_The General Features of Endoplasmic Streaming_
The streaming of the endoplasm is the most conspicuous feature of
ameboid movement. It is even more noticeable than the movement of the
pseudopods themselves, because of its greater speed and because it
occurs in all parts of the ameba. Its importance in movement is
essential, for no continued locomotion can be observed unless
accompanied by streaming. It may be profitable therefore to enquire into
the general features of streaming, and to observe some of the necessary
consequences streaming imposes upon such an animal as the ameba.
Let us take as an example an _Amoeba proteus_ (Pallas, ’66, emend.
Leidy, ’79, emend. Schaeffer, ’16) in characteristic movement (see
Figure 11, p. 37). The main streams of endoplasm are in the same
direction as that in which the ameba moves. In the withdrawing
pseudopods the current is, of course, toward the main mass of the ameba.
The endoplasmic stream is continuous from the posterior end to the tips
of the advancing pseudopods. The retracting pseudopods flow into the
main stream as tributaries. If, as often happens, the ameba is without
pseudopods, there is then a single stream arising in the posterior end
and flowing to the anterior end. In such a case it is readily observed
how absolutely dependent locomotion is upon endoplasmic streaming.
It often happens, such as when the ameba is receiving a strong stimulus,
that streaming is arrested and brought to a stop for a few seconds, more
or less. Presently however the endoplasm begins to flow as before. At
what point, in such a case, is the first movement of endoplasm
detectible? Is it at the free end of the pseudopod, at its middle
region, at its base, or at the posterior end of the ameba? Bütschli
(’80, p. 116) observed that in a withdrawing pseudopod the streaming
begins at the free end of the pseudopod; but his (’92, p. 201) later
explanation of ameboid movement seems to require that the endoplasm
must begin to move at the base of the withdrawing pseudopod. Jennings
(’04, p. 157) observed that in a withdrawing pseudopod the current of
endoplasm begins at the base of the pseudopod.
From numerous observations directed toward this point, it appears that
the conditions under which streaming is resumed after a pause, whether
in the same or in the reverse direction, are of great variety. The
shape, size, slenderness, and the position on the ameba of the
pseudopod, as well as the strength and character of the stimulus, are
among the factors capable of changing in whole or in part the flow of
endoplasm in a pseudopod. In an ameba that has been moving along a
homogeneous flat surface, as nearly unstimulated as may be, the
endoplasm first begins to flow out of the lower half of the retracting
pseudopod, if the pseudopod is more or less uniformly conical in shape
and rather slender. In such a case it may be said that the retracting
pseudopod was withdrawn “by the ameba,” and that it did not itself
receive an external stimulus producing retraction. If, however, the tip
of a pseudopod as described receives a strong negative stimulus, the
endoplasm frequently flows back from the tip while it is still flowing
into the pseudopod at the base. But very soon thereafter, in such an
event, the streaming becomes unified and the pseudopod is withdrawn. In
broad pseudopods about to be withdrawn, the endoplasm may begin to move
anywhere along its length. This is undoubtedly due to the continuous
local changes in the walls of the pseudopod, which are characteristic of
this species of ameba (see p. 20).
In an ameba which has been brought to a standstill, as by a sudden flash
of light, the first sign of recurring streaming is in the anterior half,
whether the original direction of streaming is resumed or reversed. If
the direction is reversed, the active pseudopods retract for a
considerable distance before a new one is projected. The endoplasmic
stream in a slender withdrawing pseudopod may not reach to the tip for
from several seconds to a minute, if the tip is slightly positively
stimulated. One may then observe ectoplasm streaming toward the tip and
toward the base, in the respective regions, at the same time, with
considerable fluctuation back and forth of the neutral zone separating
the two streams. The fate of such a pseudopod depends on its size, on
its position on the ameba, and the strength of the stimulus affecting it
and the rest of the ameba. That is, if the pseudopod is small or on the
posterior half of the ameba, or only slightly stimulated, it will be
retracted; but if it is large, or on the anterior end of the ameba, or
more strongly stimulated than the rest of the ameba, it may again become
active.
The fact that protoplasm is practically incompressible makes it clear
that if streaming can be observed to begin after a pause at some point
after it begins at others, the ectoplasmic walls of the ameba must give
way in the region where streaming begins. Since it has been established
by observation that the ectoplasm may give way at any point, it follows
that one of the principal factors affecting streaming is the elasticity
and liquefiability of the ectoplasm.
The streaming in an ameba is coordinated. The direction in which the
endoplasm flows in the several pseudopods, when there are no stimuli
received externally that produce visible changes in behavior, gives one
the impression that there is a “centre” controlling movement. The
several pseudopods do not act at all capriciously. The ameba seems to
move the pseudopods, not the pseudopods the ameba. If this impression of
coordination is correct, it is of the first importance in a study of
ameboid movement. Further on, this point will be taken up at length in
connection with the character of the path an externally unstimulated
ameba describes (p. 109); but there are certain observations which aid
in the analysis of the problem of coördination from the point of view of
the pseudopod, instead of that of the ameba as a whole, and to these
observations we may now direct our attention.
The mass of endoplasm within a pseudopod moves practically always in one
direction. In any cross-section of a pseudopod that is more or less
cylindrical in shape, the endoplasm in the center moves most rapidly,
that near to it less rapidly, while that near the ectoplasm moves very
slowly. One never observes a forward stream on one side of the pseudopod
and a backward stream on the other. Nor does one observe parallel
streams of endoplasm flowing in opposite directions within the same
[Illustration: Figure 1. Illustrating the various directions of
endoplasmic streaming in growing and retracting pseudopods. _a_, two
oppositely directed streams in a pseudopod, one directed toward the base
and the other toward the tip of the pseudopod, with a neutral zone
between. _b_, two streams flowing toward each other. Cases _c_ to _r_
are self explanatory. _s_, rotational currents observed occasionally in
various species of amebas. _t_, “fountain currents,” sometimes observed
in _Amoeba blattae_, and rarely in other forms. _u_ and _v_ represent
cases of streaming which have not been observed and which probably do
not occur. _w_, similar to _v_, but with a wide neutral zone between the
streams, represents an actual observed case. _m_ and _r_ probably occur
only very rarely; no such cases have been seen, but there seems to be no
reason why they do not sometimes occur. Excepting _m_, _u_, _r_ and _v_,
all these figures were drawn from observed cases of streaming.]
ectoplasmic tube, in an ameba of several pseudopods, excepting where
there is a wide zone of stationary endoplasm between the streams (Figure
1, _v_, _w_). But in “fountain currents,” such as Rhumbler (’98, p. 190)
described and figured for _Amoeba blattae_ Bütschli, and which may
readily be observed in most species of amebas if immersed in a solution
of gelatin thick enough to keep the amebas from sinking, there is a
central stream of endoplasm flowing forward, and a peripheral stream of
ectoplasm flowing backward, with a thin neutral zone between (Figure 29,
_d_). As we shall see further on, however, these fountain currents are
in principle the same as the currents observed in ordinary locomotion,
the apparent difference being due to the fact that there is no
locomotion. It is true, then, that within the same pseudopod at any
cross section the endoplasm always streams in one direction, and the
streaming is unified.
When new pseudopods are formed, or when old ones are retracted, and
especially when both these phenomena occur at the same time and close
together on a part of an older pseudopod, some of the details of
coordination in streaming are readily made out. In Figure 1 are shown a
number of observed cases of pseudopod formation and retraction, with the
direction of endoplasmic streams indicated at a given instant. For the
purpose of illustration, several (presumably) possible but unobserved
cases, _m_ and _r_, are sketched, and also two cases, _u_ and _v_, which
have not been observed and which probably do not occur. The general
conclusion to be drawn from these observations is that, while the
endoplasm in the body of an ameba as a whole may be streaming in several
different directions at any given instant, that is almost never the case
with an individual pseudopod, especially if the pseudopod is of small or
medium size and not too flat or otherwise irregular in shape. The
pseudopod is therefore the unit of coordinated protoplasmic streaming.
Another general observation which undoubtedly is connected in some way
with the problem of coordinated streaming is the following. In
externally unstimulated amebas, the new pseudopods are almost without
exception directed 60° or less from the direction in which the parent
pseudopods are moving.
It is a matter of common observation that an ameba may throw out a
pseudopod in any direction whatsoever when stimulated. The ameba may
reverse its direction of movement completely, or it may move in scores
of different directions at one time for awhile, if properly stimulated.
There is no restraint or limit imposed upon the ameba insofar as the
direction of movement is concerned. Why then should a great majority of
new pseudopods in an unstimulated ameba be projected at an angle of
approximately 60° to the parent pseudopod? It might seem at first sight
as if the merely physical aspect of the streaming would be a sufficient
explanation, in that less resistance would be met with in sending a
stream off at a small angle than at a large. But it is probable that
inertia plays no part in maintaining the direction of streaming (see p.
123, footnote, for further discussion). It requires perhaps more energy
for a pseudopod to flow off from the main stream at an angle of 120°
than at an angle of 30°. But it is plain that as many pseudopods are
withdrawn as are thrown out, and they are withdrawn at an angle against
the main stream of endoplasm in the ameba that is the complement of the
angle at which they were projected. Whatever energy might be saved
therefore in the projection of a new pseudopod at a small angle with the
main stream is lost in withdrawing the pseudopod against the stream at a
correspondingly large angle. It is clear therefore that the physics of
moving viscous fluids cannot solve the problem. It is probable that the
mechanism which controls the direction of locomotion as exemplified in
the wavy path of the ameba (see p. 109) is also involved in the
direction in which pseudopods are projected.
Some very interesting special cases of endoplasmic streaming are
observed during the process of feeding. As is well known, amebas capture
their food by the protoplasm flowing around it and engulfing it. If the
object is large the protoplasm may flow around it, in contact with it,
so that the shape of the object determines the direction in which the
enveloping protoplasm flows. If the object is small, particularly if it
is a live organism, the behavior of the ameba is quite different (Kepner
and Taliaferro, ’13, Schaeffer, ’16). To capture such a food object a
cup of protoplasm is gradually formed over it so as to imprison it
(Figure 2). If the food organism lies against some flat object, the food
cup is brought down to the surface of the object all around, thus making
escape impossible, before the protoplasm comes into contact with the
food organism. Schaeffer (’16, ’18) by experimental methods has shown
that the stimulus calling forth the formation of food cups as just
described, is the mechanical vibration of the water. At least the same
response was produced on the part
[Illustration: Figure 2. Endoplasmic streaming involved in the formation
of a typical food cup. _a_, the ameba is shown moving toward a live food
organism that is resting quietly on the bottom. _b_, the main pseudopod
forks, being the first indication that the feeding process has set in.
At _c_ the pseudopods have half-way surrounded the prey, but without
having come into contact with it. At _d_ the upper sheet of protoplasm,
_f_, (stippled), is flowing dome-like over the prey, while the
pseudopods continue to surround it. At _e_ the pseudopods have met and
fused with each other and the upper sheet of protoplasm has completely
covered the space encircled by the pseudopods, and has fused with the
pseudopods. _g_, sheets of protoplasm which are thrown out along the
_lower_ surface _under_ the prey, to form a floor to the food cup. Up to
stage _e_ the ameba has not come into physical contact with the prey,
but is just about to do so. With the completion of the floor of the food
cup, the process of feeding is completed.]
of the ameba when the ameba was carefully stimulated by means of very
fine clean glass needles. The conclusion is unavoidable therefore that
the shape of the food cup and the method of its formation is a racial
characteristic and is hereditary. The streaming endoplasm therefore,
upon suitable stimulation, takes on a definite form, that of a food cup.
This indicates again that the endoplasm is something more than the
ordinary fluids of physics, for out of an apparently structureless
fluid, organization is effected.
The fact that food cups are formed by amebas implies of course that
stimuli are received whose effect cannot be explained as a direct
physical reaction. Rhumbler (’10) has attempted to explain the formation
of food cups as the direct physical result of the stimulation by the
food body; but in recent experiments Schaeffer (’16) has shown that food
cups are formed over diffusing solutions of tyrosin, where the solutions
were quite as concentrated outside as inside the cup. These results
prove convincingly that the shape and size of the food cup are not
determined by direct action of the stimulating agent, but by hereditary
factors within the protoplasm of the ameba.
Other stimuli also affect streaming characteristically, though not so
strikingly perhaps as food stimuli. One of the most widely observed
effects on streaming is the momentary pause following stimulation of
many sorts. If an ameba that is moving along unstimulated externally,
suddenly comes near a food object, it frequently stops forward streaming
for about a second, and then begins again, usually at increased speed.
The ameba behaves as if it were startled. A similar reaction is observed
if a small perpendicular beam of light is flashed near the anterior end
of the ameba. Here also streaming is resumed with accelerated speed
toward the beam of light. Harrington and Leaming (’00) showed that if
strong light, especially at the blue end of the spectrum, is suddenly
thrown on the ameba, movement is arrested for a short time. Miss Hyman
(’17) has shown recently that if an ameba is strongly stimulated with a
glass needle, streaming is arrested momentarily, but the direction of
streaming when resumed subsequently, depends partly upon the former
direction of streaming and partly upon the location of the stimulus. All
of these cases of temporarily arrested movement are strikingly similar
to what is observed in the higher animals under similar conditions.
The ingestion of a large food mass produces usually a marked change in
streaming. A more or less spherical form is assumed, and if the food
mass be a live organism such as a large ciliate, the ameba frequently
remains quiet for a considerable interval. If a large amount of food is
eaten, as for example a dozen or two colpidia, the ameba may suspend
concerted streaming for an hour or more. During this time small
pseudopods are projected here and there, but there is no locomotion. But
if an ameba eats large masses of carmine, there is usually no pause
following ingestion, and the same thing is true when the ameba is
induced to eat bits of glass and other indigestible substances. It
follows therefore that the interrupted streaming of the endoplasm due to
feeding is not caused by the act of ingestion as such, but rather by the
onset and continuance of the normal digestive processes on a large
scale. These reactions are again strikingly similar to what is observed
in many vertebrates, in which a more or less definite body sense, whose
sense organs are in the splanchnic region, is supposed to be involved;
but what the explanation of similar behavior in ameba is, is not at all
clear.
Another factor of great importance in endoplasmic streaming is the
nucleus. It was observed by Hofer (’90) that amebas lacking nuclei did
not move in a coordinated manner. Štolc (’10) however records a number
of observations in which characteristic movement was observed in
enucleate amebas ten or more days after the enucleate ameba had been cut
off from a normal ameba. Hofer’s amebas died after nine or ten days,
while Štolc’s remained alive, some of them for over thirty days.
Recently Willis (’16) confirmed Hofer’s findings, but does not discuss
Štolc’s results.
The cutting of an ameba into two pieces, one with and the other without
a nucleus, is a very simple operation. It is also very easy to observe
that within an hour or so the enucleate ameba does not move normally,
and that there is no concerted endoplasmic streaming while the nucleate
ameba seems to behave normally. But Štolc’s contention that enucleate
amebas move characteristically (l. c., p. 159, 160, 167) is not
necessarily contradicted by these observations, for Štolc’s observations
refer to amebas that lived much longer than the enucleate amebas of
Hofer and of Willis. Even if an enucleate ameba is able to recover,
after many days, its power of concerted movement, there can be no doubt
that enucleate amebas do not move characteristically for a short time
after the operation, and that this effect is due to the lack of a
nucleus.
Very likely the action of the nucleus on the locomotory processes is
neither direct nor specific. The metabolic balance must be disturbed by
so fundamental an operation as the removal of the nucleus, and all
fundamental activities must in consequence be affected. That food
organisms (chilomonas and coleps) may be eaten and digested as Štolc
(’10) states, indicates however that the metabolic balance may after a
time be regained in some degree, for feeding undoubtedly calls for
concerted streaming, and digestion for the formation and transfer of
enzymes. Until this point receives further attention therefore, it
remains unknown in what way the removal of the nucleus disturbs
streaming for some time after the operation; but of the fact that
streaming is disorganized for some time, there can be no doubt.