6 AMEBOID MOVEMENT 



about by the bursting of protoplasmic droplets of a more 

 fluid consistency on the surface of the ameba, the con- 

 sistency of which was less fluid, thus bringing about a decrease 

 of surface tension and consequent forward streaming of the endo- 

 plasm. The necessary migration of the more fluid droplets to the 

 surface was determined by internal conditions. The direction 

 in which an ameba moves was assumed to depend therefore not 

 upon the physical character of the substrate, as suggested by 

 Berthold, but upon such internal changes as control the movement 

 of the more liquid part of the internal protoplasm to the outer 

 surface. 



Rhumbler ('98) wrote extensively on the subject of ameboid 

 movement, especially from the point of view of the feeding habits 

 of amebas. He concluded that the flow of protoplasm, while en- 

 gulfing a food object, was a direct result of the lowering of the 

 surface tension of the protoplasm by contact with the food object 

 (p. 207), thus causing its envelopment. Numerous other writers 

 of the time, including Quincke ('88), Verworn ('89, '92), Bloch- 

 mann ('94), Bernstein ('oo) and Jensen ('02), agreed in a general 

 way with Rhumbler's position that surface tension changes are 

 the cause of locomotion in ameba. 



In 1904 the general subject of ameban behavior was extensively 

 studied by Jennings, and from his observations he concluded that 

 surface tension cannot account for many of the reactions ob- 

 served. Other factors, he held, must be at work, such as contrac- 

 tility, which, acting in the posterior region, causes the endoplasm 

 to flow forward. But Jennings found it impossible to explain on 

 the same basis the extension of free pseudopods, and the creeping 

 of a pseudopod, or of the whole ameba, over a solid substratum. 



From further observations Rhumbler ('05, '10) came to modify 

 his earlier views as stated above. The rapid advances in the study 

 of the chemistry of colloids doubtless suggested to Rhumbler 

 that the change from endoplasm to ectoplasm resembled the 

 change from a sol to a gel state, and that in this process of gela- 

 tion lay the source of energy manifested in ameboid movement. 

 In thus calling attention to, and emphasizing the colloidal nature 

 of, the conversion of endoplasm into ectoplasm and vice versa, 

 the problem of ameboid movement came to be discussed from an 



