n8 AMEBOID MOVEMENT 



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 char- 

 acteristic of amebas. 



To learn something of the nature of the wave-forming mechan- 

 ism 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 tem- 

 perature 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 exam- 

 ple the pronounced waves a short distance from the end of the 

 path. All these waves are shorter but much deeer than the waves 

 made in a higher temperature. The loops in the path (all except- 

 ing the first, which is a compound loop) represent each a single 

 wave which have become so deep and contracted that they have 



