36 AMEBOID MOVEMENT 



of surface of the fluid is increased. This increase of surface pro- 

 duces the phenomenon of contractility. 



Miss Hyman is wrong however when she says (p. 90) that 

 the withdrawal and contraction of pseudopods are processes of 

 gelation. This is clearly a physical impossibility, for the ectoplasm 

 of the withdrawing pseudopod must become liquified into endo- 

 plasm, before it can be withdrawn. All writers excepting Jen- 

 nings and Hyman are agreed on the continual transformation of 

 ectoplasm into endoplasm at the posterior end while the reverse 

 process goes on at the anterior end; and Hyman herself states 

 (p. 89) that new ectoplasm is formed as the growing pseudopods 

 extend into the water. So there must be liquefaction of the ecto- 

 plasm in withdrawing pseudopods, or very soon the whole ameba 

 would be transformed into ectoplasm. As was shown in the 

 preceding pages, liquefaction of the ectoplasm at the posterior 

 end goes on at the same rate as gelation of the endoplasm at the 

 anterior end. But at another place Hyman says : 



In fact according to Jennings, Bellinger, Grnber, and Schaeffer 

 the surface of the ectoplasm actually flows forward at about the 

 same rate as the forward advance, and this indicates that the ad- 

 vancing ectoplasm at the tip of the pseudopodium is derived from 

 the surface ectoplasm and not from a transformation of endo- 

 plasm into ectoplasm at the end of the pseudopodium as Rhumb- 

 ler supposed" (p. 89). < 



This quotation is not strictly accurate. Jennings says: "The 

 pseudopodium grows chiefly from the base, so that any part of 

 the surface retains nearly its original distance from the tip" (p. 

 156). Bellinger in a general way confirmed Jennings' conclu- 

 sions. Gruber concluded that the outer layer was gelatinous, not 

 protoplasmic. Schaeffer held the third layer to be extremely thin, 

 "too thin to be seen easily," so it is impossible that the ectoplasm 

 at the tip of a pseudopod, the thickness of which is readily seen, 

 can be derived from the surface film. 



The main conclusion however in Miss Hyman's paper is that 

 there exists a metabolic gradient in the pseudopods of advancing 

 amebas, the highest rate of metabolism being at the tip and the 

 lowest at the. base, for any one pseudopod. This conclusion 

 is bound to be of the first importance in the explanation of ame- 



