AMEBOID MOVEMENT 9 



movement seems to require that the endoplasm must begin to 

 move at the base of the withdrawing pseudopod. Jennings ('04, 

 p. 157) observed that in a withdrawing pseudopod the current 

 of endoplasm begins at the base of the pseudopod. 



From numerous observations directed toward this point, it 

 appears that the conditions under which streaming is resumed 

 after a pause, whether in the same or in the reverse direction, 

 are of great variety. The shape, size, slenderness, and the posi- 

 tion on the ameba of the pseudopod, as well as the strength and 

 character of the stimulus, are among the factors capable of chang- 

 ing in whole or in part the flow of endoplasm in a pseudopod. In 

 an ameba that has been moving along a homogeneous flat surface, 

 as nearly unstimulated as may be, the endoplasm first begins to 

 flow out of the lower half of the retracting pseudopod, if the 

 pseudopod is more or less uniformly conical in shape and rather 

 slender. In such a case it may be said that the retracting pseudo- 

 pod was withdrawn "by the ameba," and that it did not itself 

 receive an external stimulus producing retraction. If, however, 

 the tip of a pseudopod as described receives a strong negative 

 stimulus, the endoplasm frequently flows back from the tip while 

 it is still flowing into the pseudopod at the base. But very soon 

 thereafter, in such an event, the streaming becomes unified and 

 the pseudopod is withdrawn. In broad pseudopods about to be 

 withdrawn, the endoplasm may begin to move anywhere along its 

 length. This is undoubtedly due to the continuous local changes 

 in the walls of the pseudopod, which are characteristic of this 

 species of ameba (see p. 20). 



In an ameba which has been brought to a standstill, as by a 

 sudden flash of light, the first sign of recurring streaming is in the 

 anterior half, whether the original direction of streaming is re- 

 sumed or reversed. If the direction is reversed, the active pseu- 

 dopods retract for a considerable distance before a new one is 

 projected. The endoplasmic stream in a slender withdrawing 

 pseudopod may not reach to the tip for from several seconds to a 

 minute, if the tip is slightly positively stimulated. One may then 

 observe ectoplasm streaming toward the tip and toward the base, 

 in the respective regions, at the same time, with considerable fluc- 

 tuation back and forth of the neutral zone separating the two 



