[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.]