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.
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.
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.