52 AMEBOID MOVEMENT 



move toward that part of the anterior edge that is advancing 

 most rapidly. Figures 17 and 18 illustrate this point. Figure 

 17 shows an ameba with two particles on its back, and with an 



Figure 17. Illustrating the different speeds with which particles move 

 when attached to the surface film of an Amoeba sphacronuclcosus, depend- 

 ing upon their location. Particle a moved 3.5 times as fast as the ameba 

 and b 2.7 times as fast. Length of ameba, no microns. 



unequally advancing anterior edge. Particle a moved more rapidly 

 than b because: (i) it was moving away from a more rapidly 

 receding posterior region ; (2) the right anterior edge was ad- 

 vancing more rapidly than the left anterior edge; (3) the par- 

 ticle was nearer the anterior edge. The rapidly advancing right 

 edge in stage 4 accounts for the veering of the particle a to the 

 right. The more rapid advance of b from stage 3 to 5 is due to 

 the remoteness of the anterior right edge, which, because of its 

 nearness to particle a pulls on it to a much greater extent than on 

 particle b. That is to say, when a particle lies somewhere bctiveen 



