their paths, we should
hardly be able to understand the apparently aimless path of _dubia_. But
having seen how a regular succession of smooth waves appears under
favorable conditions in the paths of these amebas, there can be little
question but that the staggering path of a _dubia_ also is to be
interpreted as a succession of waves, although they are somewhat
irregular.
These four species of amebas, _proteus_, _dubia_, _discoides_ and
_bigemma_ are the only species that have been specially investigated as
to their paths, and they all show such paths, as we have seen. The
presumption is strong therefore that it is a common characteristic of
amebas.
To learn something of the nature of the wave-forming mechanism in the
ameba, it is necessary to find some agencies that modify the activity of
this mechanism. That there are such factors is of course evident enough
from what has been said already about wavy paths, and from the
appearance of the paths themselves. But the factors which influence the
formation of waves in so far as they may be known or reasonably
suspected, are internal and therefore difficult to make use of
experimentally.
One of the most readily applied stimuli that is known to affect the
character of ameboid movement is temperature. In general, the lower the
temperature, the slower the movement. This has frequently been observed
and recorded. Such behavior is to be expected from a viscous fluid like
protoplasm. This may therefore be a purely physical phenomenon. But the
lowering of the temperature has also another effect on the movement of
amebas: it creates in them a tendency to cross their paths more
frequently. Figure 33 is a typical example of the path of an ameba in a
high temperature (28° C.). It did not cross its path at all during the
hour and a half it was under observation. When the temperature is low
(20° C.) the path becomes contracted and the ameba seems unable to get
away from the place it happens to be in. Movement of course continues
but it is slower, and a large number of loops occur in the path. Figure
39 indicates the general path of an ameba under controlled conditions in
a temperature a little lower than room temperature, that is, about 20°
C. During the four hours that it was under observation the ameba crossed
its path eight times and made a number of very short turns besides.
Leaving out of account the loops in the path there are a number of
sections which may be interpreted as waves, such as for example the
pronounced waves a short distance from the end of the path. All these
waves are shorter but much deeper than the waves made in a higher
temperature. The loops in the path (all excepting the first, which is a
compound loop) represent each a single wave which have become so deep
and contracted that they have become transformed into circles. As the
temperature decreases, the crests of the waves rise higher and higher,
and the bases contract more and more, until the two sides of the waves
come together, resulting in the formation of circles (Figure 40). The
actual size of the wave also decreases at the same time from about eight
times the length until it is only two or three times the length of the
ameba. Temperature affects therefore the wave mechanism independently of
the mere viscosity of the endoplasm. The speed of movement is not merely
slowed down, but the character of the waves themselves is changed.
[Illustration: Figure 39. The path of an _Amoeba dubia_ in comparatively
low temperature (18° C.). The large number of loops and deep waves in
the path are due to the low temperature. The experiment was performed
under light controlled conditions. Length of the ameba, 350 microns.]
Amebas sometimes react to stimuli by moving around the source of the
stimulus at a more or less uniform distance through one or more
quadrants of a circle, instead of reacting positively, negatively, or
indifferently, in a definite manner, to the source of the stimulus
(Schaeffer ’16, ’17). See Figure 41. The explanation that has been given
by this investigator is that the encircling is due to a balance between
the tendency to move ahead in the original direction (“Functional
inertia”) and a tendency to react positively. But now that we know that
amebas tend to form waves in their paths, the explanation of encircling
becomes simpler and perhaps also more convincing.
[Illustration: Figure 40. Diagram illustrating how the deepening of the
waves in the path of an ameba due to decreased temperature may lead to
loops in the path. The heavy line represents the wavy path, and the
light lines with the arrows indicate the direction taken when loops are
formed. The point in the path where the direction is most easily changed
is where one wave grades off into the next, as indicated by the letter
_a_.]
In the first place, instances of encircling are relatively rare in the
reaction of amebas, much rarer than one would expect if it depended
merely upon a balance between two tendencies, one to move ahead and the
other to move toward the source of the stimulus. Any explanation of this
phenomenon has therefore to account for the rarity with which it occurs
as well as the operation of the phenomenon itself. This the explanation
based upon the position of the source of the stimulus with reference to
the configuration of the wavy path can do satisfactorily.
In the experiments with temperature it was found that when the
temperature is 20° C. or lower, the waves tend to curl up, to become
transformed into circles. That is, the base of one
[Illustration: Figure 41. Showing the phenomenon of encircling, after
Schaeffer. The ameba moved around a perpendicular beam of red light. The
reaction was neither distinctly positive nor negative.]
wave, instead of running into the base of the next wave is reflected
backwards to form a circular curve. All the evidence thus indicates that
the weakest point is at the base of the wave. A constantly acting
stimulus may therefore break the wave here if it cannot break into it
elsewhere, and so change the direction of the path. In Figure 42 are
shown a few diagramatic waves in the path of an ameba together with
several reflected curves at 1, 2 and 3 indicating the points at which
the direction is most easily changed as evidenced by the temperature
experiments. If a particle within sensing range of the ameba lie at _a_,
_b_, or _c_, and stimulate the ameba only slightly but still enough to
break up the wave formation, the ameba will take a curved path around
the particle as indicated by the dotted lines. But if the same particle
lay at any other point with reference to the position of the wave, as at
_e_, _f_, or _g_, the ameba would not have changed its course. Briefly,
the following conditions must be satisfied to enable the phenomenon of
encircling to appear: (1) The particle must lie a little to the side of
the ameba’s path. (2) It must lie abreast of the point at which the
ameba begins to change its direction of movement (i. e., at the base of
the wave) when describing
[Illustration: Figure 42. Diagram illustrating the relation of the
phenomenon of encircling to the wavy path of the ameba. The weakest
point in the path, i. e., where the wave may be broken most easily, is
where one wave merges into another, as indicated by the experiments with
low temperature, _a_, _b_, _c_. The direction of movement of the ameba
is the reverse of what it would have been had there been no stimulus
producing encircling. The same stimulus at _e_, _f_, or _g_ would not
produce encircling because it is more difficult for the ameba to move
away from the concave side of the wave.]
waves. (3) It must stimulate the ameba just strong enough positively to
break into the wave-forming process. Encircling then is due to a
“balance” between a positive stimulus and a tendency to move in a curve.
This explanation conforms with all the data at hand and explains also
the rarity of the phenomenon, for the chances of encircling occurring on
this view are rather less than one-fifth as frequent as if encircling
took place whenever a balance between a tendency to react positively and
to move straight ahead occurred.
That the wavy path is broken up by the receipt of a stimulus, that is,
by a true sensation, rather than by direct effect of some agency
radiating from the particle, is indicated by the fact that stimuli
proceeding from various substances, such as keratin, glass, carbon,
light beams, etc., have all the same effect.
In attempting to explain the characteristic nature of the path of the
ameba, one’s attention centers first, perhaps, upon its orderliness; a
result undoubtedly of the general impression propagated through hastily
written textbooks and general papers, that an ameba’s whole life is a
direct response to its environment. As the recorded facts of the life of
this organism are accumulating, it is coming to be seen that the ameba
possesses all the fundamental attributes of animals generally, in
addition to many special ones. So that as a matter of fact, if the ameba
did not show some character and orderliness in its locomotion, then for
the first time should we be especially interested in what would have to
be regarded as a very striking and exceptional characteristic.
For it is very well known, and it is generally recognized by everybody,
that moving organisms usually move in an orderly manner; it is
recognized that organisms tend to move in straight paths excepting where
interrupted by the action of some special stimulus. When an organism
changes its direction of motion frequently and abruptly, we call it
erratic. The mad dashings-about of the hunter-cilitae _Didinium_ and the
unceasing gyrations of the whirligig beetle excite one’s curiosity
because these organisms do not move as other organisms do; they
contradict our expectation of movement in a straight line.
But why should organisms generally tend to move in straight paths? This
fundamentally important question has received almost no attention,
excepting that rapidly moving animals like birds, flying insects, fishes
and other rapidly swimming animals of various kinds and rapidly running
animals tend to move in straight paths because of the physical inertia
of the mass of the organism. It is easier for a rapidly moving organism
to move in a straight line than to change its direction of movement
frequently and abruptly.
The ameba however is a very slow moving animal, as animals go, for it
(_proteus_) moves only about 600 microns per minute. Under the
microscope, however, which magnifies speed as well as size, the
endoplasmic particles rush along rapidly enough to suggest that even
here mere physical inertia might be a determining factor in the path of
the ameba, which for considerable segments is often very nearly
straight. Such suspicion is not justified, however, for the viscosity of
the endoplasm taken in connection with the heterogeneous composition of
the ameba, makes it improbable that mere physical inertia can affect the
path of the ameba.[5]
It is not even necessary that movement of the endoplasmic stream be
interrupted in order that a straight path may be maintained. An ameba
may stop movement for a minute or more and then be much more apt to
resume movement in the original direction than in any other. This is
brought out by the following series of experiments.
Of sixty cases of feeding on various kinds of particles, by as many
different amebas, in which the direction of movement before and after a
particle was eaten was recorded, thirty-nine moved off in the same
direction after eating as before eating. By moving off in the same
direction is meant that the ameba did not move more than 22½° to the
right or to the left of the direction of movement before feeding. The
circle was thus divided into octants, and the expectation of movement in
the same direction after eating a particle, if it were a matter of
chance, would have been seven and one-half cases instead of thirty-nine.
But it is not only the process of feeding that has to be considered in
this connection, for feeding occasionally is affected by a side
pseudopod while the main body of the ameba moves on without being
visibly affected as to its direction of movement. No such case is
included in the figures just given. In each of these sixty cases the
endoplasmic streams of locomotion were completely stopped, from about
twenty seconds to seventeen minutes. In most cases the endoplasmic
stream was also completely disorganized, the ameba assuming a nearly
spherical form in which more or less well marked though small cross
currents of endoplasm could be detected. The direction of the light was
without effect, for the paths extended in every direction with respect
to the light both before and after feeding. Further, it has been shown
that ordinary diffuse light is without effect on the movements of the
ameba (Schaeffer, ’17). It may be concluded therefore that the ameba
tends to keep on moving in straight paths even if the highly
disorganizing act of feeding and the consequent resting period of a few
seconds to many minutes supervenes at some point in its path. To what
this induction of the original path is due is not clear, thought it is
possible that the physical condition of the ectoplasm at the anterior
end is different from that elsewhere and that it requires less energy in
consequence, or for some other reason, to flow in the original
direction. This explanation is based on the observation that it is
easier for the ameba to activate the remnants of old pseudopods than to
form new ones (Schaeffer, ’17).
CHAPTER XIII
THE WAVY PATH OF THE AMEBA AND THE SPIRAL PATHS OF CILIATES AND OTHER
ORGANISMS
The most interesting feature of the path of the ameba is of course the
waves. The path of an ameba closely resembles the projection of a
helical spiral on a plane surface, and this at once calls to mind the
spiral swimming of flagellates, ciliates, rotifers, larvae or various
groups of animals, swarm spores and zoöspores of various algae and
fungi. But before we take up the general subject of spiral movement, it
will be worth while to see what other evidence there is beside the wavy
path, that indicates that the “spiral urge” is present in the ameba.
It is well known that in a number of the small amebas, especially the
soil amebas, there are two trophic stages, an ameboid stage and a free
swimming flagellate stage. The change from one stage to the other is a
matter of a few minutes only. In the flagellate stage (Figure 43) the
amebas resemble a small flagellate like chilomonas, very closely. Their
manner of swimming is very similar. And it is especially noteworthy in
this respect that they revolve on their long axis and describe a well
marked, regular spiral path, just as do the flagellates and ciliates.
Unfortunately no records have yet been made of the paths these amebas
describe when in the true ameboid stage. Since, therefore, as we shall
see later, the slightly unsymmetrical shape of the flagellate stage is
not the cause of the spiral path, it is probable that the mechanism
controlling the activity of the flagellum can produce orderly locomotion
only when the organism follows a spiral path.
Much has been written about the fundamental similarity or identity
between flagella and pseudopods. All writers who have expressed
themselves on this point incline to think that there is such similarity,
that flagella are really very slender and very agile pseudopods. I am
not going to record here the evidence for this conclusion, for I have
recently had the good fortune to make some very convincing observations
on a hitherto undescribed ameba
[Illustration: Figure 43. The flagellate stage of a soil ameba, after
Wilson. _a_, stained preparation showing the two flagella arising from
the blepharoplast, _d_, which is connected with the caryosome, _c_, the
central chromatin mass. Much of the chromatin is deposited on the
nuclear membrane. _b_, a drawing from a live flagellate showing
flagella, nucleus, _c_, and a vacuole.]
(which for the sake of reference will here be called _flagellipodia_,
Figure 44) whose pseudopodia stand about midway between typical flagella
and typical pseudopods in their activity. In its general characteristics
it stands near _A. radiosa_, but quite unlike the stiff, static
pseudopods which _radiosa_ very frequently forms, this ameba has usually
five or more slender pseudopods of which one or two or more are in slow
flagellate motion. The distal third or half of the pseudopod is in the
shape of a corkscrew. The free end of the pseudopod travels around in a
circle (anti-clockwise in all instances observed), making one revolution
in about three seconds. If this motion were very rapid it would act like
a propeller and the ameba would swim through the water. The part of the
pseudopod back of the mobile portion is usually also thrown into a
spiral of gradually decreasing diameter until the spirality disappears.
This portion of the pseudopod is not mobile in the same way that the
distal portion is. Sometimes the whole of a pseudopod is thrown into a
spiral, all of the turns being of equal size and only slightly motile.
More than half of all the pseudopods formed become spiralized at one
time or another of their existence, the greater number of these being
however relatively immotile. Pseudopods frequently fall into spirals
while they are being extended.
[Illustration: Figure 44. _Amoeba flagellipodia._ _a_, showing nucleus,
4 microns in diameter, and four vacuoles. _b_, a pseudopod of three
spiral turns which in a few seconds grew into one of six spiral turns,
_c_. _d_, a pseudopod of a number of spiral turns, which a few seconds
later took on a shape shown at _e_. The tip of the pseudopod at _f_
turned screw-like anti-clockwise, when looking at the tip and at the
main body of the ameba. The tip made one complete revolution in about
three seconds.]
A better transition form between pseudopods and such flagella as are
found, for example, in the peranemas, could hardly be imagined. The
difference between crawling and swimming would seem to be merely a
matter of speed of movement of the pseudopod.[6] But important as such a
transition form is for theoretical purposes in understanding the nature
of both flagella and pseudopods, it is of special importance for our
present purpose because it shows a strong tendency for pseudopods to
fall into spirals and to move in spirals. This tendency is found not
only in this species of ameba but is observed also occasionally in
_radiosa_ (Figure 7, p. 30) and in several other species. In these
latter species the pseudopods are stiff and not capable of waving about
in the water, as are those of _flagellipodia_, whether in the spiral
shape or not. In _radiosa_ the pseudopods may become spiralized only as
a preliminary to withdrawal. It is evident therefore that the spiral
urge can express itself best in a plastic pseudopod.
Taking all these observations together, the tendency of pseudopods to
move in a spiral manner, the tendency of the ameba as a whole to move in
a spiral path when in the flagellate stage, and the wavy path of amebas
which is smoothest when in the clavate stage, all these observations
seem to confirm the supposition that the wavy path is in reality a
flattened spiral, and that the spiral urge in ameba is a very
fundamental factor in the process of locomotion. In other words, there
is present in ameba an automatic regulating mechanism controlling the
direction of movement so that when free from stimulation a spiral path
is followed.
Where can such a mechanism be located? In organisms of fixed form, such
as vertebrates, the mechanism controlling and coördinating locomotion is
in the central nervous system. Even in some protozoa (_Euplotes_) a
motorium has been found whose function apparently is that of
coördinating the action of at least some of the motile organs (Sharp,
’13, Yocom, ’18). But in ameba there is no fixed form. The ameba is
continually mixing itself up. No two masses of protoplasm ever occupy
the same space relations to each other for more than a moment, excepting
perhaps within the nucleus. But the nucleus as a whole is continually
changing its position with regard to the rest of the ameba, and almost
certainly its position at any given moment in the ameba is the result,
not of its own activity, but of the endoplasm and the ectoplasm. A
formed nucleus, moreover, is not necessary to concerted movement, for
_Protamoeba_, in which no granules of chromatin have been found, and
there certainly is no formed nucleus present, moves in a concerted
manner, though I am unable to state definitely whether it moves in a
wavy path. (I have seen this organism only a few times, and on none of
these occasions was I able to make the test). It seems therefore
possible that the agency responsible for the movement of amebas in
flattened spiral paths can be located at any particular point within the
ameba. It seems more likely that this mechanism is a spatial aspect of
the intimate colloidal activity occurring in such changes of phase as
are associated with the phenomenon of contractility and streaming.
Seeing then that movement in spiral paths is possible in animals not
possessed of fixed morphology, it becomes of great interest to see
whether the spiral paths of free swimming ciliates, flagellates, etc.,
are similar to those observed in amebas.
Although the spiral paths of flagellates and swarm spores were first
studied by Naegli in 1860, and subsequently discussed by numerous
botanists and zoölogists, it was not until Jennings in a number of
papers (’98-’04) on the spiral paths of numerous species of one-celled
organisms and rotifers, described the essential facts underlying spiral
movement, that the significance of this method of locomotion began to be
realized. His work marked the beginning of a healthy reaction against
the conception of ridiculous simplicity of structure and function which
had for several decades been settling upon these organisms. He showed
that the spiral path is not a purposeless, senseless reaction on the
part of these small organisms, but that it is fraught with meaning, and
that it may be regarded as one of the most important of their many
activities.
In a paper “On the significance of the spiral swimming of organisms”
Jennings (’01) develops the thesis that spiral swimming is an acquired
habit, an adaptation which has become fixed in these organisms so that
they would not be condemned to swim in circles, which would necessarily
follow from their asymmetrical form. The organism, in other words, swims
in a spiral in order to be able to swim in a generally straight course.
This explanation involves of course the supposition that the
unsymmetrical shape of the body was developed first, and then, since
this led to circular paths, revolution on the long axis became necessary
in order that a straight course might be maintained.
But in the explanation of body form in one of the rotifers he (l. c., p.
376) says: “In some of these primitively bilateral animals this spiral
method of swimming has resulted in the production of an unsymmetrical
form analogous to that of the infusoria.”
It is of course quite possible theoretically, that some of the
unsymmetrical structures on an organism that habitually swims in
spirals, are the _result_ of its spiral swimming, and that other
structures which go to make the organism unsymmetrical, are the cause of
the spiral swimming. This hypothesis is not an attractive one, however,
for, because of the endless variety of asymmetrical differentiation in
spiral swimming organisms, it would be impossible to tell for the large
majority of organs or organelles whether they were the cause or the
effect of spiral swimming.
Before taking up the hypothesis that _all moving organisms are subject
to the tendency to move in spiral paths_, a hypothesis which accords
with all the known pertinent facts, it may be well to examine the thesis
that rotation on the long axis is an adaptation which has been developed
to compensate for the effect of an unsymmetrical shape of the body.
It will be noted first that this question cannot be decided by direct
observation or experiment. The entire body of real evidence is written
in phylogeny, and that is for this purpose a closed book. It is only the
interpretations of observations that bear on this problem, and it is
these interpretations that it is of interest to examine.
Referring now only to the ciliates, all of which have numerous motile
organs, it has been observed by numerous writers that cilia are not
confined to one or two methods of contraction, but that there is great
latitude in the extent and direction of their activity. This is very
well illustrated by a paramecium or a stentor whose ciliary systems
enable these animals to execute a great variety of maneuvers depending
upon the character of stimulation, the amount of food in the body, etc.
(Jennings, ’06, Schaeffer, ’10). The cilia are under the control of the
animal in the same way as the legs and arms of a man are under his
control. Now supposing that the bodies of these organisms became
unsymmetrical during the phylogenetic history and as a result became
unable to continue to swim in a straight path, the pertinent question to
ask is: Was it easier for these organisms to learn to revolve on their
long axis than to learn to beat their cilia a little harder on the side
toward which they swerved? Observation of the forms before us does not
afford any evidence that rotation was the easiest solution. Moreover, if
it was an acquired habit, is it not strange that it should have been
easier to acquire the rotating habit for every single species of the six
or seven thousand unicellulars which now obey the spiral urge, as well
as the swarm spores and zoöspores, than to change the beat of the cilia
in some other way, in at least a few species? This explanation also
makes inevitable the assumption that the ancestors of our present
unsymmetrical protozoans were symmetrical and swam in straight courses
without revolving, a condition of affairs which contrasts strongly with
present conditions, for none of the most nearly symmetrical unicellulars
and swarm spores now swims without revolving on the long axis. It is
therefore exceedingly improbable that spiral swimming is the result of
an acquired habit.
Now what evidence is there in support of the hypothesis that the spiral
path is a necessary accompaniment of locomotion, except as it may be
broken by the effect of stimulation?
As a problem in engineering, it is clear that the shape of the body is
not responsible for the spiral course, for almost every conceivable
shape is met with in organisms swimming in spiral paths. The frequent
spiral turns in the path of stylonychia cannot be the result of the
shape of the body, which is almost, if not quite, as well adapted for
swimming through the water as is that of a euglena or a fish, but for
revolution on its long axis it is not nearly so well adapted. Moreover,
some of the euglenas turn the ventral or smaller lip out in the spiral
turns, while others turn the dorsal or larger lip out (Mast, ’10). Since
there is no other asymmetry of shape in these euglenas, it is clear that
the shape of the body has nothing to do with causing the spiral path.
The immediate cause of spirality must therefore be the work of the
motile organ, and not the shape of the body.
Similar observations on paramecium have shown that it is the special
action of the cilia of a paramecium that causes it to rotate and not the
shape of the body. Again the shape of a _Stentor caeruleus_ is subject
to very great variation due to varying amounts of food eaten, and to
surgical operation, but a spiral path is nevertheless maintained while
the body shape undergoes marked changes.
Although all free-swimming unicellular organisms revolve on their long
(antero-posterior) axis, an occasional one does not move in spirals.
This is observed in the large colonial flagellate _Volvox_ occasionally,
but not always (Mast, ’10). Since it is more frequently seen in the
larger individuals, it is probable that the formation of spirals is
prevented because of the increased physical inertia of the colony; for
the older and larger colonies are much more unsymmetrical than the
younger and smaller, owing to the unequal distribution of the
reproductive elements. _Spondylomorum_ and several other colonial forms
describe smaller spirals than smaller solitary organisms. These colonial
organisms consisting of from four to twenty thousand cells, each of
which may be possessed of cilia, are marvels of locomotory coördination,
but it is not at all clear how this coördination is brought about. Since
the colonies are symmetrical however, the spirality of the path is
clearly due to the special action of the cilia.
Some organisms possess body shapes that seem to be due to the habit of
spiral swimming. Jennings (’01) describes a species of rotifer whose
body forms a segment of a spiral. When swimming a spiral path is
described, “of which its own twisted body forms a part” (p. 376).
Elsewhere he has pointed out that the oral groove of a paramecium
likewise coincides with its own spiral path. Indications of such
correspondence between the axis of a structure and the spiral path the
organism possessing it, describes, are numerous among free swimming
animals. But such correspondence (with an imaginary spiral path) is also
found in organisms that do not swim freely. One of the most interesting
of such cases is found in the _Oscillatoriaceae_. In a previous chapter
it was seen that many of these organisms are capable of moving about by
means of a film of what is probably protoplasm, which moves spirally
around the filament. A particle attached to this film describes a spiral
path like that of a flagellate or a ciliate. Most of the
_Oscillatoriaceae_ that are capable of movement, consist of straight
filaments; but two of the genera, _Arthrospira_ and _Spirulina_, are
spirally twisted in such a way that the spiral axis of the filament
corresponds approximately to the spiral path of a particle attached to
the surface film of an _Oscillatoria_ filament, except, of course, in
size. (The movement of the surface film of neither _Arthrospira_ nor
_Spirulina_ has been studied).
That the spiral shape of a rotifer, for example, may be caused by
swimming in a spiral path might perhaps be regarded as a plausible
explanation, but it seems to me that it would be more satisfactory to
explain the spiral shape of rotifers and _Arthrospira_, the direction of
the oral groove of paramecium and similar structures in other organisms,
as due to the same fundamental process that causes the spiral path in
locomotion. This explanation is purely mechanistic and avoids the
teleological element on which the other explanation ultimately depends.
Most of the asymmetrical shapes of the flagellates, ciliates, rotifers,
etc., have originated in phylogeny without regard to swimming in spiral
paths, and indeed in spite of it. In spindle-shaped organisms like
euglena or paramecium the amount of energy required to revolve on the
long axis, as compared with that required for forward movement, is
small. But in stylonychia, a dorso-ventrally flattened ciliate, much
more energy is required to revolve the animal, proportionally, than is
needed for forward movement. It is of course perfectly evident that as a
problem in engineering it requires much more energy to revolve a flat
plate on its long axis than a spindle-shaped solid, in a dense medium
like water. But in spite of all the obstacles to revolution which
asymmetry of body form presents, none of them are serious enough to
prevent revolution from occurring, unless the keeled rotifer _Euchlanis_
(Jennings, ’01) presents such a case. Observation would lead one to
believe, however, that the compressed body forms of some of the
hypotrichans and some of the flagellates such as phacus, have made
revolution on the long axis very difficult; but not difficult enough to
destroy the tendency to revolve and describe spirals. In short, these
organisms spiralize in spite of asymmetry, not because of it.
A simple but decisive experiment by Jennings (’06) showed that the
revolution and the forward movement of a paramecium is due to the
oblique stroke of the cilia, for the severed posterior portion of a
paramecium, which is symmetrical, nevertheless still revolves during
progression. The question now arises whether this oblique stroke is
analyzable into components in another way than by local stimulation; for
example, can one increase or decrease the amount of revolution faster
than the amount of progression? Observation of paramecium and euglena in
different temperatures answers this question affirmatively. Organisms
from the same culture were subjected to two temperatures, the culture
temperature of 21° C. and 8° C. At temperatures lower than 8° C. the
paramecia quickly precipitated to the bottom of the dish.
In 21° C. paramecia revolve once while swimming 5.5 body lengths.
In 21° C. euglenas revolve once while swimming 4.2 body lengths.
In 8° C. paramecia revolve once while swimming 3.6 body lengths.
In 8° C. euglenas revolve once while swimming ¼ to 2 body lengths.
The effect of decreased temperature is therefore to retard forward
movement and to increase proportionally the number of spiral turns, for
a revolution of the body on the long axis is the equivalent of one turn
in the spiral path. It will be recalled that a similar result was
obtained with amebas; in the lower temperature the rate of forward
movement was reduced and the tendency to deepen the waves increased. In
both these classes of organisms, differences in temperature enable one
to separate the forward movement component from the spiral component, in
the same way and in general to the same extent.
In clear water of optimum temperature or somewhere near it, paramecia
and euglena (_Euglena gracilis_, which does not readily react to light)
often swim for long stretches without change of direction. When the
temperature is lowered, however, the stretches of straight paths become
much shorter. In a temperature of 8° C. changes of direction become very
frequent. In paramecium some of these changes are probably due to shock
of some sort, judging from mere appearance; but in many cases the change
of direction is preceded by a slowing up of forward movement and the
swinging of the anterior end in a wide circle one or more times around.
Occasionally one observes slow forward movement with wide swinging of
the anterior end, for considerable distances. In euglena this condition
is more marked than in paramecium; frequently the anterior end spins
around with the posterior end as a pivot for several minutes at a time,
in low temperatures.
These observations are strikingly analogous to the circles formed in the
paths of amebas in low temperatures, and geometrically they bear the
same relation to the spiral paths of ciliates and flagellates as the
circles do to the wavy path of the ameba.
Besides the effect of temperature on paramecium and euglena, effects
which are continuous and automatic, it is of course well known that the
spiral path may be readily broken into by appropriate stimulation of the
sense organs. The automatic locomotory mechanism is then for the time
being controlled with reference to the character of the stimulus and the
experience of the organism. But as soon as the effect of the stimulus
has disappeared, the automatic mechanism again controls locomotion.
Sense organs of orientation, including organs of equilibration, break in
upon the spiral mechanism controlling direction of movement, and
eliminate its effect. It thus happens that no animals with image-forming
eyes or equilibrating organs move in spirals in three-dimensional space
when these organs are functional. Conversely, animals without
image-forming eyes or equilibrating organs move in spiral paths. In
addition to the ciliates, flagellates, protophyta, swarm spores and
zoöspores of algae and fungi, Oscillatoriaceae and rotifers, may also be
mentioned the larvae of many worms, echinoderms and molluscs. All these
are within the grip of the spiral urge. The grip is indeed slight, as we
have seen, but in the absence of stimulation it is none the less
absolute.
The movements of none of the animals in the higher groups have been
studied in any detail. Excepting the movements of some of the ciliates,
flagellates, amebas, rotifers, a few scattered protophyta and swarm
spores our knowledge of the movements of spiral swimming organisms is
of the most casual and fragmentary sort. Nothing beyond the mere fact
that these organisms describe some kind of a spiral swimming, is known.
[Illustration: Figure 45. Showing the path walked by a normal
right-handed man (J. N.), blindfolded and counting his paces. The whole
path was 546 paces long. The command given was to walk in the direction
of the arrow until halted. The field was slightly rolling. The stump
made necessary a termination of the experiment.]
That a spiralizing mechanism is probably also present in organisms with
highly developed equilibrating and orienting senses would be the logical
expectation from what has been said regarding the presence of such a
mechanism in the lower forms of life; but the effect of such a mechanism
would naturally be suppressed when the orienting senses are functioning.
To test this point, man was selected for experiment. With eyes
blindfolded and ears plugged (this latter precaution was subsequently
found to be unnecessary) so as to render the orienting senses
ineffective, a normal man was directed to walk straight ahead over a
large field towards an object he had just looked at. Although a number
of experiments were made with several individuals, _none of them was
able to walk a straight path. All of them walked true spirals or series
of circles with remarkably smooth curves_ (Figures 45, 46). The spirals
were right and left handed in the same individual, and sometimes in the
same experiment. In these experiments the subject was totally
unconscious of the direction in which he was walking. No effort of
consciousness seemed capable of changing the _degree of curvature_ of
the spiral or circle and keep it smooth, though one could of course at
any time break into the spiral or circle and walk off in another
direction. (The writer himself walked in several experiments.) If one
has one’s mind _strongly_ on the direction of walking, thinking of each
step, the curve of the path shows small “wabbles”; but if one recites
something or counts his paces, the curves are quite smooth.
[Illustration: Figure 46. Illustrating a path walked by a normal
right-handed man (J. D.), blindfolded and counting his paces. The path
was 560 paces long and was walked over the same field as the path
illustrated in figure 45. The path had to be terminated because of a
clump of brush.]
Considerable unevenness of the ground has no effect on the curvature of
the spiral. Structural differences in the legs are also without effect,
for a person with one artificial leg walks quite as smooth a spiral as
one with two normal limbs.[7]
From these experiments on man, it follows that there is a “centre” in
the central nervous system which automatically coördinates and controls
movement during locomotion and, particularly from the point of view of
this discussion, the direction of locomotion when the orienting senses
are not functioning. This center must be very deep seated and automatic,
and in so far as its influencing the direction of locomotion is
concerned, it is of no discoverable use to man. It may be presumed to
have existed before the present orienting senses originated in man, for
there is very good evidence that horses and perhaps dogs, too, possess
this mechanism. For these animals, like man, tend to walk in circles
when lost, a peculiarity of behavior undoubtedly due to the activity of
this mechanism and not to stronger right or left legs, etc., as has
often been suggested (e. g., Thompson, ’17, p. 498). According to the
accounts of experienced hunters, rabbits also run in circles when hard
pressed by hounds, which may possibly be due to the suppression of the
functioning of the orienting senses by fear, thus allowing the automatic
directing mechanism to operate.
The facts are therefore that all organisms without orienting senses or
equilibrating organs, or animals possessing such organs which are
rendered ineffective by some means, will not move in straight paths nor
in any kind of irregular path, but _in orderly paths_, so that a given
segment of the path serves as a basis for predicting the further
direction of the path. And the degree of accuracy to which such
prediction may attain is proportional to the extent to which the
activity of the automatic regulating mechanism may be kept free from
outside interference. The organisms of which this holds true include,
as far as known, all the free-swimming unicellulars, swarm spores of
algae and fungi, uni-and multinucleate zoöspores, rotifers, a large
number of worms and worm larvae of all classes (excepting the nematodes)
and the larvae of many molluscs, echinoderms and copepods as well as
some adult copepods. Organisms restricted to two dimensions of space in
their movements, in which orderly paths have been recorded, are ameba
and man and perhaps we may include the horse and the dog. This is indeed
only a small number of organisms compared with all that can move; but
there are representatives in the list of all the large groups excepting
the higher plants, and without doubt observation will greatly extend the
list, for there are mentioned here only such organisms whose movements
have been definitely recorded or personally observed. As far as now
known, no organism lacking orienting organs moves in a straight line.
Many spermatozoa with flagellate tails seem, however, to do so, but no
careful studies of their paths have yet been made.[8]
The orderliness of the paths of these organisms when moving under such
conditions as described above, is itself orderly; that is, the path of
all these organisms is a spiral of one kind or another: (1) a helical
spiral, as in the free-swimming unicellulars; (2) a true spiral in one
plane, as in man; (3) a helical spiral projected on a plane surface, as
in ameba.
These facts point inevitably to the hypothesis that the movements of
these and all other moving organisms are controlled by an automatic
regulating mechanism, which is of essentially similar nature in all
organisms, as is indicated by the tendency to spiralize the path. This
mechanism, being automatic, absolutely controls the direction of the
path so long as outside interferences permit; but when sensory
stimulation occurs, or when changes in temperature, etc. occur, the
mechanism is no longer able to operate automatically or smoothly. The
direction of the path then depends upon the nature and direction from
which stimulation was received, and upon the degree and direction of
change of temperature, etc.
The importance of this conception of movement lies in the fact that it
enables us to look at a large mass of otherwise unrelated data from a
single point of view. Secondly, it permits of a mathematical treatment
of the whole subject of movement in organisms. And third, it replaces a
teleological explanation of spiral movement in unicellulars, swarm
spores, rotifers, etc., with a purely mechanistic explanation.
CHAPTER XIV
CONCLUSIONS
One of the most important results of recent work on the movements of
ameba and of streaming endoplasm in plant cells is the rapidly growing
conviction that the streaming of protoplasm, wherever it is found, is
due to the same fundamental cause. The value of this conception lies in
the greatly widened front that is presented for attacking the general
problem of streaming. The many special aspects of streaming, which in
the past have been thought to be essential or fundamental processes, may
thus be placed against each other, following what is known as the
comparative method, and the main problem will thus be freed of much that
is not strictly relevant. In this way we come at once to the heart of
the problem.
One of these special aspects of streaming in amebas is the formation of
ectoplasm. For ectoplasm formation is not essential to streaming. But it
is almost certainly essential to locomotion, for locomotion has not been
observed in amebas where ectoplasm was not formed. But, on the other
hand, ectoplasm, as known in the amebas, is not formed without
streaming, although observations indicate that ectoplasm may suddenly
and temporarily pass into the gel state (_Vallisneria_). Streaming is
therefore the fundamental process in ameboid locomotion.
The surface layer of the ameba is physiologically distinct from the
ectoplasm, although it differs from ectoplasm chiefly, if not wholly, by
virtue of its position only. That is, the surface layer is a true
surface tension film. There are no observations recorded which actually
show that the surface film of the ameba is a semi-permeable or plasma
membrane; but, on the other hand, there are no observations which speak
against such a supposition. On theoretical grounds the conclusion is
justifiable that the surface film as demonstrated by the movements of
attached particles is the plasma membrane.
The similarity of the movements of the surface film in ameba with the
movements of the superficial films of _Oscillatoria_ filaments, diatoms,
crawling euglenas, and probably also Gregarinidas, indicates that the
superficial films of all these organisms, including amebas, are all
activated by surface tension changes. Thus instead of postulating
several methods of locomotion which are fundamentally different from
each other, for these respective organisms (excepting the ameba), one
explanation serves the purpose; and it has the further merit of agreeing
more nearly with observation than the various other theories proposed.
From the point of view of ameboid movement, the discovery of the surface
film and its activities narrows down the problem very considerably. It
does not help _directly_ perhaps, in the solution of ameboid movement,
but it shows clearly that the region where ectoplasm is most rapidly
formed (at the anterior ends of pseudopods) is also the region where the
superficial tension is increased. This therefore gives us somewhat of an
insight into what must take place during the transformation of endoplasm
into ectoplasm.
Although the wavy path of the ameba does not at present relate itself to
any other process in the ameba, it is bound to be of the greatest
significance in investigating the intimate nature of protoplasm while in
movement. In so far as the wavy path concerns the ameba, it effectively
disproves the presence of that scientific monstrosity, random movement.
The path of the ameba is orderly.
The wavy path of the ameba represents a projection on a plane surface of
a helical spiral. The path of the ameba is thus geometrically related to
the spiral paths of free-swimming organisms such as ciliates,
flagellates, rotifers, swarm spores, worm larvae, etc. But the paths are
more closely related than merely geometrically. The effects produced by
temperature on amebas and ciliates and flagellates indicate a
relationship between the physical processes underlying the control of
the direction of the paths traveled over in free movement. No causal
distinction can yet be made between rotation on the long axis and the
spiral swinging.
The spiral path is not an acquired habit. It is not a habit that has
been developed to overcome asymmetry of body shape, for some spirally
swimming organisms are not asymmetrical enough to make swimming in
spirals necessary. It is also unlikely that so many thousands of species
of animals and plants of widely different groups would hit upon the same
complex habit to solve widely different problems; for it is not equally
important that all animals should swim in straight paths. It also
necessitates supposing that the ancestors of our present ciliates,
flagellates, rotifers, swarm spores, zoöspores, etc., were symmetrical
and swam without revolving on the long axis and without forming spirals.
Such an assumption is too formidable and makes the explanation
top-heavy.
Spiral swimming is supposed to be due to an automatic regulating
mechanism which is present in all moving organisms. It is held to be a
spatial aspect of the physical processes originating and controlling
movement. The property of moving automatically in an orderly path is
inherent in organisms in the same way, e.g., as the property of growth
is. A spiral path will be followed whenever an organism is free to move,
that is, when not disturbed by sensory stimulation. Slight stimulation
is often without effect. The justification of supposing that probably
all moving organisms are within the grip of the spiral urge is found in
the fact that the amebas, ciliates, flagellates, swarm spores,
zoöspores, Oscillatoria, diatoms, rotifers, larvae of worms, molluscs
and echinoderms, oligochaets, copepods, as well as man, all move in
regular smooth spirals of one kind or another when free from strong
stimulation, and that no organism that is free to move as these are,
moves in a straight or irregular path.
The observations indicate that the same type of mechanism that controls
the direction of the path of an organism also unifies and coördinates
the streaming of the protoplasm of the ameba, the action of the cilia of
the paramecium, or the contraction of the muscles of man, as the case
may be. Why the automatic mechanism controlling the direction of
movement should produce a helical spiral in paramecium, a wavy path or
flattened spiral in ameba, and a series of spirals in man, is not yet
subject to profitable discussion, except of course to point out that
paramecium is not restricted to two dimensions of space as is ameba and
man. In the nature of the case there can be no question but that the
mechanism is one that attaches to the fundamental structure of
protoplasm rather than to the gross morphology. As a mathematical
question, however, the circles occurring in the path of an ameba in low
temperature may serve to connect up the flattened spiral path of the
ameba under optimum conditions with the circular path often observed in
man.
The movement of the ameba thus becomes related to crawling euglenas,
Oscillatoria filaments, diatoms, and perhaps Gregarinidas, because of
the movements of its surface layer; to leucocytes, streaming protoplasm
in the higher plant cells, etc., because of its streaming endoplasm; and
to the locomotory movements of all organisms because of the wavy
character of its path, which betrays the activity of an automatic
regulating mechanism, a type of which is held to be present in every
moving organism.
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FOOTNOTES:
[1] Wilson (’00) describes it as _Pelomyxa_, but it has much closer
affinities with _Amoeba_. It is in fact perhaps the closest relative
of _Amoeba proteus_. Ectoplasm formation, and especially the formation
of ectoplasmic ridges in _carolinensis_, is exactly like that in
_proteus_.
[2] This is shown by the fact that after this ameba has taken on a
spherical shape due to some disturbance in the water, the number of
small ridgeless pseudopods thrown out upon resuming movement, is about
the same as in _dubia_; but after ridges begin to form, the number of
pseudopods decreases.
[3] That is, resemblances in nuclear division stages are not
correlated with corresponding degrees of resemblance in somatic
characters. It is not generally held that the shape or size or number
of chromosomes is correlated with any external characters. It is
the presence of hypothetical factors or genes which are held to be
correlated with somatic characters and their number or arrangement in
a chromosome is not in any way related to their character.
[4] It is possible that Gruber was led to suggest a gelatinous
composition for the layer in question on the strength of assertions
made by several writers that amebas secrete mucus. It is true that
amebas may be displaced by threads of mucus hanging to glass needles
which has collected on the needles while manipulating the amebas in
the culture medium, but that is not to be taken as evidence that the
mucus is secreted by the amebas. Ameba cultures are always full of
gelatinous material formed by bacteria. I have not thus far been able
to convince myself that amebas actually secrete mucus.
[5] According to Ewart (’03) the viscosity of streaming protoplasm
in plant cells lies between η = .04 and η = .2. But the velocity of
streaming endoplasm in ameba is considerably slower than that in
the plant cells which formed the basis for Ewart’s calculations. In
comparison, we may estimate the viscosity of the endoplasm of ameba
as η = .1 dynes per sq. cm. The velocity of streaming endoplasm, as
ascertained by observations on _Amoeba dubia_ (in which the endoplasm
flows usually rapidly)
1
is---- cm. per second.
880
Now, given a unit mass of endoplasm moving at a given instant with a 1
velocity of---- cm. per second against viscosity of η = .1 dynes per
sq. 880 cm., how far will the unit mass travel before coming to rest?
Velocity
Force = Mass × Acceleration, and Acceleration = ---------.
Time
MV
η = Viscosity = F = MA = ----.
T
V
Now if M = 1, η = F = A = ----.
T
1
V ---- 1
T = ---- = 880 = ----.
η ---- 88
.1
The space travelled over in uniformly accelerated motion equals
1 1 1 1 1
S = ---- AT^{2} = ---- × ---- × (----)^{2} = ------ = .00000645 cm.
2 2 10 88 154880
If, therefore, the force moving the central stream of endoplasm should
suddenly be discontinued, the resistance offered by the viscosity of
the enveloping endoplasm would allow it to move only .0000645 mm.
before coming to rest. But the ameba as a whole moves more slowly than
the central stream of endoplasm, the average rate of movement being
about
1
----- mm. per second. The effect of the streaming endoplasm on the
300
forward movement of the whole ameba would therefore be correspondingly
decreased. Now if the ameba was perfectly homogeneous and perfectly
symmetrical, and free from external stimulation, and moved in a
perfectly homogeneous liquid on a perfectly plane surface, the
excessively small amount of mechanical inertia would then be
sufficient, theoretically, to cause the ameba to move in a straight
instead of an irregular path. But these conditions are never realized.
The ameba is unsymmetrical in form, heterogeneous in composition and
always unsymmetrically stimulated; hence it is impossible that the
excessively small amount of mechanical inertia can be considered a
factor in determining the direction of the ameba’s path.
[6] The gap between the rate of movement of a pseudopod and that of
a flagellum is however very wide. Insofar as the _character_ of the
movement is concerned, pseudopods such as those of _flagellipodia_,
probably resemble the flagella of the soil ameba and of flagellates.
But the very much greater speed of contraction of a flagellum and
the presence of a special organ (blepharoplast) at the base of the
flagellum, and their connection with the nucleus, indicates that a
special mechanism is necessary to cause the rapid contraction. A
flagellum appears to be a pseudopod supplied with something like nerve
tissue and a ganglion capable of setting free a rapid succession of
impulses.
[7] It should be added here that since this paragraph was written I
have been very fortunate to secure numerous records of paths swam
by blindfolded swimmers, which strikingly resemble those of persons
walking blindfolded as described above. Most of the common swimming
strokes were employed in these observations and occasionally several
strokes were employed in a single experiment. In a few cases the
spiral path was made up of over twenty turns, and in one case of
over fifty turns. A fuller discussion of these results does not seem
pertinent here, and must be deferred to a later date.
[8] Since this was written I have been able to examine the movement
of live sperm cells in a number of representative animals, including
the jellyfish _Aurelia_; the molluscs _Ostrea_, _Solemya_, _Pandora_;
the arthropods _Limulus_ and _Anisolabia_, and the vertebrates frog,
turtle, snake, cat, dog and man, with the result that all these
spermatozoa revolve on their long axes and swim in spiral paths
resembling those of flagellates. Owing to their minute size their
movements are made out only with great difficulty, but so far as could
be determined all the sperms of any one species turn on their axes in
the same way, that is, either right-handed or left-handed. Recently
there has also come to my notice the very informing paper of W. D.
Hoyt, 1910, in the _Botanical Gazette_, in which it is stated that
fern sperms of various species swim in spiral paths.
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