AMEBOID MOVEMENT 59 



Figure 25. Amoeba proteus. Rate of movement of the surface layer 

 as compared with the rate of movement of the ameba. The pseudopod 

 on the right was extended to stage 5 ; from then on it was retracted, as 

 indicated by the outlines. Length of the ameba, 400 microns. 



retract. The particle moved 1.43 times as fast as the ameba from 

 i to 4. But from 4 to 7 the particle moved only 1.06 times as 

 fast as the ameba. 



In the earlier stages the outer layer was pulled toward the tip 

 of both pseudopods, in the later stages only toward one, and in this 

 lies the explanation for a more rapid movement of the particles 

 in the earlier, and a slower movement in the later stages. This 

 effect was also observed in discoides, but the fact that the particle 

 in the later stages moved only very little faster than the ameba 

 is due to a narrow anterior edge and to the formation of ectoplasm 

 in the ridges over the surface of the ameba. The effect of ridge 

 formation on the movement of particles attached to the surface 

 film is well seen when an ameba has two forward moving regions 

 opposite each other. Under such conditions particles located 

 equidistant or nearly so between such regions, move only very 

 slowly or not at all, the pull upon the film being nearly or quite 

 equal. In a similar manner the ridges which are constantly form- 

 ing on a proteus are continually competing with the anterior end 



