120 AMEBOID MOVEMENT 



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. 



waves in their paths, the explanation of encircling becomes simpler 

 and perhaps also more convincing. 



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 opera- 

 tion 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 



