AMEBOID MOVEMENT 65 



Now we have seen that if a particle becomes attached to the 

 outer layer of such an ameba as discoides, which has nearly 

 symmetrical pseudopods, at some considerable distance from the 

 tip of the pseudopod, it moves forward until the tip of the pseudo- 

 pod is reached. It does not tend to come to rest near the tip of 

 the pseudopod, where the rate of ectoplasm formation is much 

 higher than at the sides of the pseudopod, though not as high as 

 at the tip, but it moves on until the tip is reached. That is, the 

 movement of particles on the surface film is toward that small 

 area at the extreme anterior end where the rate of ectoplasm 

 formation is highest. 



In such an ameba as verrucosa, however, the highest rate of 

 ectoplasm formation would be, not at a small circular area, but 

 a very narrow strip along the anterior edge; for the rate of ecto- 

 plasm formation over a considerable portion of the width of the 

 anterior end of the ameba is practically the same, according to 

 observation. Consequently we do not find particles which are 

 attached to the outer layer tending to move to a point lying on 

 the longitudinal axis, but their paths are found to be straight and 

 parallel with the longitudinal axis, if headed toward any point 

 over a considerable stretch of the anterior edge on either side of 

 the longitudinal axis. 



All the evidence that is at hand therefore points to the con- 

 clusion that the direction of movement of the surface film in a 

 moving ameba is toward that point where ectoplasm is formed 

 most rapidly. 



But where do the particles come from? At exactly what re- 

 gions of the ameba do they start to travel toward the anterior 

 ends of the ameba? In sphaeronucleosus and its congeners, it 

 is very difficult to determine just when the particles begin to move 

 toward the forward edge. Particles near the posterior end on the 

 upper sprface of these amebas moved forward slowly, much more 

 slowly than particles near the middle. Sometimes particles near 

 the posterior end seem to be motionless for some time, but the 

 incessant though slow kneading process going on at the posterior 

 end makes accurate observation difficult. Only in a general way 

 it may be stated that particles begin their forward march at or 

 near the posterior end. In amebas that habitually form pseudo- 

 pods more accurate information can be obtained. 



