THE WAVY PATH OF THE AMEBA
In the preceding chapters we discussed the various factors which
characterize ameboid movement: the streaming of the endoplasm, the
formation of ectoplasm, and the behavior of the surface film. The
discussion has involved only momentary cross-sections of the life of an
ameba, following the method of investigation in general use for the
solving of problems connected with ameboid movement. It has been tacitly
assumed that if one could explain ameboid movement at any particular
cross-section in time, one understood the whole process of ameboid
movement no matter how long it continued, excepting, of course, the
action of various kinds of stimuli that produced changes in direction,
speed, etc., of streaming. It was not assumed that time was an element
in the practical sense in the explanation of locomotion. A few seconds’
or a few minutes’ comprehensive observation was supposed to furnish
sufficient basis for an explanation.
Sometime ago I discovered however that the path of an ameba as it moves
over a flat surface free from particles possesses character; it is not
an aimless irregular zigzagging here and there, such as has been
generally supposed, and in occasional instances asserted, to be the
case. On the contrary, the path of an ameba during the course of an hour
or two consists of a succession of gentle right and left-hand curves
alternating with each other. The general appearance of the path is that
of a flattened spiral. Having observed a part of an ameba’s path,
therefore, one can predict with considerable accuracy in what direction
the ameba will continue to move. Thus that scientific bugaboo “Random
Movement” is evicted from that strongest of his strongholds, the aimless
wanderings of the ameba.
The mechanism producing the sinuosities in the path of the amebas is
easily disturbed by external or internal stimulation of various sorts,
resembling in this respect the spiral path of a paramecium, which is
also easily changed by the presence of various solid and dissolved
substances in the culture medium. But the mechanism controlling the
direction of the path of an ameba is apparently much more delicate than
that in paramecium, for it is only occasionally that a considerable
succession of regular sinuosities are described by an ameba in moving
over a flat surface. On the other hand, a few fair curves are found in
the path of practically every ameba if carefully observed for an hour or
more under favorable conditions.
To observe the path an ameba describes in moving over a flat surface,
the following conditions must be fulfilled. One must have a small glass
dish with a flat bottom, polished preferably, but not necessarily, of
the size of a small petri dish, but square so as to fit into a
mechanical stage. The dish should be filled with culture fluid free from
solid particles. Centrifuging the culture medium, or dialyzing distilled
water in the culture medium, will yield a satisfactory medium. It is
only by experience that one can pick out an ameba that seems to be in an
optimum condition for this purpose, that is, free from strong internal
stimuli, such as those from a large mass of food, etc. Just as we speak
of “clean-limbed” athletes, meaning thereby a high degree of muscular
coördination, so one who has worked with these animals for some time
acquires the capacity to pick out “clean-limbed” amebas; though how
these differ from others is just as impossible to describe adequately as
to tell what a clean-limbed athlete is. But having selected two or three
amebas that move in a well coördinated manner and passed them through
two or three changes of water free from particles, they are placed in
the middle of a dish and allowed to remain for ten or fifteen minutes
before observations are begun. A small shade should be placed in front
of the dish if very strong light can reach it. It does not matter if
diffuse light reaches the dish. A camera lucida with its appurtenances
is absolutely essential. In addition to the ordinary precautions the
edge of the paper must be laid parallel with the side of the mechanical
stage, for a number of sheets of paper will have to be used up in the
course of an hour or two and these must be pasted together properly to
reconstruct the path. The best magnification shows the ameba two to five
cm. long on the paper. Drawings should be made quickly but carefully,
beginning and ending with the posterior end of the ameba.
One of the best examples of the sinuous path of an ameba is shown in
Figure 33. It is the path of an _Amoeba bigemma_ from a natural out-door
culture. The observations were made under the conditions outlined above.
The temperature, which is an important factor, was unfortunately not
recorded, but it was about 28° C.
Practically the whole of the path of this ameba consists of right and
left-hand curves which are nearly uniform in length, each wave being
about eight to ten times the length of the ameba. Since the drawings
were made at intervals of a minute, the waves are therefore from eight
to ten minutes long in time, measuring from crest to crest. Some of the
waves are flatter than others, for example wave No. 4, but otherwise it
is like the others. Wave 7 is a double wave due to a change of
direction. Instead of turning to the right at 9:23 the ameba changed its
direction and turned to the left. The smoothness with which this turn
was made indicates that it originated in the mechanism producing the
sinuous course itself, or that it proceeded from a very slight stimulus
external to it. At 9:49 the direction of movement was changed again, but
just enough to disturb the formation of a smooth wave. The general
direction of locomotion was not changed. It may be assumed that this
change was produced by a stimulus external to the wave-producing
mechanism. The irregularity and shortness of wave 13 was probably due to
the same stimulus that disturbed wave 11. Shortly after 9:58 the ameba
came within sensing range of a mass of debris which it pushed away and
followed, thus causing a change in the direction of movement. Although
waves begin to appear again after this, some of them very smooth, they
are not typical for they are too short, ranging from a little over three
lengths (wave 16) to a little over six lengths (wave 23). It is likely
that the disturbance caused by the mass of debris at 9:58 together with
the onset of the division crisis produced the succession of atypical
waves. An external disturbance that is sufficiently strong to change the
direction of locomotion usually persists for the duration of at least
one wave length thereafter. It will be noted that
[Illustration: Figure 33. Illustrating the path of an _Amoeba bigemma_
under controlled conditions, in ordinary diffuse light from a north
window. For convenience of reference the waves in the path are numbered
from 1 to 23. The figures were drawn with the aid of a camera lucida at
intervals of one minute. The path was recorded from 8:58 to 10:26, when
fission occurred. After fission, the path of one of the daughter cells
was followed for a short time, but the ingestion of a mass of debris
destroyed the regularity of the path. The vertical lines at the point
where fission occurred indicate that the figures _above_ the lines were
moved up the length of the lines. The first figure beyond wave 13 was
influenced in its movements by a mass of debris lying to its left.
Average length of the ameba, 150 microns.]
the approach of the division crisis did not tend to destroy the action
of the wave mechanism, but only slowed down movement and shortened the
waves. The path of one of the daughter amebas was followed for a short
time, in which there is evidence of a wavy path, but it soon came upon a
small mass of debris which it ingested and soon thereafter reversed its
direction of movement. This behavior made it unprofitable to continue
further observation of this ameba. For the gradual change in direction
to the left from wave 1 to 6 in the path of the parent ameba, no
adequate explanation has suggested itself.
That amebas react to light has been shown by Verworn (’89), Davenport
(’97), Harrington and Leaming (’00), Mast (’10) and especially Schaeffer
(’17). It appeared desirable therefore to control the rays of light, for
it was thought possible that light might be a factor in the formation of
the wavy path. Since no method has yet been devised that permits of the
observation of the path of the ameba other than a succession of camera
lucida outlines, it is impossible to omit light altogether in the
experiments. The next best procedure was therefore followed, viz., the
alternation of periods of darkness of a few minutes’ duration with
brief--ten-second--periods of light, to permit the drawing of camera
lucida outlines. The dish in which the amebas were observed was placed
in a light-tight box and all light excluded except that which passed
through “Daylite” glass with an opal surface on both sides between the
condenser and the light source. None but parallel beams, passed through
a condenser, reached the ameba. The metal parts of the objective were
also blackened. The work was done in a dark room.
Figures 34 and 35 show sections of the path of two _Amoeba discoides_
under these experimental conditions. The amebas were for the most part
in clavate shape, which is the most favorable shape for the formation of
smooth waves. In figure 34, from 2:29¼ to 2:42¼ the ameba was in
continuous light. A section of a little more than one wave is shown.
Although pseudopods were thrown out at considerable distances to the
right and left of the path, a smooth, wavy path was nevertheless
maintained. At 2:42½ the light was turned off until 2:58½ except for two
ten-second flashes at 2:47 and 2:52. During the first period of
[Illustration: Figure 34. The path of an _Amoeba discoides_ under light
controlled conditions. At 2:42¼ the light was turned off until 2:52,
except for a ten-second flash at 2:47. The smoothness of the wavy path
was thus maintained in complete darkness. Length of the ameba, about 400
microns.]
[Illustration: Figure 35. Path of an _Amoeba discoides_ showing that
continuous light is unnecessary for the maintenance of a wavy path. The
ameba moved under light controlled conditions from 3:02¼ to 3:13¼. From
then until 3:43 the light was turned off except for 10-second flashes at
3:18, 3:22½, 3:24, and so on. The ameba had probably come to rest for
some reason between 3:24 and 3:30½, for an unexpectedly small amount of
space was covered in that time. In spite of this disturbance, however,
the evidence indicates that light is without causal effect in the wavy
path of the ameba. Length of the ameba, 450 microns.]
darkness the ameba merely kept on in the direction it was going when the
light was turned off. But during the second period of darkness the ameba
changed its course in such a way as to make a smooth curve. In the third
period of darkness the ameba continued on its course completing the
wave. It is thus apparent that continuous light is not necessary to the
formation of waves nor is it detrimental to their formation. Figure 35
shows essentially the same thing as Figure 34. The light was turned on
from 3:30½ to 3:32. During this time the behavior of the ameba was
irregular, but whether this was caused by the light or not, cannot be
stated. At 3:43 the ameba came into contact with a small particle which
changed its course. The slow speed of movement of these two amebas was
due to the low temperature (20° C.), the experiments being performed in
January. The apparent connection between longer waves and darkness has
not yet been investigated.
Figure 36 shows the path of an _Amoeba proteus_ under controlled light
conditions as above described, but instead of moving over a polished
plate glass surface as in the previous experiments, the ameba in this
experiment moved over a fine ground-glass surface. It will be observed
that for the first twenty minutes the path shows smooth waves, although
at 11:53 and 11:55 there was a slight disturbance which was associated
with the formation of numerous pseudopods. From 12:07 on, however, the
path becomes irregular, the wave-like character being almost
obliterated. Associated with this irregularity is the presence of
numerous pseudopods. This is a sample of a number of records which
indicates that in _proteus_ and _discoides_ the presence of numerous
pseudopods in some way prevents the ameba from moving in a path marked
by smooth and conspicuous waves.
[Illustration: Figure 36. Showing how smooth waves in the path of an
_Amoeba proteus_ in clavate shape become disturbed by the projection of
prominent pseudopods. Although there was considerable disturbance due to
the formation of pseudopods at 11:53 the conformation of the wave was
not changed until the stage preceding the one at 12:07, when the
formation of numerous pseudopods resulted in an irregular path. Length
of the ameba, about 600 microns.]
When a wave in the path for some reason becomes unusually long, there is
likely to be a very abrupt and decided change in the direction of
movement, which is away from the _convex_ side of the wave. Figure 37 is
inserted here to illustrate this point. The ameba should have turned to
the left at 3:43 to keep the waves of typical size, and at 3:45 a
pseudopod was extended in this direction a short distance, but again the
curve toward the right persisted. But at 3:48½ the ameba broke up into
several pseudopods at right angles with the main axis, and through one
[Illustration: Figure 37. An illustration showing that sudden changes
from the expected direction of the wavy path are centrifugal, not
centripetal; that is, away from the focus of the wave, not toward it.
The tendency to break away from the first smooth wave became apparent at
3:45, as indicated by the extension of pseudopods near the anterior end.
In the stage about two minutes later, a number of small pseudopods were
thrown out in various directions. At 3:48½ several large pseudopods were
thrown out near the anterior end almost at right angles to the main
axis, instead of at an angle of about sixty degrees or less, as is the
usual case. The same thing occurred at 3:57, except that the angle of
the pseudopod was not so large.]
of these the ameba moved on with the reestablishment of the wavy path.
The tendency to wave formation evidently has to overcome resistance of
some sort.
[Illustration: Figure 38. _Amoeba dubia_ usually moves with numerous
large pseudopods, but this illustration shows that there is very good
reason for concluding that there is a tendency in this ameba to move in
wavy paths. Length of the ameba, 400 microns.]
Although amebas in clavate shapes describe the smoothest waves in their
paths, waves may also be detected in the paths of amebas that habitually
form many pseudopods. The path of an _Amoeba dubia_ is shown in Figure
38. The ameba moved on an opal surface under light-controlled
conditions. If we had not already seen how _proteus_, _discoides_, and
especially _bigemma_ formed smooth waves in their paths, we should
hardly be able to understand the apparently aimless path of _dubia_. But
having seen how a regular succession of smooth waves appears under
favorable conditions in the paths of these amebas, there can be little
question but that the staggering path of a _dubia_ also is to be
interpreted as a succession of waves, although they are somewhat
irregular.
These four species of amebas, _proteus_, _dubia_, _discoides_ and
_bigemma_ are the only species that have been specially investigated as
to their paths, and they all show such paths, as we have seen. The
presumption is strong therefore that it is a common characteristic of
amebas.
To learn something of the nature of the wave-forming mechanism in the
ameba, it is necessary to find some agencies that modify the activity of
this mechanism. That there are such factors is of course evident enough
from what has been said already about wavy paths, and from the
appearance of the paths themselves. But the factors which influence the
formation of waves in so far as they may be known or reasonably
suspected, are internal and therefore difficult to make use of
experimentally.
One of the most readily applied stimuli that is known to affect the
character of ameboid movement is temperature. In general, the lower the
temperature, the slower the movement. This has frequently been observed
and recorded. Such behavior is to be expected from a viscous fluid like
protoplasm. This may therefore be a purely physical phenomenon. But the
lowering of the temperature has also another effect on the movement of
amebas: it creates in them a tendency to cross their paths more
frequently. Figure 33 is a typical example of the path of an ameba in a
high temperature (28° C.). It did not cross its path at all during the
hour and a half it was under observation. When the temperature is low
(20° C.) the path becomes contracted and the ameba seems unable to get
away from the place it happens to be in. Movement of course continues
but it is slower, and a large number of loops occur in the path. Figure
39 indicates the general path of an ameba under controlled conditions in
a temperature a little lower than room temperature, that is, about 20°
C. During the four hours that it was under observation the ameba crossed
its path eight times and made a number of very short turns besides.
Leaving out of account the loops in the path there are a number of
sections which may be interpreted as waves, such as for example the
pronounced waves a short distance from the end of the path. All these
waves are shorter but much deeper than the waves made in a higher
temperature. The loops in the path (all excepting the first, which is a
compound loop) represent each a single wave which have become so deep
and contracted that they have become transformed into circles. As the
temperature decreases, the crests of the waves rise higher and higher,
and the bases contract more and more, until the two sides of the waves
come together, resulting in the formation of circles (Figure 40). The
actual size of the wave also decreases at the same time from about eight
times the length until it is only two or three times the length of the
ameba. Temperature affects therefore the wave mechanism independently of
the mere viscosity of the endoplasm. The speed of movement is not merely
slowed down, but the character of the waves themselves is changed.
[Illustration: Figure 39. The path of an _Amoeba dubia_ in comparatively
low temperature (18° C.). The large number of loops and deep waves in
the path are due to the low temperature. The experiment was performed
under light controlled conditions. Length of the ameba, 350 microns.]
Amebas sometimes react to stimuli by moving around the source of the
stimulus at a more or less uniform distance through one or more
quadrants of a circle, instead of reacting positively, negatively, or
indifferently, in a definite manner, to the source of the stimulus
(Schaeffer ’16, ’17). See Figure 41. The explanation that has been given
by this investigator is that the encircling is due to a balance between
the tendency to move ahead in the original direction (“Functional
inertia”) and a tendency to react positively. But now that we know that
amebas tend to form waves in their paths, the explanation of encircling
becomes simpler and perhaps also more convincing.
[Illustration: Figure 40. Diagram illustrating how the deepening of the
waves in the path of an ameba due to decreased temperature may lead to
loops in the path. The heavy line represents the wavy path, and the
light lines with the arrows indicate the direction taken when loops are
formed. The point in the path where the direction is most easily changed
is where one wave grades off into the next, as indicated by the letter
_a_.]
In the first place, instances of encircling are relatively rare in the
reaction of amebas, much rarer than one would expect if it depended
merely upon a balance between two tendencies, one to move ahead and the
other to move toward the source of the stimulus. Any explanation of this
phenomenon has therefore to account for the rarity with which it occurs
as well as the operation of the phenomenon itself. This the explanation
based upon the position of the source of the stimulus with reference to
the configuration of the wavy path can do satisfactorily.
In the experiments with temperature it was found that when the
temperature is 20° C. or lower, the waves tend to curl up, to become
transformed into circles. That is, the base of one
[Illustration: Figure 41. Showing the phenomenon of encircling, after
Schaeffer. The ameba moved around a perpendicular beam of red light. The
reaction was neither distinctly positive nor negative.]
wave, instead of running into the base of the next wave is reflected
backwards to form a circular curve. All the evidence thus indicates that
the weakest point is at the base of the wave. A constantly acting
stimulus may therefore break the wave here if it cannot break into it
elsewhere, and so change the direction of the path. In Figure 42 are
shown a few diagramatic waves in the path of an ameba together with
several reflected curves at 1, 2 and 3 indicating the points at which
the direction is most easily changed as evidenced by the temperature
experiments. If a particle within sensing range of the ameba lie at _a_,
_b_, or _c_, and stimulate the ameba only slightly but still enough to
break up the wave formation, the ameba will take a curved path around
the particle as indicated by the dotted lines. But if the same particle
lay at any other point with reference to the position of the wave, as at
_e_, _f_, or _g_, the ameba would not have changed its course. Briefly,
the following conditions must be satisfied to enable the phenomenon of
encircling to appear: (1) The particle must lie a little to the side of
the ameba’s path. (2) It must lie abreast of the point at which the
ameba begins to change its direction of movement (i. e., at the base of
the wave) when describing
[Illustration: Figure 42. Diagram illustrating the relation of the
phenomenon of encircling to the wavy path of the ameba. The weakest
point in the path, i. e., where the wave may be broken most easily, is
where one wave merges into another, as indicated by the experiments with
low temperature, _a_, _b_, _c_. The direction of movement of the ameba
is the reverse of what it would have been had there been no stimulus
producing encircling. The same stimulus at _e_, _f_, or _g_ would not
produce encircling because it is more difficult for the ameba to move
away from the concave side of the wave.]
waves. (3) It must stimulate the ameba just strong enough positively to
break into the wave-forming process. Encircling then is due to a
“balance” between a positive stimulus and a tendency to move in a curve.
This explanation conforms with all the data at hand and explains also
the rarity of the phenomenon, for the chances of encircling occurring on
this view are rather less than one-fifth as frequent as if encircling
took place whenever a balance between a tendency to react positively and
to move straight ahead occurred.
That the wavy path is broken up by the receipt of a stimulus, that is,
by a true sensation, rather than by direct effect of some agency
radiating from the particle, is indicated by the fact that stimuli
proceeding from various substances, such as keratin, glass, carbon,
light beams, etc., have all the same effect.
In attempting to explain the characteristic nature of the path of the
ameba, one’s attention centers first, perhaps, upon its orderliness; a
result undoubtedly of the general impression propagated through hastily
written textbooks and general papers, that an ameba’s whole life is a
direct response to its environment. As the recorded facts of the life of
this organism are accumulating, it is coming to be seen that the ameba
possesses all the fundamental attributes of animals generally, in
addition to many special ones. So that as a matter of fact, if the ameba
did not show some character and orderliness in its locomotion, then for
the first time should we be especially interested in what would have to
be regarded as a very striking and exceptional characteristic.
For it is very well known, and it is generally recognized by everybody,
that moving organisms usually move in an orderly manner; it is
recognized that organisms tend to move in straight paths excepting where
interrupted by the action of some special stimulus. When an organism
changes its direction of motion frequently and abruptly, we call it
erratic. The mad dashings-about of the hunter-cilitae _Didinium_ and the
unceasing gyrations of the whirligig beetle excite one’s curiosity
because these organisms do not move as other organisms do; they
contradict our expectation of movement in a straight line.
But why should organisms generally tend to move in straight paths? This
fundamentally important question has received almost no attention,
excepting that rapidly moving animals like birds, flying insects, fishes
and other rapidly swimming animals of various kinds and rapidly running
animals tend to move in straight paths because of the physical inertia
of the mass of the organism. It is easier for a rapidly moving organism
to move in a straight line than to change its direction of movement
frequently and abruptly.
The ameba however is a very slow moving animal, as animals go, for it
(_proteus_) moves only about 600 microns per minute. Under the
microscope, however, which magnifies speed as well as size, the
endoplasmic particles rush along rapidly enough to suggest that even
here mere physical inertia might be a determining factor in the path of
the ameba, which for considerable segments is often very nearly
straight. Such suspicion is not justified, however, for the viscosity of
the endoplasm taken in connection with the heterogeneous composition of
the ameba, makes it improbable that mere physical inertia can affect the
path of the ameba.[5]
It is not even necessary that movement of the endoplasmic stream be
interrupted in order that a straight path may be maintained. An ameba
may stop movement for a minute or more and then be much more apt to
resume movement in the original direction than in any other. This is
brought out by the following series of experiments.
Of sixty cases of feeding on various kinds of particles, by as many
different amebas, in which the direction of movement before and after a
particle was eaten was recorded, thirty-nine moved off in the same
direction after eating as before eating. By moving off in the same
direction is meant that the ameba did not move more than 22½° to the
right or to the left of the direction of movement before feeding. The
circle was thus divided into octants, and the expectation of movement in
the same direction after eating a particle, if it were a matter of
chance, would have been seven and one-half cases instead of thirty-nine.
But it is not only the process of feeding that has to be considered in
this connection, for feeding occasionally is affected by a side
pseudopod while the main body of the ameba moves on without being
visibly affected as to its direction of movement. No such case is
included in the figures just given. In each of these sixty cases the
endoplasmic streams of locomotion were completely stopped, from about
twenty seconds to seventeen minutes. In most cases the endoplasmic
stream was also completely disorganized, the ameba assuming a nearly
spherical form in which more or less well marked though small cross
currents of endoplasm could be detected. The direction of the light was
without effect, for the paths extended in every direction with respect
to the light both before and after feeding. Further, it has been shown
that ordinary diffuse light is without effect on the movements of the
ameba (Schaeffer, ’17). It may be concluded therefore that the ameba
tends to keep on moving in straight paths even if the highly
disorganizing act of feeding and the consequent resting period of a few
seconds to many minutes supervenes at some point in its path. To what
this induction of the original path is due is not clear, thought it is
possible that the physical condition of the ectoplasm at the anterior
end is different from that elsewhere and that it requires less energy in
consequence, or for some other reason, to flow in the original
direction. This explanation is based on the observation that it is
easier for the ameba to activate the remnants of old pseudopods than to
form new ones (Schaeffer, ’17).