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AMEBOID MOVEMENT ~*~

BY
ASA A. SCHAEFFER, Pu.D.

PROFESSOR OF ZOOLOGY, UNIVERSITY
OF TENNESSEE

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PRINCETON UNIVERSITY PRESS
PRINCETON

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~

~ LONDON: HUMPHREY MILFORD
~ OXFORD UNIVERSITY PRESS

1920

Copyright, 1920, by
PRINCETON UNIVERSITY PRESS

Published 1920
Printed in the United States of America

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 psychol-
ogist 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 com-
mon to other organisms. ;

But in addition to presenting an account of the main facts con-
cerned 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 pre-
sent 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 sur-
face, therefore serve as an introductory study to the movements
of organisms generally. For the presumption is strong that there

~

v

vi PREFACE

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 ob-
servation 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 funda-
mental question, if it is to be solved at all, can be solved only
by persistent observation and experiment on the ameba and re-
lated organisms themselves. “All knowledge is vain and erron-
eous excepting that brought into the world by sense perception,

‘the mother of all certainty” (Leonardo).

CONTENTS

CHAPTER I
NN oo sng y a9 oa oes vg oo eRe ees wa cule kas:
Cuapter II
PAIStOT iCal OREN oa ia pias Gus aa ow Ba 8 8 ede aes
CHAPTER III
The General Features of Endoplasmic Streaming.........
CHAPTER IV
The Transformation of Endoplasm into Ectoplasm........
CHAPTER V
Pseudopods and the Nature of the Ectoplasm.............
CHAPTER VI
The Species Question........... ‘a Gee Saas ee P es Cute
CHAPTER VII
Experiments on the Surface Layer of the Ameba.........
Cuapter VIII
On the Nature of the Surface Layer: 23. .:....02. 60.3% “

CHAPTER IX
The Surface Layer and Theories of Ameboid Movement...

CHAPTER X

Streaming, Contractility and Ameboid Movement.........

CHAPTER XI
The Surface Layer as a Locomotor Organ................

CHAPTER XII
mue Wavy Path of the Ameba. 3 0.6 Sos pee oa eee

CuapTerR XIII
The Wavy Path of the Ameba and the Spiral Paths of
Ciliates and Other Organisms... 2.4. 2.000.565 oe eee

CHAPTER XIV
RMN ete Sng 2 te To OO eit ec ne A aa

IIE SP ooo ew gies GS eary sss snp ein Sain Bate wee SIS RE

CHAPTER I

INTRODUCTION

The manner of movement common to amebas has attracted
the attention of biologists ever since the discovery of ameba by
Résel 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 ob-
servation 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, be-
cause 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 stream-
ing, so it came to be generally accepted that the really fundamental
feature of ameboid movement was the streaming of the proto-
plasm.

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

I

2 AMEBOID MOVEMENT

') of movement, feeding habits and gross structure. The great im-

portance 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 in-
stances 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 unre-
lated 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 sur-
face film of the ameba is analogous to that of some blue-green
algae, diatoms and crawling euglenas, in which organisms the sur-
face film seems to be the vehicle of movement. Thus the ameba
finds itself related to these organisms by new ties. More im-
portant 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

HIstTorRICAL SKETCH

For the purpose of presenting in brief compass the main pub-
lished 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 in-
_ creases 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 re-
mained for Biitschli (’80, p. 115) to point out that in a moving
ameba endopasm 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 continu-
ally at the posterior end of the ameba. He describes the relation
of ectoplasm to endoplasm as a “circulation” ; the endoplasm, ar-
riving 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 ame-
boid movement.

3

4 AMEBOID MOVEMENT

Jennings (’04), however, from extended study of the physi-
ology 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 Butschli 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 Bitschli (’92, p. 219) while examining a
pelomyxa. To his great surprise he found that there were cur-
rents of water, as evidenced by the movement of suspended par-
ticles, 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 out-
side of pelomyxas, that the surface of the pelomyxa actually
moves forward during active locomotion. He did not state de-
finitely 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 at-
tached particles of soot and other substances, is continuous with
the ectoplasm, and is really the ectoplasm. The rate of move-
ment of this layer was stated to be about the same as that of the

AMEBOID MOVEMENT 5

ameba as a whole. He denied the validity of Bitschli’s sugges-
tion 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 at-
traction 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 pseu-
dopods 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.

Bitschli (’92, p. 187) pointed out that it was highly improb-
able that pseudopods in contact with a solid substratum were pro-
jected in a fundamentally different way from that in which free
pseudopods were extended, as explained by Berthold. Biitschli
assumed that all ameboid movement was due to the same funda-
mental 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 Biitschli assumed that the decrease in
surface tension at the anterior end of the ameba was brought

6 AMEBOID MOVEMENT

about by the bursting of protoplasmic droplets of a more
fluid consistency on the surface of the ameba, the con-
sistency of which was less fluid, thus bringing about a decrease
of surface tension and consequent forward streaming of the endo- —
plasm. 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 en-
gulfing 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), Bloch-
mann (’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 ob-
served. Other factors, he held, must be at work, such as contrac-
tility, 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 gela-
tion 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

AMEBOID MOVEMENT 7

entirely new angle. Certain phases of Rhumbler’s theory are de-
veloped 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 state-
ment, wherein he says that changes in behavior are directly de-
ducible 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 move-
ment 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 ob-
served 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 move-
ment (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 tribu-
-taries. 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? Biitschli (’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
8

AMEBOID -MOVEMENT 9

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 posi-
tion on the ameba of the pseudopod, as well as the strength and
character of the stimulus, are among the factors capable of chang-
ing 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 pseudo-
pod 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 re-
sumed or reversed. If the direction is reversed, the active pseu-
dopods 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 fluc-
tuation back and forth of the neutral zone separating the two

10 AMEBOID MOVEMENT

streams. The fate of such a pseudopod depends on its size, on
its position on the ameba, and the strength of the stimulus af-
fecting 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” con-
trolling 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 move-
ment. Further on, this point will be taken up at length in con-
nection 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 coordination 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 atten-
tion.

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 ecto-

AMEBOID MOVEMENT II

plasmic 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

Ae
iPhoto
Dua
FOES

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. 5b, two streams flowing
toward each other. Cases c to r are self explanatory. s, rotational cur-
rents observed occasionally in various species of amebas. ?¢, “fountain
currents,” sometimes observed in Amoeba blattae, and rarely in other
forms. wu and wv represent cases of streaming which have not been ob-
served 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. Ex-
cepting m, u, r and v, all these figures were drawn from observed cases

of streaming.

(798, p. 190) described and figured for Amoeba blattae Butschli,
and which may readily be observed in most species of amebas if
immersed in a solution of gelatin thick enough to keep the amebas

12 AMEBOID MOVEMENT

from sinking, there is a central stream of endoplasm flowing for-
ward, 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 ap-
parent difference being due to the fact that there is no locomo-
tion. 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 re-
tracted, 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 pseudo-
pod formation and retraction, with the direction of endoplasmic
streams indicated at a given instant. For the purpose of illus-
tration, 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 con-
clusion 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 other-
wise 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 pseudo-
pods 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 un-

AMEBOID MOVEMENT ge

stimulated 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 ex-
planation, 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 stream-
ing (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 prob-
lem. It is probable that the mechanism which controls the direc-
tion 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 de-
termines the direction in which the enveloping protoplasm flows.
If the object is small, particularly if it is a live organism, the be-
havior of the ameba is quite different (Kepner and Taliaferro, ’13,
Schaeffer, ’16). To capture such a food object a cup of proto-
plasm 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 con-
tact 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

14 AMEBOID MOVEMENT

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 pseudo-
pods 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 sur-
round 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 proto-
plasm 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 com-
pleted.

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

AMEBOID MOVEMENT 15

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 ex-
plain 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 dif-
fusing solutions of tyrosin, where the solutions were quite as
concentrated outside as inside the cup. These results prove con-
vincingly that the shape and size of the food cup are not deter-
mined 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 re-
sumed 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 sim-
ilar to what is observed in the higher animals under similar con-
ditions.

16 AMEBOID MOVEMENT

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 be-
havior 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 lack-
ing nuclei did not move in a coordinated manner. Stole (’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 Stolc’s remained
alive, some of them for over thirty days. Recently Willis (16)
confirmed Hofer’s findings, but does not discuss Stolc’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 stream-
ing while the nucleate ameba seems to behave normally. But
Stole’s contention that enucleate amebas move characteristically
(1. c., p. 159, 160, 167) is not necessarily contradicted by these
observations, for Stole’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,

AMEBOID MOVEMENT 17

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 pro-
- cesses 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 Stole (’10) states, indicates however that
the metabolic balance may after a time be regained in some de-
gree, 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 dis-
organized 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 in-
timated 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 role 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 pro-
toplasm 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
(704) 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 proper-
ties of the substances to which they refer, but only as labels.

The demonstration of the most conspicuous and important prop-
erty of ectoplasm in Amoeba proteus is easily made. With the

18

AMEBOID MOVEMENT 19

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 po-
sition 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 com-
plicated process than in almost any other ameba, excepting the
large species Amoeba carolinensis’ discovered by Wilson (’oo).
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 pseudo-
pods 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 for-
mation at the anterior ends of pseudopods only, we find this pro-
cess 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 sympa-

1 Wilson (’oo) describes it as Pelomyxa, but it has much closer affin-
ities 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.

20 AMEBOID MOVEMENT

B C

Figure 3. Formation of longitudinal ridges and grooves in the ecto-
plasm of Amoeba proteus. A, B, C, D, showing stages in the development
of a single pseudopod. a, b, c, d, d1, cross sections of pseudopods at the
levels indicated. ‘The arrows show the direction of endoplasmic stream-
ing 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.

thetically-minded naturalist, that of the large number of subse-
quent writers on ameboid movement only one (Penard, ’o2, 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 ap-
peared 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 longi-
tudinally folded (p. 326).” Penard (’o2) confirms Leidy’s obser-
vation as to the existence of these folds: “The current (of endo-
plasm) indeed is not unified, but there exist many currents at the
same time because of the fact that the endosarc is divided into a

AMEBOID MOVEMENT 21

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 de-
ceives 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 satisfac-
tion 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 cen-
tral 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 how-
ever 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 endo-
plasmic stream, impinge against the imperfectly solidified sides of
the tube of ectoplasm, the innermost strands of the spongy net-
work of ectoplasm snap, usually with readiness, allowing the large
object to pass by.

The surface of a young pseudopod is smooth, a cross section

22 AMEBOID MOVEMENT

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 for-
ward. 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 pseudo-
pod 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 hyloplasm 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 coales-
cence is however exceptional. Once a ridge is formed, it retains

AMEBOID MOVEMENT 23

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, 4). 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 evi-
dent 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 ap-
pearance, 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 ob-
served in verrucosa, sphaeronucleosus (Figure 13) and their con-
geners, especially while the latter are in locomotion. A sphaero-
nucleosus is especially favorable for study in this connection be-
cause,of its greater activity. This ameba has four or more longi-
tudinal 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

24 AMEBOID MOVEMENT

extended at their anterior ends continually, and unless the direc-
tion of locomotion is changed, the ridges may retain their iden-
tity 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 sphaero-
nucleosus, verrucosa and the rest of this group, is much firmer
than in most other amebas.

a ae ee a ee I eee TL ee re

CHAPTER V
PsEUDOPOoDS 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 Jaureata 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 ecto-
plasm during locomotion.

Of all the amebas with smooth surfaces, the most favorable for
observation as to the formation of ectoplasm, is the giant Jaureata
(Figure 4), though it is unfortunately of infrequent occurrence.

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.

This species is as often found in clavate form as with pseudo-

pods. 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.
25

26 AMEBOID MOVEMENT

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 condi-
tion.

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 ap-
proached. 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 trans-
formation 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

ee ee ee ae ee Or Mg Ee ee NT rere, Grim pt

ae eee
— “al ,

AMEBOID MOVEMENT 27

stream). But because of friction against the sides of the ecto-
plasmic tube, there is a layer of endoplasm of appreciable thick-
ness 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 distin-
guished 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 fric-
tion layer would be about one-twenty-third of the diameter of
the pseudopod. These observations therefore point to the con- |
clusion that the tendency in Jaureata 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 Pelomy.xa
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). Ob-
servation indicates also that the ectoplasmic tube is thinner than
would be the case were there complete transformation of endo-
plasm into ectoplasm at the anterior end. The origin of pseudo-
pods in these pelomyxas is not steady and under control as in
laureata, but sudden and eruptive, indicating a less coherent ecto-
plasm.

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

28 AMEBOID MOVEMENT

(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 characteriezd by the possession, usually, of numerous
pseudopods extending from a central mass of protoplasm. In
this stagé it possesses no main pseudopod as does proteus, dis-
coides, laureata and other species, but there are three or four
pseudopods extending actively in the general direction of loco-
motion. 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 endo-
plasm 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.

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 sel-
dom. 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

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.

AMEBOID MOVEMENT 29

12:51

12:52

vas
aS.

12:54

=

2)

¢
SO
iM"
°

J

\
1)
je,

=F,

12.59

Figure 6. Pelomyxa schiedti, after Schaeffer. b, bacterial rods char-
acteristic of the genus Pelomyxa. c, v, contractile vacuole. g, glycogen
bodies. », nucleus. wu, uroidal projections. At the left is shown a series
of outlines of the animal during locomotion. Length, about 75 microns.

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 con-
siderable distance, thus leaving a part of the old ectoplasmic
wall stand for a few seconds between the reflected wave of ecto-
plasm 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.

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

30 AMEBOID MOVEMENT

bodies (Stolc, ’00) which fill it to capacity, lie so close to the sur-
face that it is frequently impossible to see any protoplasm between
them and the exterior. The contractile vacuoles which are nu-
merous, 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.

Another yery interesting feature of Pelomyxa-schiedti is the
-uroid (Figure 6, «), 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 sub-
stratum and in some way aid in locomotion. These uroidal pro-
jections are of considerable length, and may persist for a consid-
erable length of time. Thus while schiedti is unable to form
pseudopods at its anterior end, it forms uroidal projections with

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 pre-
ceding the trophic stage shown at d.

AMEBOID MOVEMENT 31

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 cane
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) anda
new species which for convenience will be referred to as Dilzi. —

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

Figure 8. Amoeba bigemma, after Schaeffer. a, usual form in locomo-
tion, 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. ¢, a small sphere at-
tached to a crystal. f, a twin crystal unattached to a sphere. Length of
a, 150 microns; of d, 12 microns; of f, 2 microns.

32 AMEBOID MOVEMENT

(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, ’o2,
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 with-
drawal proceeds, the spiral becomes irregular, but parts of some
of the turns persist in the last vestiges preceding complete with-
drawal (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 as-
sumed. 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 proto-
plasm. The trophic stage (Figure 8, a) is much the more com-
mon. In this condition pseudopods are formed in large number.
They are small, conical or linear, and blunt,-and they do not de-
termine 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 con-
dition ; 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 ob-
servation 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.

AMEBOID MOVEMENT 33

Figure 9. Amoeba bilzi. a, the ameba in locomotion, showing the
ectoplasmic ridges, nucleus, contractile vacuole. 6, 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, ex-
cepting 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

34 AMEBOID MOVEMENT

locomotion but of position; they are not dynamic but static struc-
tures. 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 fol-’
lowing representative list of amebas: limicola, villosa, dubia, |
proteus, discoides, laureata, bigemma, bilzi, radiosa, sphaeronu-.
cleosus, verrucosa, the given order indicates a progressively’
thicker and firmer ectoplasm as one passes from limicola to ver-
rucosa, 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 bDi-
gemma and bilzi while in radiosa they occur at almost. all times.
Iu sphaeronucleosus and verrucosa no distinctive pseudopods of
any kind occur. paH Ise

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 pseudo-
pods than would be the case if no ridges were formed®. It may

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 de-
creases.

Pee a ee eee

limicola
proteus
discoides
laureata
nucleosus

villosa
dubia
bigemma
“) bilzi
radiosa
sphaero

verrucosa

q os ez :
\

\ oe

d eee
Se — v

_

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 pseudo-
pods directing locomotion in the different species of amebas. b, the num-
ber 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 verru-
cosa as 6,

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.

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 fac-
tors 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 per-
sistence when once formed, in the ratio of length to average
diameter, in the general shape, in the frequency with whicli
straight pseudopods are formed, in the speed of their formation

Bos AMEBOID MOVEMENT

and withdrawal, in the manner of their withdrawal, in their dis-
position with respect to geometrical pattern, in the charac-
ter of the bases of the pseudopods, in the form of the free ends,
and so on. Many of these characteristics are still further analyz-_
able 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 ex-
tended 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 fun-
damental in amebas than has been believed to be the case hith-
erto (cf. Calkins, 12). The intimate structure of amebas is in-
deed similar to that of higher animals where the precipitin reac-
tions (Richet, ’02, ’12; Reichert and Brown, ’09; Dale, ’12; Nut-
tal, ’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 like-
wise 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

AMEBOID MOVEMENT 37

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.
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

Figure 11. A, Amoeba proteus in locomotion. Note especially the’
longitudinal ridges. a1, equatorial view of the discoid nucleus. a?, a polar
view of the nucleus. a, equatorial view of a folded or crushed nucleus
frequently found in large individuals. a*, shape of crystals found in this
species. B, Amoeba discoides in locomotion. lb, b?, equatorial and polar
views of the discoid nucleus. b3, shape of the crystals found in the ameba.
C, Amoeba dubia in locomotion. ct and c?, equatorial and polar views of
the ovoid nucleus. c3-c1°, 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; a1, 46 x 12; 51, 40 x 18; cl, 40 x 32; a*, maximum, 4.5; b?, maxi-
mum, 2.5; c3-cl°, maxima, 10 to 30.

38 AMEBOID
oe Amoeba |
Characteristics discoides

MOVEMENT

Amoeba
proteus

1
|.
}
1

Size in locomotion 450 microns
Pseudopods cylindrical

smooth ectoplasm

“main” pseudopod

| present

‘cross section
circular

average number
in locomotion,

j
}
|
|. three
|
{

Crystals very numerous
|
‘all uniform trun-
cated bipyramids
maximum size
| 2.5p
Fission ‘slower than
proteus
Maximum time be-
tween divisions 20 days

Multinuclearity “binucleate
- occasionally

|biconcave disc,
| never folded

Nucleus, shape

size (40u x 18p
General resistance |

to same condi- |

tions. slight
Surface of

| free from debris

i

’ posterior end
Effect of mechan-

ical stimuli slightly responsive responsive
Food cups ‘small large
Reaction to ‘readily eaten; readily eaten;
carmine | rejected in a rejected in a
few minutes few minutes
Distribution sporadic, small very common

|
| numbers
}

600 microns

dorso-ventrally

| flattened

folded ectoplasm

“main” pseudopod
present

cross section an
irregular oval

average number

| in locomotion,

| five .

‘less than in
discoides

‘all uniform trun-
cated bi-pyra-
mids; rarely a
few flat plates

maximum size
4-5p.

average I division
in 48 hours at
Sor

8 days
binucleate fre-
quent; tetranu-
cleate occasional
biconcave disc, fre-
quently folded
464 X I2p,

very great

free from debris

Amoeba
dubia

400 microns
dorsoventrally
flattened
smooth ectoplasm
no “main” pseudo-
pod
cross section
oval
average number
in locomotion,
twelve
relatively few

at least four
varieties present ;
few perfect

crystals
maximum size
10u, I2y, 30pu

faster than proteus

6 days
binucleate very
rarely

ovoidal
40pm X 32

greater than
discoides

carries debris

very responsive
often enormous
eaten only occa-
sionally; often
retained for hrs.
sporadic, frequent-
ly 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 con-
tractile vacuoles, the general character of endoplasmic streaming,

AMEBOID MOVEMENT 30

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 di-
versity 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 crystalliza-
tion. 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 re-
stricted to a certain range of shape, which must be a direct re-
sult 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 shape-
less 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 ana-
lyzed into smaller and smaller details, is in itself conclusive evi-
dence that the protoplasms of the amebas are chemically different ;
the resemblance between the gross anatomy and physiology be-
tween two different species is due to the greater conspicuousness
of such characters as are the result of the action of physical pro-
_cesses. That is to say, chemically or molecularly different masses
of matter may resemble each other in their molar aspects.

40 AMEBOID MOVEMENT

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 consid- -
ered 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 exam-
ination of the views expressed to the contrary. And it is to this
side of the discussion that we may now briefly direct our atten-
tion.

CHAPTER VI
THE SPECIES QUESTION

After the discovery of the ameba by Rosel 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 in-
clined 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 rhizo-
pods of the Leman Basin, in which he describes forty-five species
of amebas (including Gloidium, Protamoeba, Amoeba, Dinamoe-
ba, Pelomyxa), paying attention mainly to the readily observed
ectoplasmic and endoplasmic characters, and the appearance of
the resting nucleus.

The remarkable discoveries of Vahlkampf (’o5) of the nuclear
changes during the division process turned the attention of nu-
merous investigators to this field, and the ectoplasmic and endo-
plasmic 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

41

42 AMEBOID MOVEMENT

vespertilio cannot be distinguished from those of verrucosa, radi-
osa, polypodia, limax and guttula. Schepotieff (710) in a similar
vein, writes: “Wir werden demnach so bekannte und so lange Zeit
als selbststandige und typische Amdbenarten aufgefasste Formen
wie A. limax, A. polypodia, und A. radiosa nur als Umwand-
lungsstadien andrer Arten bezeichnen diirfen.” Glaser (’12) re-
marks: “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 ecto-
plasmic 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 di-
vision 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 organ-
isms 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 arti-
ficial system. For in no group of organisms has it been found
possible thus far to use the nuclear changes as a basis of classifi-
cation. 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.* In
- 8 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 cor-
related 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.

eo AMEBOID MOVEMENT 43

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 ex~.
ception 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 oc-
curring during the division process. Cf. Dobell (’14) and Hart-
mann (’14) im re Amoeba lacertae; Nagler (’09), Glaser (12)
and Wilson (’16). on the presence of a centriole in amebas; etc.
The work of Schardinger (99), Wherry (713) and Wilson (’16)
on the nuclear stages of amebas was done with care, yet Wilson
(716) still remains in doubt as to whether or not these investi-
gators 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 Actino-
sphaerium; there being thus greater correspondence in the nu-
clear changes between species belonging to different orders than
there is between species in the same genus. Logically therefore
Actinosphaerium would have to be : oy 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 in-
vestigator (Dobell, ’10). Experimental work, which is usually
done with one of the larger species, would thus be greatly handi-
- capped 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 fore-
doomed 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 minute-

. 44 AMEBOID MOVEMENT

ness; such amebas could be given specific names for reference
but they could not be classified in a natural system excepting per-
haps as a group.

But the definiteness and the consistency with which the nuclear
division stages occur in any given species of ameba, lends sup-
port 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 prac-
tically unanimous that the elaborate mechanism involved in nu-
clear 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 in-
herited from one generation to the next with as little variation as
is observed in other organisms (Schaeffer, ’16). The recent
work of Jennings (716) 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 dif-
fering 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

AMEBOID MOVEMENT 45

highly characteristic nature of the pseudopods formed by the
amebas of any species, it is seen, is to be referred to the funda-
mental 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 stream-
ing 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 organ-
isms, 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 move-
ment 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 cir-
culation of the endoplasm,—a true streaming process,— is an in-
voluntary act. But in Frontonia, another large ciliate, the cir-
culation 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 circu-
lates 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 accom-
panied by the formation of ectoplasm as observed in ameba. In
plant cells the ectoplasm is practically stationary, while the endo-
plasm 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 con-

46 AMEBOID MOVEMENT

tact 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 move-
men 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 ecto-
plasm of the kind observed in amebas. The middle of the pseudo-
pod, 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 de-
tailed 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 fre-
quently happens that knobs are found on a slender pseudopod.
These knobs may move back and forth with great rapidity with-
out 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 themseslves 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 ac-
ceptance 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

AMEBOID MOVEMENT 47°

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.

_pseudopods, is undoubtedly conditioned by the structure of the
protoplasm, both physically and chemically, but more especially
the latter.

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 obser-
vations prove that movement in ameba is always associated with
streaming, and streaming (in locomotion) with ectoplasm forma-
tion. It follows therefore that the form of movement observed
in amebas depends. invariably upon the streaming of the endo-
plasm 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
Butschli (°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, con-
cluded 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 how-
ever 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 per-
manently 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.

48

AMEBOID MOVEMENT 49

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 ex-
perience 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.

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 observa-
tions on Amoeba sphaeronucleosus (Figure 13). This ameba re-

Figure 13. Amoeba sphaeronucleosus. In locomotion. Note the nucleus,
contractile vacuole, ectoplasmic ridges. This ameba is not known to form
pseudopods. Length, 120 microns.

sembles 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

50 AMEBOID MOVEMENT

Figure 14. Illustrating the movement of a particle on the upper surface
layer of an Amoeba sphaeronucleosus. Length of the ameba, 120 microns.

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 I to 4, and
from ameba outline 1 to 4, it is seen that the rate of move-
ment of the particle compares with the rate of movement of the
ameba as 2.48 to I.

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

Figure 15. An Amoeba sphacronucleosus 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.

AMEBOID MOVEMENT 51

particle. The speed ratio of particle a to particle b was as 1.26
to I.

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 pos-

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.

terior 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

52 AMEBOID MOVEMENT

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

Figure 17. Illustrating the different speeds with which particles move
when attached to the surface film of an Amoeba sphaeronucleosus, depend-
ing upon their location. Particle a moved 3.5 times as fast as the ameba
and 6b 2.7 times as fast. Length of ameba, 110 microns.

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 ad-
vancing more rapidly than the left anterior edge; (3) the par-
ticle 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

AMEBOID MOVEMENT 53

two rapidly growing regions on the anterior edge, leading in di‘-
ferent 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 D 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 an-
terior 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.

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

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.

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 direc-
tion of the particle was again a response to the waves of ecto-
plasm thrown out at the left anterior edge, which thus became the
anterior end.

54 AMEBOID MOVEMENT

Figure 19. Illustrating the rapid movement of the upper surface of an
Amoeba sphaeronucleosus under the most favorable conditions. The par-
ticle 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 de-
termined. The movement of these particles was considerably
slower than the movement of the ameba. In another ameba at-
tached 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 surround-
ing 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 “rota-
tion” of particles adhering to an ameba as described by Jennings
(04) and Dellinger (’06), though the explanation is different

AMEBOID MOVEMENT 55

—

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 # moved slowly forward. 6, the
under side of the ameba, showing the attached parts stippled. The par-
ticles suspended in the water at + moved slowly forward with the ameba.
c, a cross section of an ameba of shape shown in 5, showing the ridges on
the surface. Length of the ameba, about 100 microns.

from that given by Jennings (1. 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.

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

50 AMEBOID MOVEMENT

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 rela-
tive positions as the ameba (verrucosa) changes its direction of move-
ment. Length of the ameba, 150 microns.

cross section. It is also more in the habit of loping along the sur-
face 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).

Go eee KC ee

45s c b gc d

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
I to 10 was 1.36 times as fast as that of the ameba, a much
slower rate than was observed in sphaeronucleosus. At 6 anew
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 to the anterior end again spread out and again

AMEBOID MOVEMENT ; 57

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.

the particle moved faster—twice as fast as the ameba from 9 to
10. Stages II, 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. |

In another observation the effect of a narrowing of the ad-
vancing tip of the ameba is shown very well. In figure 24 the
ameba was advancing with a broad anterior end, as shown at I
and 2. From 2 to 4, the region where new ectoplasm was made,

58 AMEBOID MOVEMENT

/ ge 22

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.

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.

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, fre-
quently 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

AMEBOID MOVEMENT 59

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.

retract. The particle moved 1.43 times as fast as the ameba from
I to 4. But from 4 to 7 the particle moved only 1.06 times as
fast as the ameba.

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. Ina similar manner the ridges which are constantly form-
ing on a proteus are continually competing with the anterior end

60 AMEBOID MOVEMENT

in their pull upon the surface layer, thus preventing rapid forward
movement.

Figure 26 shows that the surface layer flows away from the tip
of a retracting pseudopod that is located near the anterior end.

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. ~

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 pro-
portion 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 psesudopods. Three particles marked a, b, c and connected

AMEBOID MOVEMENT 61

by a line for convenience of reference, were in the position in-
dicated at 1 when the forward end of the ameba occupied the
position indicated by outline 1. As the ameba moved forward

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. 5
and c were drawn back toward the main body of the ameba while c re-
mained behind, moving only very slowly. Thus the relative positions of
these particles was completely changed.

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

62 AMEBOID MOVEMENT

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 pseudo-
pod. 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 rélative 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, aled. No further demon-
stration 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, interest-
ing enough, for a number of contradictory statements by various
students of the amebas are satisfactorily cleared up by these ob-
servations. 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 be-
comes 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 par-
ticles 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 indi-

63

64 AMEBOID MOVEMENT

cates 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 endo-
plasmic 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 bs the surface layer broaa a 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 ad-
vancing 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 forma-
tion complicates to some extent the longitudial gradient of ecto-
plasm 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.

AMEBOID MOVEMENT 65

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 pseudo-
pod 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 ecto-
plasm 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 con-
clusion 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 re-
gions 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 sprface 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 pseudo-
pods more accurate information can be obtained. F

66 AMEBOID MOVEMENT

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 de-
scribed 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 par-
ticles located some distance away from the tip of the pseudopod.
If. the particle has reached the tip of the pseudopod before re-
versal of the endoplasmic stream takes place, the particle often
remains at the tip until the pseudopod is almost completely ‘with-
drawn 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 ised 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 par-
ticle on the distal end moves slowly toward the tip while a par-
ticle in the proximal region moves toward the base of the pseudo-
pod. Particles over the neutral zone are motionless. In these
cases, however, changes in the direction and speed of the ecto-
plasmic 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 kine-
matograph is essential for this purpose), so no figures of the
speed of movement of such particles are given. Nevertheless

~AMEBOID MOVEMENT 67

the general results of the observations are as stated. It might be
added that in some cases the neutral zone for the particles at-
tached 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, be-
cause 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 indicat-
ing 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

68 AMEBOID MOVEMENT

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 ecto-
plasm 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 ob-
servations described on p. 56) possesses mobile surface proto-
plasm. 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 attach-
ment is some distance back, as indicated in figure 20. The effect
- of this is to increase the amount of forming ectoplasm in propor-
tion 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, there-
fore, that there is more ectoplasm formed at the anterior end of a
Sphaeronucleosus than would be the case were the upper surfatce
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,

AMEBOID MOVEMENT 69

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 propor-
tional 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 there-
fore 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.

Since the movement of the surface film is obviously a shbface
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.

Figure 28. A clavate Amoeba discoides, showing the amount of ecto-
plasm that is constantly being made at the anterior end. Length of the
ameba, 310 microns.

Three outlines of the anterior end only are given; the rear por-
tion 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 repre-
sents forming ectoplasm, and ten-elevenths formed ectoplasm.
(One-twenty-second of the total surface was deducted for surface

“

70 AMEBOID MOVEMENT

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 oc-
cupied by forming ectoplasm is 1/5.8 of the total surface, and the
ratio of formed to forming ectoplasm is 4.8 to I.

(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 sphaero-
nucleosus 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 at-

AMEBOID MOVEMENT 71

tachment 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 ac-
cumulation 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 tem-
perature 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 sur-
face. Almost any liquid may be used for this purpose. But it is
comparatively very difficult to increase the surface tension at some

72 AMEBOID MOVEMENT

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 nec-
essary 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 suface 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 sur-

AMEBOID MOVEMENT 73

face 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 sur-
face 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 in-
gredients 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 sugges-
tion. 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 re-
mains 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

74 AMEBOID MOVEMENT

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.*

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 gen-
eral and in detail the behavior of a surface tension layer in an in-
ert 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, conrespanding 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 struc-
ture 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 Hdber’s
(11) view that the plasma membranes generally are living struc-
tures.. But it may be regarded as certain that if lipoids.are present
in the protoplasm of the ameba, these substances, according to the

“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 dis-
placed 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 secréte mucus..

AMEBOID MOVEMENT 75

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 dif-
ficulties. An ameba moves with the electric current, when a cur-
rent 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 direc-
tion 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 low-
ered 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 move-
ment 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 con-
cerned.

The transformation of endoplasm into ectoplasm is unfortun-
ately 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 un-
known, and it is quite open to criticism whether analogies from the
behavior of pure solutions of colloids, suchas 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

76

~

AMEBOID MOVEMENT 77

causal manner. It does not at present give. us any greater in-
sight 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 de-
mands 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 prop-
erties of the outer layer are here described in detail for the first
time, it becomes necessary to enquire to what extent the more com-
monly held theories of ameboid movement conform with the ob-
served behavior of the surface film. Although the surface tension
theory was the first detailed theory proposed toward an explana-
tion of ameboid movement, I shall discuss Jennings’ (’04) ob-—
servations 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 pro-
teus and verrucosa “types,” as follows:

“In an advancing Amoeba substance flows forward on the up-
per 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 ro-
tation 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 ex-

“8 - AMEBOID MOVEMENT

tends 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 move-
ment 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 con-
gruent 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.

AMEBOID MOVEMENT. - 79

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. Jen-
nings’ 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 inter-
pretations.

To illustrate this point graphically so as to avoid as far as
possible future confusion Figure 29 is appended. In ais 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. If
illustrates the backward moving ectoplasm in ,an ameba that is
suspended in a jelly medium that prevents the ameba from sink-
ing to the bottom. It must be admitted that in thus considering

80 AMEBOID MOVEMENT
"ati

ae,
a b dies ras

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 longi-
‘tudinal 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.

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.

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 be- -
tween 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 be-
comes slower as the ectoplasm is approached on ail sides.

We said above that if the ectoplasm were a more or less
permanent skin in which the ameba rolled as described by Jen-
nings, 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 sur-

AMEBOID MOVEMENT 81

face 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 im-
portant factor is a more or less permanent ectoplasm in which
the ameba rolls along, would unquestionably produce rotary cur-
rents. Rhumbler (’98) recognized this and after full considera- .
tion rejected the idea that the ectoplasm is a permanent skin in
which the ameba rolls along in locomotion, because rotary cur-
rents 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 reason-
ing 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

82 AMEBOID MOVEMENT

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, in-
dicates clearly enough that this theory does not really give a
very adequate idea of the processes involved in ameboid move-
ment after all, and in so far as feeding processes are concerned,
the theory does not seem to apply at all according to Schaeffer
(716, °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 obser-
vational 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 biolo-
gists, 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 inter-
esting reading for those who want to know first of all what
amebas can do. Only the more important modifications of the
surface tension theory are applied to ameboid movements will
therefore be discussed and these modifications will be considered
important in proportion to the amount of observation or experi-
ment 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 imme--
diate environment.

AMEBOID MOVEMENT 83

One of the most elaborate attempts that has been made toward
explaining ameboid movement on the basis of surface tension
phenomena was that of Biitschli (’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 migra-
tion 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. Biitschli described many experiments with
fluids on which the surface tension was changed by appropriate
means to simulate the process of movement. After Biitschli 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
Biitschli himself probably concluded that the surface tension
theory of movement as he developed it, is not of general applica-
tion or is nothing more: than a step in the development of such
a theory. .

Rhumbler has writtten a number of papers on the mechanics
of ameboid movement, most of which are concerned with elabora-
tions and modifications of a surface tension theory very similar
to Butschli’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 (ecto-
plasm) of the ameba. The presence of the food body, he held,

84 AMEBOID MOVEMENT

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 con-
cluded (’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 phys-
ical models for the explanation of various ameboid activities such
as feeding, defecation, movement and so forth.

In general agreement with Butschli and Rhumbler were Ver-
worn (’92), Blochmann (’94), Bernstein (’00), Jensen (’oI, ’o2),
and recently Hirschfeld (’09) and McClendon (’12). All these
authors held that ameboid movement is a surface tension phenom-
enon. The application of the surface tension theory in explain-
ing ameboid movement demands a fluid surface and a fluid in-
terior and it is perhaps unnecessary to add that Bitschli, Rhumb-
ler 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), Dellin-
ger (06) and others opposing the group of authors just men-
tioned, 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

AMEBOID MOVEMENT 85

cells, and it is not surprising therefore that explanations of
streaming and ameboid movement have taken a different direc-
tion 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 Biitschli, as we saw, believed
that protoplasm has an emulsoid structure; but according to
Biitschli’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 contrac-
tion. According to Ewart’s hypothesis the emulsion globules (dis-
perse phase) have their surface tension lowered at correspond-
ing points by electrical currents traversing the endoplasm, the
electrical currents themselves originating in chemical actions.

While all available evidence from the study of colloidal solu-
tions 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

86 AMEBOID MOVEMENT

of surface of the fluid is increased. This increase of surface pro-
duces 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 endo-
plasm, before it can be withdrawn. All writers excepting Jen-
nings 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 ecto-
plasm 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 ad-
vancing ectoplasm at the tip of the pseudopodium is derived from
the surface ectoplasm.and not from a transformation of endo-
plasm into ectoplasm at the end of the peendopeeian as Rhumb- °
ler supposed”? (p. 89).

This quotation is. not spt i 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’ conclu-
sions. 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 inthe 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 ame-

AMEBOID * MOVEMENT 87

boid movement. It will give our first real insight’ into the chem- .
istry of ameboid movement. The fact that her method of demon-
strating gradients has yielded uniform results in the hands of
Child (715), who originated it, as well as in her own when applied
to a great many different organisms, onttles her conclusions to
careful examination. :

Of the stitaiatens there can. ‘s no doubt, écas3 in many desis
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 (Figtire 30).- The

i aes aes c d ;

Figure 30. Disintegration of an ameba in. % molecular KNC. After
Hyman. a, ameba flowing in the direction of the arrow. 6, 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 +, disintegrated first. d, the next youngest pseudo-
pod, 2, disintegrated next. Pseudopod 1, the oldest, disintégrated last.

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

‘

88 AMEBOID MOVEMENT

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 ob-
servation 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 pro-
duction of a pseudopodium, that the metabolic change which is the
cause of the liquefaction should continue to occur at the pseudo-
podial 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, ac-
cording 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. Lique-
faction 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 demon-
strated 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

AMEBOID MOVEMENT “a

itself to new investigations and observations is the surface tension
theory. The earlier theories under this head were mistaken how-
ever 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 proto-
plasm; 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 prob-
able 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 stream-
ing 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 de-
pends 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 pro-
portion 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 |

90 AMEBOID MOVEMENT

and Pelomyxa scheidti; in the next group, with less fluid proto-
plasm 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 pro-
gressive 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 longi-
tudinal 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, flagel-
lipodia.) 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 prac-
tically incapable of contraction. The high development of con-
tractility 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 com-
pares the different species, whether they form very little ecto-
plasm 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 endo-
plasmic stream effective for the purpose of orderly movement.
It requires very little imagination to seé 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

AMEBOID MOVEMENT gI

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 ecto-
plasm 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 stream-
ing. There is however one observation which may, upon further
investigation, negative this possibility. Frequently in a pseudo-
pod 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. Observa-
tion 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 Diffilugia and Arcella and the foraminifera.

Although these waves are frequently not to be seen during
locomotion in Amoeba proteus and other large amebas, particu-
larly 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 pro-
jection of these waves is the purest expression of ameboid move-
ment. 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 move-
ment 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 endo-
plasm 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 loco-
motion 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 ab-
sence 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 occasion-
ally 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 Diffiugia 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 Diffiugia is thick and

92

AMEBOID MOVEMENT 93

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 consider-
ably faster than the tip of the pseudopod advances through the
water. But when a large pseudopod is cut off from a Diffiugia
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 Diffilugia. 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 ecto-
plasm. 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 pseudo-
pods of this organism, for since only about one-fifth of the cir-
cumference can be in contact with the solid substratum, the pseu-
dopods must attach themselves, contract so as to pull the Acan-
thocystis along, and relax their hold, all in the space of two
seconds.

Among pseudopod forming organisms, the highest develop-
ment 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 proto-
plasmic strands. The terminal sections of these strands are fre-
quently so thin and transparent that they cannot be seen easily
with the microscope. As a rule the granular endoplasm is ob-
servable only in the main body of the organism and in the larger
trunks of the pseudopods. Much the larger part of the pseudo-
pods, as measured lineally, is devoid of granular endoplasm. The

94 AMEBOID MOVEMENT

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 im-
posed 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 trans-
parency 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 6f 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 stream-
ing 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 con-

AMEBOID MOVEMENT | 95

tinued presumably as long as the myxomycete is in the plasmodial
stage. With every change in the direction of movement of stream-
ing, 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 Bar-
anetzsky (’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 an-
terior 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 appar-
ently only to a slight degree. Too little is known of the stream-
ing 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 es-
sential 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 proto-
plasts as, for example, ameba. Inside of the cell wall is a layer
of ectoplasm which has essentially the same properties as the ecto-
plasm of amebas. In some cells such as those of Chara, the ecto-
plasmic 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

96 AMEBOID MOVEMENT

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 ecto-
plasm. 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 rota-
tional; 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 rota-
tional or circulatory, from the rotational streaming so commonly
found in protozoa.
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

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.

while the layer near the ectoplasm moves very slowly and the layer
in contact with the ectoplasm moves hardly at all. But the endo-
plasm in contact with the central vacuole moves only a little more

AMEBOID MOVEMENT 97

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 strearn 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), Schaefer (’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 ve-
locity 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 lim-
iting ectoplasm, when they act as obstructions to the flow of the
endoplasm. The velocity of the endoplasmic stream in para-
mecium 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

98 AMEBOID MOVEMENT

in feeding. Frontonia feeds mostly, if not entirely, on large par-
ticles. It has no oral groove like paramecium has, and when
swimming no ciliary vortex is produced such as is seen in para-
mecium. Frontonia feeds mostly by “browsing,” that is by eat-
ing 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 theibody. Pieces of Oscillatoria six to eight times as long
as the-Frontonia are readily eaten in this way.
As airule the end ofa 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 postetior end the fila-
ment is brought up along the oral wall... The rotational streaming
continues until the entire filament is wound up, which in excep-
tional 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 endo-
plasm 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 Fron-

AMEBOID MOVEMENT 99

peas re)
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.

tonia and’ paramecium rapid endoplasmic steaming precedes for
a short distance the forward end of the ingested filament or the
food vacuole (Figure 32, a).

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 un-
winding is thrown out of the body. The viscosity of the endo-
plasm 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 Fron-
tonia and in paramecium are: (1) It is under the control of the
organism in Frontonia while in paramecium it is a continuous re-

versible process. (2) It is much more rapid in Frontonia than in
paramecium. On the other hand, the physics of streaming in

100 AMEBOID MOVEMENT

both organisms is essentially the same, so far as could be de-
tected. 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 men-
tioned; 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 phago-
cytes and wandering cells of a great many animals; in the nuclei
of some animal cells; and in the intestinal epithelial cells of per-
haps 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 tend-
ing 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 com-
plete, in the colloidal system which is held to be the chief char-
acteristic 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

AMEBOID MOVEMENT IOI

phase resulting in gelation, for observation has failed to detect
this process going on to any extent whatever in streaming proto-
plasm. Further, an increase in the amount of water in the proto-
plasm 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 ac-
counting 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), Kithne (64), Briicke (’64), Hanstein (’80), in one form
or another also have expressed their adherence to the contrac-
tility theory. More recently Dellinger (’06, p. 356) postulated
contractile fibrillae in rhizopods similar to those postulated by
Brticke to explain protoplasmic streaming. The contractility
theories are no longer considered tenable, for no waves of con-
tractility can be demonstrated, as the theories of Corti, Heiden-
hain, et al. demand, and, contractile fibers can neither be demon-
strated 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)

102 AMEBOID MOVEMENT

and Englemann (’79) conceived of streaming as being caused
by certain constituents of the cell imbibing water and later dis-
charging it. Sachs and Hoffmeister thought that waves of im-
bibition 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 ob-
serving 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 proto-
plasm toward the point of lowered tension. The lowering of the
surface tension is brought about by a union of the Biogen Mole-
cule 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 Ver-
worn’s theory; but the very hypothetical nature of his theory
prevents one from discussing this possibility.

-~AMEBOID MOVEMENT 103

(4) The electrical theories. These fall into three classes:
(a) The galvanic theory. Amici (’18) suggested that the chloro-
plastids 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 funda-
mentally different from that in other cells. (b) The electro-
magnetic theory. Velten (’72, 73) and Hoérmann (’98) are
chiefly responsible for the development of the electromagnetic
theory. They hold that chloroplastids have an independent move-
ment of their own; but the principal postulate of this theory is
that there is electric repulsion between the ectoplasm and the en-
doplasm. 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 dif-
ficult 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 endo-
plasmic streaming in plants, Ewart (’03) has come to the con-
clusion 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 pass-
_ ing 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 correspond-
ing ends decreased when electric currents traverse the endoplasmic
stream, the particles and, of necessity, the whole stream of endo-
plasm would move in the direction of lowered surface tension
(Figure 31, p. 96). Continuous chemical actions would be

104 AMEBOID MOVEMENT

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 sur-
face energy in the interfaces in the colloidal system which comes
to be regarded as the primary source of the energy. But all at-
tempts 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 occa-
sional 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 gen-
eral search for an explanation of streaming along this line is
proceeding in the right direction. The researches of Bancroft
(713, 14) and especially of Clowes (’13, 16) on the nature of the
change of phase in emulsions are very instructive in this connec-
tion ; 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 stream-
ing 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 pos-
sesses 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 Oscilla-
toria 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 move-
ment is caused by the rapid and forcible exudation of mucus is ex-
ceedingly 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 move-
ment 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 move-

105

106 AMEBOID MOVEMENT

ment 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 sphaeronucle-
osus 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 aegis thas by its particle-car-
rying capacity.

~The main features of the movements of diatoms are very
similar to those of Oscillatoria.. Muller (’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 as-
sumption 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 be-
hind. 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 con-
cerning the excretion of mucus, but the source of the locomotive
energy is probably here also surface energy, though the observa-
tional 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

AMEBOID MOVEMENT 107

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 cohesive-
ness 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 cir-
cumstance that crawling euglenas, diatoms and Oscillatoria
threads are much more refractory to galvanic currents than flagel-
late euglenas or other flagellates or ciliates: The electrical ap-
paratus at my disposal was rather crude, but I was unable to
find that I could influence the direction or character of move-
ment 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 what-
ever on amebas, diatoms, Oscillatoria or crawling euglenas. Dia-
toms 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, des-
mids, or crawling euglenas, is that all derive their motive power
from the energy in the superficial films of these organisms; while

108 AMEBOID MOVEMENT

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, Forami-
nifera) 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, ex-
cepting flagellated or ciliated organisms (of whose motor mechan-
ism we have no detailed knowledge), depend upon surface energy
as the source of the energy of movement.

CHAPTER XII
Tue Wavy PatH or 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 para-
mecium, which is also easily changed by the presence of various

109

110 AMEBOID MOVEMENT

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 sur-
face. 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 coordination, 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 codrdinated 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

AMEBOID MOVEMENT ae

made quickly but carefully, beginning and ending with the pos-
terior 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 pro-
ducing 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 locomo-
tion was not changed. It may be assumed that.this change was
produced by a stimulus external to the wave-producing mechan-
ism. 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 dura-
tion of at least one wave length thereafter. It will be noted that

112 AMEBOID MOVEMENT

Figure 33. Illustrating the path of an Amoeba bigemma under con-
trolled 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 fol-
lowed 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, I50 microns.

AMEBOID MOVEMENT 113

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 experi-
ments. 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 dis-
coides 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
2:29% to 2:42% 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 2:42% the light was turned off until 2:5814 except for two
ten-second flashes at 2:47 and 2:52. During the first period of

114 AMEBOID MOVEMENT

ee > 2295
ge f 424

22 a,

Figure 34. The path of an Amoeba discoides under light controlled
conditions. At 2:42% 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.

5:24

) * S we 3:223
wy SS ee .

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 3:02% to 3:13%. From then
until 3:43 the light was turned off except for 10-second flashes at 3:18, -
3:22%, 3:24, and so on. The ameba had probably come to rest for
some reason between 3:24 and 3:30%, 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 con-
tinued 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 essen-
tially the same thing as Figure 34. The light was turned on from
3:30% 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

AMEBOID MOVEMENT 115

apparent connection between longer waves and darkness has not
yet been investigated.

Figure 36 shows the path of an Amoeba proteus under con-
trolled 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

2

1:45

Figure 36. Showing how smooth waves in the path of an Amoeba pro-
teus 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 forma-
tion of numerous pseudopods resulted in an irregular path. Length of the
ameba, about 600 microns.

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 obliter-
ated. Associated with this irregularity is the presence of numer-
ous 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.

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 3:48%4 the ameba broke up into several
pseudopods at right angles with the main axis, and through one

116 AMEBOID MOVEMENT
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. ,

4:003

SF: 57

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 3:48% 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.

AMEBOID MOVEMENT 117

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 sur-
face under light-controlled conditions. If we had not already
seen how proteus, discoides, and especially bigemma formed

Nyyl:40

Figure 38. Amoeba dubia usually moves with numerous large pseudo-
pods, but this illustration shows that there is very good reason for con-
cluding that there is a tendency in this ameba to move in wavy paths.
Length of the ameba, 400 microns.

smooth waves in their paths, we should hardly be able to under-
stand the apparently aimless path of dubia. But having seen
how a regular succession of smooth waves appears under favor-
able 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

118 AMEBOID MOVEMENT

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 char-
acteristic of amebas.

To learn something of the nature of the wave-forming mechan-
ism 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 tem-
perature 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 exam-
ple the pronounced waves a short distance from the end of the
path. All these waves are shorter but much deeer than the waves
made in a higher temperature. The loops in the path (all except-
ing the first, which is a compound loop) represent each a single
wave which have become so deep and contracted that they have

AMEBOID MOVEMENT 119

2:2/

Figure 39. The path of an Amoeba dubia in comparatively low tem-
perature (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.

become transformed into circles: As the temperature decreases,
the crests of the waves rise higher and higher, and the bases con-
tract more and more, until the two sides of the waves come to-
gether, 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, batt the
character of the waves themselves is changed:

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, nega-
tively, 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

120 AMEBOID MOVEMENT

cs a oo"

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.

waves in their paths, the explanation of encircling becomes simpler
and perhaps also more convincing.

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 opera-
tion 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

AMEBOID MOVEMENT 121

MIME GS

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 re-
flected 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
I, 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 en-
able 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 direc-
tion of movement (i. e., at the base of the wave) when describing

122 AMEBOID MOVEMENT

Figure 42. Diagram illustrating the relation of the phenomenon of en-
circling 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 tempera-
ture, 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 ten-
dency 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 when-
ever 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 pro-
pagated 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 accumulat-
ing, it is coming to be seen that the ameba possesses all the funda-
mental attributes of animals generally, in addition to many special
ones. So that as a matter of fact, if the ameba did not show

AMEBOID MOVEMENT 123

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 Didiniwm and the unceasing gyrations
of the whirligig beetle excite one’s curiosity because these organ-
isms 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 fre-
quently 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 deter-
mining 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 intertia can affect the
path of the ameba.°®

It is not even necessary that movement of the endoplasmic

5 According to Ewart (’03) the viscosity of streaming protoplasm in
plant cells lies between n = .04 and » = .2. But the velocity of stream-
ing 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 n = -I dynes

{24 AMEBOID MOVEMENT

stream be interrupted in order that a straight path may be main-
tained. An ameba may stop movement for a minute or more
and then be much more apt to resume movement in the original

per sq. cm. The velocity of streaming endoplasm, as ascertained by ob-
servations on Amoeba dubia (in which the endoplasm flows usually rap-

I
idly) is —— cm. per second.
880

Now, given a unit mass of endoplasm moving at a given instant with a

; I
velocity of oom cm. per second against viscosity of n = -1 dynes per sq.
cm., how far will the unit mass travel before coming to rest?

: Velocit
Force = Mass X Acceleration, and Acceleration = een casi
Time
; ; MV
n = Viscosity = F = MA = ——.,
§

Vv
Piatt Mose aay waite Ase

I

v— I

vi, iret 880 = 3
7 |

The space travelled over in uniformly accelerated motion equals
I I I I I

S-=)— AT? = — X —— X (——)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

I
— mm. per second. The effect of the streaming. endoplasm on the

300

forward movement of the whole ameba would therefore be correspond-
ingly decreased. Now if the ameba was perfectly homogeneous and per-
fectly 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.

ee,

AMEBOID MOVEMENT 125

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 mov-
ing off in the same direction is meant that the ameba did not
move more than 2214° 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 con-—
sidered 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 min-
utes. 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 endo-
plasm 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
fo the ectoplasm at the anterior end is different from that else-
where 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

Tue 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 zoospores
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 weeds is beside the wavy path, that indicates rane
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 loco-
motion only when the organism follows a spiral path.

Much has been written about the fundamental similarity or iden-.
tity between flagella and pseudopods. All writers who have ex-
pressed 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

126

AMEBOID MOVEMENT 127

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 draw-
ing 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-clock-
wise 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

e.g AMEBOID MOVEMENT

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-clock-
wise, when looking at the tip and at the main body of the ameba. The
tip made one complete revolution in about three seconds.

pseudopods formed become spiralized at one time or another of
their existence, the greater number of these being however rela-
tively immotile. Pseudopods frequently fall into spirals while
they are being extended.

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 pseudo-
pod.® But important as such a transition form is for theoretical

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 move-
ment is concerned, pseudopods such as those of flagellipodia, probably re-
semble 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 con-
nection 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 sneer capable of setting
free a rapid succession of impulses.

AMEBOID MOVEMENT 129

purposes in understanding the nature of both flagella and pseudo-
pods, 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 pseudo-
pods 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 loco-
motion. 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
coordinating locomotion is in the central nervous system. Even
in some protozoa (Euplotes) a motorium has been found whose
function apparently is that of coordinating 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 occpy the same
space relations to each other for more than a moment, excepting
perhaps within the nucleus. But the nucleus as a whole is con-
tinually changing its position with regard to the rest of the ameba,
and almost certainly its position at any given moment in the ameba
is he result, not of its own acticity, but of the endoplasm and the
ectoplasm. A formed nucleus, moreover, is not necessary to con-
certed 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 un-

\

130 AMEBOID MOVEMENT

able to state definitely whether it moves in a wavy path. (1 have
seen this organism only a few times, and on none of these occa-
sions was I able to make the test). It seems therefore possible
that the agency responsible for the movement of ambas in flat-
ined 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 zoologists, 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 essen-
tial 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 ridicu-
lous 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 or-
ganisms” Jennings (’o1) 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 asym-
metrical 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 unsym-
metrical 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.

AMEBOID MOVEMENT 131

But in the explanation of body form in one of the rotifers he
(1. c., p. 376) says: “In some of these primatively bilateral ani-
mals this spiral method of swimming has resulted in the pro-
duction of an unsymmetrical form analogous to that of the in-
fusoria.”’

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 at-
tractive 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 exam-
ine the thesis that rotation on the long axis is an adaptation which
has been developed to compensate for the effect of an unsym-
metrical 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 evi-
dence 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 stimula-
tion, the amount of food in the body, etc. (Jennings, ’06, Schaef-
fer, *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 unsym-

132 AMEBOID MOVEMENT

metrical 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? Observa-
tion 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 zodspores, than to change the beat of
the cilia in some other way, in at least a few species? This ex-
planation 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 ex-
ceedingly 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, ex-
cept 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 can-
not 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 assymmetry 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

AMEBOID MOVEMENT 133:

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 fre-
quently seen in the larger individuals, it is probable that the forma-
tion of spirals is prevented because of the increased physical in-
ertia 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. Spondylo-
morum and several other colonial forms describe smaller spirals
than smaller solitary organisms. ‘These colonial organisms con-
sisting of from four to twenty thousand cells, each of which may
be possessed of cilia, are marvels of locomotory coérdination,
but it is not at all clear how this codrdination 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 (’o1) describes a species
of rotifer whose body forms a segment of a spiral. When swim-
ming 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. Ina
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

134 AMEBOID MOVEMENT

oraciliate. Most of the Oscillatoriaceae that are capable of move-
ment, consist of straight filaments; but two of the genera, Arthro-
spira 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 Oscilla-
toria 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 Arthro-
spira, 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 explana-
tion 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 stylo-
-nychia, 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 flate 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. Observa-
tion 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 dif-
ficult; 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.

AMEBOID MOVEMENT 135

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 de-
crease the amount of revolution faster than the amount of pro-
gression? 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 %4 to 2 body
lengths.

The effect of decreased temperature is therefore to retard for-
ward 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 tem-
perature the rate of forward movement was reduced and the ten-
dency 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 tempera-
ture of 8° C. changes of direction become very frequent. In

136 AMEBOID MOVEMENT

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 for-
ward 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 geomet-
rically 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 appro-
priate 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 organ-
ism. 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 move-
ment, 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, flagel-
lates, protophyta, swarm spores and zodspores 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 scat-
tered protophyta and swarm spores our knowledge of the move-

AMEBOID MOVEMENT 137

ments of spiral swimming organisms is of the most casual and
fragmentary sort. Nothing beyond the mere fact that these or-
ganisms describe some kind of a spiral swimming, is known.
That a spiralizing mechanism is probably also present in organ-
isms with highly developed equilibrating and orienting senses
would be the logical expectation from what has been said regard-
ing 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 num-
ber 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 (Fig-

UD) TUMP - A. L

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.

138 AMEBOID MOVEMENT

ures 45, 46). The spirals were right and left handed in the same
individual, and sometimes in the same experiment. In these ex-
periments 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

brush

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.

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 some-
thing or counts his paces, the curves are quite smooth.
Considerable unevenness of the ground has no effect on the
curvature of the spiral. Structural differences in the legs are also

» AMEBOID MOVEMENT . 139

without effect, for a person with one artificial leg walks quite as
smooth a spiral as one with two normal limbs.” .

From these experiments on man, it follows that there is a
“centre” in the central nervous system which automatically coor-
dinates and controls movement during locomotion and, partic-
ularly 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 im 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

7It should be added here that since this paragraph was written I have
been very fortunate to secure numerous records of paths swam by blind-
folded 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.

140 AMEBOID MOVEMENT —

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 zodspores, 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 dimen-
sions 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 rep-.
resentatives 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.®

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 unicel-
lulars; (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 move-
ments of these and all other moving organisms are controlled
by an automatic regulating mechanism, which is of essentially

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 vertbrates 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.

RD, (eRe eee

AMEBOID MOVEMENT gee se

similar nature in all organisms, as is indicated by the tendency
to spiralize the path. This mechanism, being automatic, abso-
lutely controls the direction of the path so long as outside inter-
ferences 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 un-
related 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 move-
ments 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 com-
parative 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 observa-
tions indicate that ectoplasm may suddenly and temporarily pass
into the gel state (Vallisneria). Streaming is therefore the funda-
mental 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

142

—

AMEBOID MOVEMENT 143

with the movements of the superficial films of Oscillatoria fila-
ments, diatoms, crawling euglenas, and probably also Gregari-
nidas, 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 solu-
tion 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 proto-
plasm 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 geo-
metrically related to the spiral paths of free-swimming organ-
isms 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 asymmetery of body shape, for

144 AMEBOID MOVEMENT

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 sup-
posing that the ancestors of our present ciliates, flagellates, roti-
fers, swarm spores, zoOspores, 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 regulat-
ing 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 orgdnisms 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 dis-
turbed 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, zodspores,
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
coordinates 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 mechan-
ism 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 profit-
able 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

AMEBOID MOVEMENT 145 -

mechanism is one that attaches to the fundamental structure of
protoplasm rather than to the gross morphology. As a mathe-
matical 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 Gregari-
nidas, because of the movements of its surface layer; to leuco-
cytes, 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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