io AMEBOID MOVEMENT 



streams. The fate of such a pseudopod depends on its size, on 

 its position on the ameba, and the strength of the stimulus af- 

 fecting it and the rest of the ameba. That is, if the pseudopod 

 is small or on the posterior half of the ameba, or only slightly 

 stimulated, it will be retracted ; but if it is large, or on the anterior 

 end of the ameba, or more strongly stimulated than the rest of 

 the ameba, it may again become active. 



The fact that protoplasm is practically incompressible makes 

 it clear that if streaming can be observed to begin after a pause 

 at some point after it begins at others, the ectoplasmic walls of 

 the ameba must give way in the region where streaming begins. 

 Since it has been established by observation that the ectoplasm 

 may give way at any point, it follows that one of the principal 

 factors affecting streaming is the elasticity and liquefiability of 

 the ectoplasm. 



The streaming in an ameba is coordinated. The direction in 

 which the endoplasm flows in the several pseudopods, when there 

 are no stimuli received externally that produce visible changes in 

 behavior, gives one the impression that there is a "centre" con- 

 trolling movement. The several pseudopods do not act at all 

 capriciously. The ameba seems to move the pseudopods, not the 

 pseudopods the ameba. If this impression of coordination is 

 correct, it is of the first importance in a study of ameboid move- 

 ment. Further on, this point will be taken up at length in con- 

 nection with the character of the path an externally unstimulated 

 ameba describes (p. 109) ; but there are certain observations 

 which aid in the analysis of the problem of coordination from the 

 point of view of the pseudopod, instead of that of the ameba as 

 a whole, and to these observations we may now direct our atten- 

 tion. 



The mass of endoplasm within a pseudopod moves practically 

 always in one direction. In any cross-section of a pseudopod 

 that is more or less cylindrical in shape, the endoplasm in the 

 center moves most rapidly, that near to it less rapidly, while that 

 near the ectoplasm moves very slowly. One never observes a 

 forward stream on one side of the pseudopod and a backward 

 stream on the other. Nor does one observe parallel streams of 

 endoplasm flowing in opposite directions within the same ecto- 



