THE SURFACE LAYER AND THEORIES OF AMEBOID MOVEMENT
The observations recorded in the preceding two chapters, while they do
not tell us anything about the direct cause of the movement of the
surface layer, nevertheless indicate clearly enough that the area where
ectoplasm is made is the area toward which the surface film flows. There
is no question therefore of the intimate relation between the
transformation of endoplasm into ectoplasm and the movement of the
surface layer.
The apparent absence of movement in the surface film of the pseudopods
of _Difflugia_ (Schaeffer, ’17) and the definitely proved absence of
movement in the surface layer in the foraminiferan _Biomyxa_ and
myxomycete plasmodia, where no ectoplasm is formed in the manner
observed in amebas, also indicates a causal relation between movement of
the surface layer and ectoplasm formation. The relation moreover seems
to be a necessary one for the movement of the surface layer is contrary
to the processes involved in locomotion. In other words, from the
standpoint of the ameba, it is a “necessary evil,” so far as locomotion
is concerned.
The transformation of endoplasm into ectoplasm is unfortunately not
understood, though from what we know in a general way of the behavior of
colloidal solutions it seems to be a surface tension effect due to (or
accompanying) a change of phase. Something akin to gelation occurs as
Kite (’13) has shown. It is a problem in the chemistry of colloids. But
the structure or composition of the protoplasm is complex and
practically unknown, and it is quite open to criticism whether analogies
from the behavior of pure solutions of colloids, such as gelatin, afford
any real basis for an explanation.
Although a knowledge of the movements of the surface layer is
interesting enough by itself, it will achieve its true importance only
when it can be related to other processes in the ameba in a causal
manner. It does not at present give us any greater insight into the
ultimate cause of ameboid movement, although it is clear that an
important step in this direction has been taken. But no theory of
ameboid movement can be accepted that demands conditions in the ameba
that are contrary to those described in the preceding chapters, in
connection with the surface layer. From this point of view therefore the
discovery of the true nature of the outside surface of the ameba is of
importance, for it widens to a very considerable extent the
observational basis with which any theory of ameboid movement must
conform. Since the properties of the outer layer are here described in
detail for the first time, it becomes necessary to enquire to what
extent the more commonly held theories of ameboid movement conform with
the observed behavior of the surface film. Although the surface tension
theory was the first detailed theory proposed toward an explanation of
ameboid movement, I shall discuss Jennings’ (’04) observations on the
movements of the ameba first, because a great part of his work deals
with the movements of the surface film, although he did not recognize it
as distinct from the ectoplasm in its movements.
It is generally recognized that Dellinger’s (’06) work proved that
Jennings’ conception of the ectoplasm as a permanent skin in which the
ameba rolls along, is probably inadequate for such amebas as _proteus_;
though singularly enough it is still supposed that _verrucosa_ and its
congeners move in the way described by Jennings (Hyman, ’17, p. 83).
Jennings (’04) describes the movements of amebas, both _proteus_ and
_verrucosa_ “types,” as follows:
“In an advancing Amoeba substance flows forward on the upper surface,
rolls over at the anterior edge, coming into contact with the
substratum, then remaining quiet until the body of the ameba has passed
over it. It then moves upward at the posterior end, and forward again on
the upper surface, continuing in rotation as long as the ameba continues
to progress. The motion of the upper surface is congruent with that of
the endosarc, the two forming a single stream (p. 148).
“We have demonstrated above, for Amoeba at least, that the forward
movement is not confined to a thin outer layer, but extends from the
outer surface to the endosarc; in other words that the outer surface
moves in continuity with the internal substance (p. 150).
“There is no regular transformation of endosarc into ectosarc at the
anterior end. On the contrary the ectosarc here retains its continuity
unbroken, moving across the anterior end in the same manner as across
other parts of the body. In the same way the ectosarc is not regularly
transformed into endosarc in the hinder part of the body.... Such
transformation is by no means a regular accompaniment of locomotion” (p.
174).
According to Jennings, locomotion is aided by the projection of waves of
hyaloplasm at the anterior edge, “an active movement of the protoplasm
of a sort which has not been physically explained.” These waves,
attaching themselves to the substratum, enable the ameba to pull itself
along by a rolling movement as described in the quotation above.
As to the rate of movement of the outer surface as compared with that of
the endoplasm, Jennings concluded:
“The direction of movement of particles on the outer surface is the same
as that of the underlying particles of endosarc. The rate is also about
the same as for the endosarc, though often, or perhaps usually, the
outer particles move a little more slowly than those in the endosarc”
(p. 142).
In view of the observations recorded in the preceding pages it is clear
that Jennings’ statement that substance after moving forward on the
upper surface, rolls over the anterior edge is quite erroneous. The
attached particles, if heavy, may do so, but the surface film itself
does not. It is, on the contrary, taken into the interior at the
anterior edge.
The statement that the movement of the outer surface is congruent with
that of the ectoplasm can likewise not be substantiated by observation,
as has been demonstrated in the preceding pages. It is difficult to
distinguish between the ectoplasm and the surface layer in such amebas
as _sphaeronucleosus_ and _verrucosa_, for there are no large crystals
or other bodies which get caught in the ectoplasm as it is formed from
endoplasm at the anterior end. But attentive observation will
demonstrate very definitely that the ectoplasm here is stationary to the
same degree as in _proteus_. The stationary properties of the ectoplasm
are however not properly a matter for discussion; for five minutes’
observation of a _proteus_, _discoides_, _annulata_, particularly a
_laureata_, under 300 diameters magnification, will convince anyone that
the ectoplasm is stationary while the surface film, with attached
particles, moves over it. No one can possibly come to any other
conclusion. Jennings’ conclusion was due undoubtedly to an error of
observation.
Jennings’ statement that the rate of movement of the outer surface is
the same as that of the endoplasm (p. 142) when taken in connection with
his other statement that the ectoplasm is a more or less permanent skin,
presents a mechanical impossibility; for unless the outer surface moves
_twice as fast_ as the endoplasm, no rolling movement would be possible.
Several of Jennings’ figures (especially Figures 38, 39, and 41)
indicate in fact that he conceived of the outer surface as moving faster
than the ameba advances, or that the upper surface moves _over the
ameba_ as the ameba moves over the substrate. Jennings’ theory requires
that the surface layer move twice as fast as the ameba advances. Hyman
(’17) also makes a similar mistake in referring to the rate of movement
of the outer surface (p. 85).
Lest the discussion of this point be suspected of being merely
verbalistic, it should be recalled that the surface layer of _proteus_
often moves at about the _same rate_ as the ameba; that the surface
layer of _discoides_ moves about _twice as fast_ as the ameba; that the
surface layer of _verrucosa_ and _sphaeronucleosus_ moves about _three
times as fast_ as the ameba; and that the ectoplasm does not move at
all. It is of course incumbent on one to discuss what is stated; one is
not at liberty to select one of several possible interpretations.
To illustrate this point graphically so as to avoid as far as possible
future confusion Figure 29 is appended. In _a_ is shown a particle
traveling on an ameba at the same rate of speed as the ameba; at _b_ is
shown a particle that moves twice as fast as the ameba; at _c_ the
attached heavy particle does not move at all. For the sake of
completeness _d_, Figure 29, is added here. It illustrates the backward
moving ectoplasm in an ameba that is suspended in a jelly medium that
prevents the ameba from sinking to the bottom. It must be admitted that
in thus considering the rate of movement of the various tissues of the
ameba from a single standpoint, a point outside of the ameba, little
room is left for confusion.
[Illustration: Figure 29. _a_, a particle attached to an ameba and
moving at the same rate as the ameba. This condition is often observed
in _proteus_ where the surface film, owing to its destruction during the
formation of the longitudinal ridges, retards the forward movement of
this layer. _b_, a particle attached to the surface film of an ameba
moving twice as fast as the ameba. This condition is seen in
_discoides_, _verrucosa_, _sphaeronucleosus_, etc. _c_, a particle on an
ameba that does not move at all although the ameba does. This is seen
when a heavy particle is laid on an ameba, too heavy for the surface
film to move. _d_, movement of ectoplasm in an ameba suspended in a
jelly medium. The vertical lines are to be considered as stationary.]
There is comparatively little friction, if any at all, between the upper
surface and the endosarc, according to Jennings’ view, since both these
layers move at the same rate and as a single stream. On the other hand
there must be very considerable friction between the endoplasm and the
lower ectoplasm, which does not move at all. This difference in the
amount of friction must show itself in the different speeds of the
endoplasm near the upper ectoplasm and near the lower ectoplasm.
Observation indicates however that the most rapid streaming of the
endoplasm is in the middle of the ameba or pseudopod and that it
gradually becomes slower as the ectoplasm is approached on _all sides_.
We said above that if the ectoplasm were a more or less permanent skin
in which the ameba rolled as described by Jennings, the upper surface
(=ectosarc, Jennings), according to a well known mechanical principle,
would have to move ahead about twice as fast as the ameba advances. Now
the upper surface of _sphaeronucleosus_ and of _verrucosa_ in
locomotion was found to move from three to three and a half times as
fast as the ameba (Chapter VII). In discussing movement in “_verrucosa_
and its relatives” Jennings says “the essential features of the movement
seem to be (1) the advance of a wave from the upper surface at the
anterior edge; (2) the pull exercised by this wave on the remainder of
the upper surface of the body, bringing it forward. Most of the other
phenomena follow as consequences of these two” (p. 146). Thus the amount
of _stretch_ of the upper surface would exceed the amount of _pull_ on
it from 50% to 75%!
Jennings’ explanation of ameboid movement in which the important factor
is a more or less permanent ectoplasm in which the ameba rolls along,
would unquestionably produce rotary currents. Rhumbler (’98) recognized
this and after full consideration rejected the idea that the ectoplasm
is a permanent skin in which the ameba rolls along in locomotion,
because rotary currents are not observed in a moving ameba. Anyone who
doubts that rotary currents would be produced under these conditions can
convince himself by putting a quantity of glycerine and some shavings in
a large transparent rubber balloon or celloidin bag and letting it roll
slowly down an incline in front of a strong light. If not too much
glycerine is placed in the balloon, the shape of an ameba is closely
enough approximated, and the rotary currents--down at the posterior and
up at the anterior end--are well shown.
From all these considerations it is quite clear that Jennings’
explanation of ameboid movement as a rolling movement can not any longer
be maintained. His “discussion of this matter (the rolling movement
hypothesis) is an excellent example of the fact that acumen and
excellent reasoning may lead one astray in scientific matters when the
observational basis for the reasoning is not secure.” (These are
Jennings’ own words in criticism of Rhumbler on the same subject!)
The surface tension theory, with its many modifications, has had a great
many more adherents than any other theory that has been advanced to
explain ameboid movement. It represents the attempt of biologists to
explain a vital phenomenon on physical grounds. The fact that it has
been held to go further in this direction than any other, and the fact
of its greater simplicity, doubtless are responsible for its wider
acceptance. The recent criticism to which this theory has been
subjected, however, indicates clearly enough that this theory does not
really give a very adequate idea of the processes involved in ameboid
movement after all, and in so far as feeding processes are concerned,
the theory does not seem to apply at all according to Schaeffer (’16,
’17). But it could hardly have been anything more than excellent
guesswork if the surface tension theory _as advanced_ by a number of
writers had been found adequate, for the observational basis was very
narrow, as the preceding pages have shown, and as the succeeding pages
further show.
Not anything like a complete historical account of this theory with its
numerous modifications will be attempted here. It would be a large
undertaking, for nearly every biologist and biochemist has expressed
himself on the subject. It does not appear that much is gained by merely
recording the opinions, even of biologists, unless they are based on
experimental or observational data, preferably their own. Scientific
questions are not decided by ballot vote, and it is not apparent what
value such a record of opinions would have except the doubtful one of
showing whether the persons involved declared for or against the surface
tension theory. Moreover such an account would not be interesting
reading for those who want to know first of all what amebas can do. Only
the more important modifications of the surface tension theory as
applied to ameboid movements will therefore be discussed and these
modifications will be considered important in proportion to the amount
of observation or experiment on which they are based.
Attention has already been called in Chapter II to Berthold’s (’86)
theory of ameboid movement, which was the first attempt to explain this
phenomenon on physical principles. As will be remembered, Berthold
thought that the nature of the ameba’s immediate environment determined
when and in what direction it should move, the source of the energy of
movement being supposed to be a decrease in the tension of the surface
film of the ameba, brought about by some factor in the ameba’s immediate
environment.
One of the most elaborate attempts that has been made toward explaining
ameboid movement on the basis of surface tension phenomena was that of
Bütschli (’92). From his extensive knowledge of the lower organisms,
especially the protozoa, he concluded that protoplasm is an emulsion of
two fluids: a more concentrated “plasma,” insoluble in water; and a
thinner fluid, “enchylema.” Ameboid movement was brought about by
migration of enchylema droplets to the surface of the ameba at the
anterior end, where they burst and spread over the surface, lowering its
tension. The effect of this change in tension was held to be a flowing
backward of the surface of the ameba and a flowing forward of the
endoplasm. This is what happens in a drop of fluid, such as oil, on
water to one side of which is brought a soapy solution. Bütschli
described many experiments with fluids on which the surface tension was
changed by appropriate means to simulate the process of movement. After
Bütschli had developed his surface tension theory of movement, he
discovered, as has already been noted, that in a pelomyxa the surface
layer moves forward instead of backward as required by the surface
tension theory. In spite of this however he still maintained that his
theory of movement could be modified to apply to amebas generally,
although so far as I have been able to find, he did not then or
subsequently state how. From this we may infer that Bütschli himself
probably concluded that the surface tension theory of movement as he
developed it, is not of general application or is nothing more than a
step in the development of such a theory.
Rhumbler has written a number of papers on the mechanics of ameboid
movement, most of which are concerned with elaborations and
modifications of a surface tension theory very similar to Bütschli’s.
Rhumbler published a general outline of his theory in 1898. The
transformation of endoplasm into ectoplasm at the anterior end, and the
reverse process at the posterior end, was stated to be an important part
of his theory of movement, but just how this was necessary to surface
tension effects was not explained in physical terms. Feeding was assumed
to be caused by the direct action of the food body on the surface layer
(ectoplasm) of the ameba. The presence of the food body, he held,
produced a lowering of the surface tension of the ameba thus causing the
ameba to flow around it (’98, p. 207). Subsequently, however, he (’14)
came to the conclusion that many amebas cannot have fluid surfaces as
usually understood, since they do not spread as a film over water when
they come into contact with the surface. From this and other
observations Rhumbler concluded (’14, pp. 501-514) that the surfaces of
amebas are not to be compared with surface tension films on drops of
inert simple fluids; but with the surface films of emulsions which take
on the properties of a solid. Since the question of ameboid movement is
not especially discussed in this later paper, it may be assumed that in
this respect his (’98, ’10) earlier views have not been materially
modified. Rhumbler has suggested a great many physical models for the
explanation of various ameboid activities such as feeding, defecation,
movement and so forth.
In general agreement with Bütschli and Rhumbler were Verworn (’92),
Blochmann (’94), Bernstein (’00), Jensen (’01, ’02), and recently
Hirschfeld (’09) and McClendon (’12). All these authors held that
ameboid movement is a surface tension phenomenon. The application of the
surface tension theory in explaining ameboid movement demands a fluid
surface and a fluid interior and it is perhaps unnecessary to add that
Bütschli, Rhumbler and the others mentioned held that the protoplasm is
fluid. The question as to whether protoplasm is a fluid or possessed of
an internal structure was however hotly debated and we find Fleming
(’96), Heidenhain (’98), Klemensievicz (’98), Dellinger (’06) and others
opposing the group of authors just mentioned, by contending that the
streaming protoplasm must have some kind of structure. This question no
longer concerns us however, owing to our rapidly increasing knowledge of
colloidal solutions, for it is undoubtedly correct to hold that
protoplasm is colloidal.
We have already insisted (p. 46) that the problem of ameboid movement is
made more difficult by narrowing it down to the movements of ameba, and
that to see the problem in its fullest aspect requires consideration of
streaming protoplasm wherever found. Now it happens that there is in
certain respects greater diversity of streaming to be found in plant
cells than in animal cells, and it is not surprising therefore that
explanations of streaming and ameboid movement have taken a different
direction among botanists than among zoologists. It is for this reason
doubtless that Ewart (’03), while espousing the surface tension theory
as explaining streaming, does not look to the superficial surface of a
plant cell as the source of the necessary energy, but to the interior of
the protoplasm. This idea is, of course not entirely original with
Ewart, for Bütschli, as we saw, believed that protoplasm has an emulsoid
structure; but according to Bütschli’s hypothesis, the surface forces
were not brought into play in movement until the droplets of enchylema
spread over the surface and so reduced the tension. Ewart, however
points out that there is very much more surface energy present in the
interior of streaming protoplasm than is required for all the movements
known to protoplasm, including muscular contraction. According to
Ewart’s hypothesis the emulsion globules (disperse phase) have their
surface tension lowered at corresponding points by electrical currents
traversing the endoplasm, the electrical currents themselves originating
in chemical actions.
While all available evidence from the study of colloidal solutions and
from observation from protoplasm confirms Ewart’s statement that more
than sufficient energy is available in the interior of colloids for all
purposes of movement, there is little or no evidence that the proper
electrical currents are present to release or transform the surface
energy into that of movement. This step in his explanation is therefore
highly hypothetical and at present unconvincing. Moreover, this step in
the theory would not be applicable to streaming as observed in amebas,
without very considerable modification.
Recently Hyman (’17) has developed the surface tension theory of
movement in the direction indicated by Ewart. The motive power is
supposed to have its source in the contractility of the ectoplasm. The
endoplasm is held to be a passive stream, not an active stream as Ewart
supposed to be the case in plant cells. The power of contractility is
held to be due to the process of gelation of endoplasm into ectoplasm,
which is due to a change of phase, the fluid part of the endoplasm
becoming dispersed and thereby developing surface energy in proportion
as the amount of surface of the fluid is increased. This increase of
surface produces the phenomenon of contractility.
Miss Hyman is wrong however when she says (p. 90) that the withdrawal
and contraction of pseudopods are processes of gelation. This is clearly
a physical impossibility, for the ectoplasm of the withdrawing pseudopod
must become liquified into endoplasm, before it can be withdrawn. All
writers excepting Jennings and Hyman are agreed on the continual
transformation of ectoplasm into endoplasm at the posterior end while
the reverse process goes on at the anterior end; and Hyman herself
states (p. 89) that new ectoplasm is formed as the growing pseudopods
extend into the water. So there must be liquefaction of the ectoplasm in
withdrawing pseudopods, or very soon the whole ameba would be
transformed into ectoplasm. As was shown in the preceding pages,
liquefaction of the ectoplasm at the posterior end goes on at the same
rate as gelation of the endoplasm at the anterior end. But at another
place Hyman says:
“In fact according to Jennings, Dellinger, Gruber, and Schaeffer the
surface of the ectoplasm actually flows forward at about the same rate
as the forward advance, and this indicates that the advancing ectoplasm
at the tip of the pseudopodium is derived from the surface ectoplasm and
not from a transformation of endoplasm into ectoplasm at the end of the
pseudopodium as Rhumbler supposed” (p. 89).
This quotation is not strictly accurate. Jennings says: “The
pseudopodium grows chiefly from the base, so that any part of the
surface retains nearly its original distance from the tip” (p. 156).
Dellinger in a general way confirmed Jennings’ conclusions. Gruber
concluded that the outer layer was gelatinous, not protoplasmic.
Schaeffer held the third layer to be extremely thin, “too thin to be
seen easily,” so it is impossible that the ectoplasm at the tip of a
pseudopod, the thickness of which is readily seen, can be derived from
the surface film.
The main conclusion however in Miss Hyman’s paper is that there exists a
metabolic gradient in the pseudopods of advancing amebas, the highest
rate of metabolism being at the tip and the lowest at the base, for any
one pseudopod. This conclusion is bound to be of the first importance in
the explanation of ameboid movement. It will give our first real
insight into the chemistry of ameboid movement. The fact that her method
of demonstrating gradients has yielded uniform results in the hands of
Child (’15), who originated it, as well as in her own when applied to a
great many different organisms, entitles her conclusions to careful
examination.
[Illustration: Figure 30. Disintegration of an ameba in ¼ molecular KNC.
After Hyman. _a_, ameba flowing in the direction of the arrow. _b_, the
ameba has abandoned pseudopod 1 and flows into pseudopod 2, which has
become reactivated. The ameba was exposed to KNC at this stage and, as
is usual in such experiments, the posterior end at _x_ becomes active.
_c_, the youngest pseudopod, at _x_, disintegrated first. _d_, the next
youngest pseudopod, 2, disintegrated next. Pseudopod 1, the oldest,
disintegrated last.]
Of the observations there can be no doubt, for in many details earlier
observations are confirmed. Her figures show that the tips of the
pseudopods disintegrate first in the potassium cyanide solution and
later the regions further back (Figure 30). The question is, what causes
the gradient of disintegration, which Miss Hyman takes to represent also
a metabolic gradient? Where is the gradient located: in the ectoplasm or
in the endoplasm; or is the gelation process synonymous with the
metabolism that gives rise to the observed gradient? Miss Hyman does not
say; but it cannot be in the endoplasm, for it is in motion along the
whole pseudopod at about the same rate and it undergoes a demonstrable
and visible change only at the anterior end of the pseudopod. While
metabolic changes might be higher at the free end of the pseudopod,
therefore, there would not be a gradient from there on back. No recorded
observations on the endoplasm along the length of a pseudopod can be
arranged so as to form a gradient which would suggest a similar
gradient in metabolic rate; and if the endoplasm is a passively moved
fluid as Hyman’s theory seems to imply, a metabolic gradient would seem
to be precluded.
In the ectoplasm however there exists a time gradient; that at the base
of a pseudopod is older than that near the tip, and observation
generally tends to confirm the view that the older it is the firmer it
becomes. This gradient in the amount or extent of gelation corresponds
with the disintegration gradient of cyanide along a forming pseudopod.
That is, the rate of disintegration is proportional to the age of the
ectoplasm. There is however no good evidence that the age of ectoplasm
corresponds to the rate of metabolism, so that the younger the ectoplasm
is the higher will be the metabolic rate in it. The following statement
seems to bear this out: “As soon as the pseudopodium extends into the
water its surfaces gelatinizes because of contact with the water”
(Hyman, ’17, p. 89). Gelation is, according to Hyman, a passive process
and therefore not distinctively metabolic. She continues: “It is
necessary therefore for the continuous production of a pseudopodium,
that the metabolic change which is the cause of the liquefaction should
continue to occur at the pseudopodial tip. There is thus produced the
metabolic gradient along the pseudopodium which I have described....”
But if the metabolic gradient is bound up with the process of
liquefaction, it is difficult to see how there can be a _gradient_ along
the pseudopod, for liquefaction takes place only at the tip, according
to her own statement. As a matter of fact, however, _gelation_ is
constantly occurring at the tip of the pseudopod and to a less degree
back along the sides of the pseudopod. Liquefaction occurs only at the
posterior end of the ameba in orderly movement.
We must conclude therefore that while Hyman’s data are of the first
importance in contributing to the structure and behavior of the ameba,
her contention that a metabolic gradient is demonstrated in the ameba is
not convincing.
From this short account of the main theories that have been advanced to
explain ameboid movement it appears that of the modern theories the only
one that has been capable of adjusting itself to new investigations and
observations is the surface tension theory. The earlier theories under
this head were mistaken however in looking to the superficial film of
the ameba as the source of energy. But with the increase in knowledge of
the chemistry of colloids, the source of the surface energy came to be
located in the interfaces between the phases of the colloidal system. As
has already been remarked, there is more than sufficient free energy
here to account for all the movements observed in protoplasm; there
remains the problem of explaining how the surface energy is transformed
into that of movement. As Graham (’61) remarked: “The colloidal is in
fact a dynamical state of matter. The colloid possesses _energia_. It
may be looked upon as the probable primary source of the force (energy)
appearing in the phenomena of vitality.”
Now, viewing streaming wherever it occurs in the protoplasm of animals
or plant cells the surface tension theory, as far as observations
permit, applies to the various conditions of streaming as follows.
In the first place we shall begin with the assumption that is generally
held, that protoplasm is a reversible colloidal solution consisting
mainly of proteins, with some carbohydrates, lipoids, etc., on the one
hand and water on the other. Its reversibility consists of course in
being able to change from a sol to a gel state and the reverse, the
water being in the disperse phase in the gel state. The consistency of
the protoplasm therefore depends upon two factors: upon the amount of
water present, and upon the degree of its dispersion; the smaller the
droplets the more solid will be the gel because of the increase in
surface of the mass. Colloids exhibit the property of contractility in
proportion as the droplets of water are decreased in size; or, which
amounts to the same thing, in proportion as the amount of the surface of
the water is increased. It appears as if the source of energy of
contractility was the free energy in the surface films of the internal
phase of the gel.
Taking the amebas as a group and applying these principles of colloidal
solutions, we find that we can arrange the amebas in a series of four or
more grades representing differences of fluidity of the protoplasm.
Among the most fluid are _Amoeba limicola_ and _Pelomyxa schiedti_; in
the next group, with less fluid protoplasm is _Amoeba dubia_; in the
third group is _A. proteus_ and _A. discoides_; in the fourth group,
with the least fluid protoplasm, come _A. radiosa_ and _A. verrucosa_.
These groups represent a progressive increase in the amount of ectoplasm
in proportion to the endoplasm. There being less water present in the
higher groups than in the lower, which follows from a stiffer endoplasm,
it is possible for them to form endoplasm, that is, to change phase,
more readily. And as a corollary to this we may add that more pseudopods
are formed, since ectoplasm can be formed more readily. (The _verrucosa_
types possess very stiff ectoplasm, and they increase their surface by
flattening out and by forming longitudinal ridges. They cannot for some
unknown reason form pseudopods). Again with the increase in the
consistency of the protoplasm, the pseudopods become more slender (and
stiffer) and more contractile, the most slender pseudopods (_radiosa_,
_flagellipodia_) being very much more contractile than the larger ones
of _proteus_ or _discoides_, for example. An additional factor operates
here, however, for some of the slender pseudopods as of _radiosa_ and
_bilzi_ are static and for a great part of their existence practically
incapable of contraction. The high development of contractility follows,
of course, from the high degree of dispersion of the internal phase in
ectoplasm, of which these pseudopods almost wholly consist. Thus, many,
if not most, of the more generalized peculiarities of form of amebas may
be traced to the amount of water in the protoplasm.
The number of pseudopods in an ameba is an important factor in its
method of locomotion, as may readily be perceived. Since amebas
generally move with great variation in speed as one compares the
different species, whether they form very little ectoplasm or very much,
and are able to maintain themselves on their paths, it follows that
ectoplasm formation by itself does not play an important part in
originating movement. But it requires only a few minutes’ observation to
see that ectoplasm is necessary to guide the ameba, so to speak, and to
make the endoplasmic stream effective for the purpose of orderly
movement. It requires very little imagination to see what would happen
if no ectoplasm were present in a _limicola_ or any very fluid ameba.
Streaming would undoubtedly occur as before, but the currents would be
rotational and irregular and no progression could take place. The
ectoplasm furnishes just that stiff tube against which the backward
action of the endoplasm can impinge so to speak in order to enable it to
flow forward. The ectoplasm is essential for orderly movement forward,
but it is not essential for streaming.
But this does not imply that the contractile power of the ectoplasm may
not be used to aid in propelling the endoplasm in streaming. It has been
demonstrated by Miss Hyman (’17) that the ectoplasm is actually
contractile when the ameba is strongly stimulated all over its exterior
by a solution of potassium cyanide. While this proves only the
contractile powers of the ectoplasm under exceptional conditions, and
when at rest, it is not impossible that under ordinary conditions of
locomotion it may aid in streaming. There is however one observation
which may, upon further investigation, negative this possibility.
Frequently in a pseudopod about to be retracted some of the endoplasm
flows toward the tip while the rest flows toward the base (Figure 1, p.
11).
One more point needs mention in this connection, and that is the small
waves of clear protoplasm which are thrown out by many amebas at their
anterior ends during locomotion. They are especially prominent in _A.
bigemma_ (Figure 7) and in _radiosa_ (Figure 8), but they are formed in
perhaps all species. Observation does not indicate that they move in
exactly the same way as the main body of the endoplasm, even if the
larger granules could be left out of account. They behave more like the
clear pseudopods of _Difflugia_ and _Arcella_ and the foraminifera.
Although these waves are frequently not to be seen during locomotion in
_Amoeba proteus_ and other large amebas, particularly in _Pelomyxa
palustris_ and _P. belevskii_, it is possible that the wave forming
process has become indistinguishably merged with endoplasmic streaming.
It is not impossible that the projection of these waves is the purest
expression of ameboid movement. But on account of their small size and
transparency, it is very much more difficult to investigate them than
streaming of the granular endoplasm, as it is observed in amebas,
ciliates and plant cells. It seems to be true however that streaming can
occur in the entire absence of these waves, so their importance in
ameboid movement is probably secondary.