12 AMEBOID MOVEMENT 



from sinking, there is a central stream of endoplasm flowing for- 

 ward, and a peripheral stream of ectoplasm flowing backward, 

 with a thin neutral zone between (Figure 29, d). As we shall 

 see further on, however, these fountain currents are in principle 

 the same as the currents observed in ordinary locomotion, the ap- 

 parent difference being due to the fact that there is no locomo- 

 tion. It is true, then, that within the same pseudopod at any 

 cross section the endoplasm always streams in one direction, and 

 the streaming is unified. 



When new pseudopods are formed, or when old ones are re- 

 tracted, and especially when both these phenomena occur at the 

 same time and close together on a part of an older pseudopod, 

 some of the details of coordination in streaming are readily made 

 out. In Figure i are shown a number of observed cases of pseudo- 

 pod formation and retraction, with the direction of endoplasmic 

 streams indicated at a given instant. For the purpose of illus- 

 tration, several (presumably) possible but unobserved cases, m 

 and r, are sketched, and also two cases, u and v, which have not 

 been observed and which probably do not occur. The general con- 

 clusion to be drawn from these observations is that, while the 

 endoplasm in the body of an ameba as a whole may be streaming 

 in several different directions at any given instant, that is almost 

 never the case with an individual pseudopod, especially if the 

 pseudopod is of small or medium size and not too flat or other- 

 wise irregular in shape. The pseudopod is therefore the unit of 

 coordinated protoplasmic streaming. 



Another general observation which undoubtedly is connected 

 in some way with the problem of coordinated streaming is the 

 following. In externally unstimulated amebas, the new pseudo- 

 pods are almost without exception directed 60 or less from the 

 direction in which the parent pseudopods are moving. 



It is a matter of common observation that an ameba may throw 

 out a pseudopod in any direction whatsoever when stimulated. 

 The ameba may reverse its direction of movement completely, or 

 it may move in scores of different directions at one time for awhile, 

 if properly stimulated. There is no restraint or limit imposed upon 

 the ameba insofar as the direction of movement is concerned. 

 Why then should a great majority of new pseudopods in an un- 



