66 AMEBOID MOVEMENT 



In proteus or disc aides, for example, projecting pseudopods are 

 often suddenly stopped and retracted, with a resultant change of 

 an anterior to a posterior end. Particles attached to the outer 

 surface on such pseudopods move toward the anterior end, of 

 course, as long as the pseudopod is building, in the manner de- 

 scribed in the preceding pages. But when the endoplasmic stream 

 is arrested, the forward movement of the particle likewise stops. 

 When the endoplasm starts to flow back into the main body of the 

 ameba, the particle also starts moving back ; but there is a period 

 of a few seconds after the endoplasmic stream is reversed during 

 which the particle remains quiet. And when it does start in to 

 move, it moves only slowly. Within a few seconds, however, 

 the average speed of movement is attained. This is true of par- 

 ticles located some distance away from the tip of the pseudopod. 

 If. the particle has reached the tip of the pseudopod before re- 

 versal of the endoplasmic stream takes place, the particle often 

 remains at the tip until the pseudopod is almost completely with- 

 drawn into the main body of the ameba (Figure 26, p. 60). At 

 other times such a particle becomes displaced, presumably by 

 irregular retraction of the tip of the pseudopod, and finds itself 

 at the side of the pseudopod. When this happens it moves slowly 

 toward the main body of the ameba, but faster than the tip of 

 the pseudopod does. 



It frequently happens, especially in annulata, but also in proteus 

 and other forms with many pseudopods, that when an advancing 

 pseudopod is about to be withdrawn, there intervenes a stage 

 where the endoplasm in the distal part moves away from the 

 ameba, while that in the proximal part moves toward the ameba, 

 with a neutral or motionless zone between. In such case a par- 

 ticle on the distal end moves slowly toward the tip while a par- 

 ticle in the proximal region moves toward the base of the pseudo- 

 pod. Particles over the neutral zone are motionless. In these 

 cases, however, changes in the direction and speed of the ecto- 

 plasmic stream are too frequent and the relative strengths of the 

 distal and proximal currents too variable, to enable one to secure 

 very accurate data by means of camera lucicla drawings (a kine- 

 matograph is essential for this purpose), so no figures of the 

 speed of movement of such particles are given. Nevertheless 



