46 AMEBOID MOVEMENT 



tact with water without stiffening. And if so, there appears to be 

 no reason why locomotion might not occur. It appears however 

 under normal conditions that a moderate tendency to ectoplasm 

 formation (proteus, dubia) leads to greater efficiency in move- 

 men than a very weak (limicola) or a very strong (vernicosa) 

 tendency to form ectoplasm. 



In the reticulose rhizopods, as is well known there is no ecto- 

 plasm of the kind observed in amebas. The middle of the pseudo- 

 pod, moreover, is not the region of most rapid streaming as in 

 ameba, but frequently becomes congealed, on the contrary, into 

 a rod-like structure. In general this axial rod has the character 

 of very stiff ectoplasm. The character of streaming in reticulose 

 rhizopods, however, has received very little attention, and de- 

 tailed comparisons are therefore impossible. 



Another interesting property of reticulose pseudopods, which 

 are formed by a streaming process, is their great power in some 

 species, of rapid contraction. If a diatom for example, in its 

 movements breaks loose a pseudopod it is often (though not 

 necessarily) contracted very rapidly, much more rapidly than 

 could be the case if it were accomplished by streaming. It fre- 

 quently happens that knobs are found on a slender pseudopod. 

 These knobs may move back and forth with great rapidity with- 

 out visibly affecting the pseudopod (Figure 12). The process 

 reminds one of a block sliding on a rope. These observations 

 indicate a very high degree of elasticity in the formed pseudopods 

 of such a rhizopod as Biomyxa as compared with a very low 

 degree of elasticity in the amebas. 



It thus appears that the process of streaming is a much more 

 fundamental phenomenon than most of the theories accounting 

 for ameboid movement would lead one to suppose; for these 

 theories concern themselves only with streaming as observed in 

 amebas, and many content themseslves with only two or three 

 species. Since the general features of streaming are similar no 

 matter where streaming occurs, no theory is likely to gain ac- 

 ceptance that explains streaming only in one group of organisms. 

 Streaming in rhizopods, myxomycetes, ciliates, plant cells, is 

 most rationally looked upon as caused by the same fundamental 

 process ; but the detailed form it takes, especially in freely formed 



