88 AMEBOID MOVEMENT 



a gradient which would suggest a similar gradient in metabolic 

 rate; and if the endoplasm is a passively moved fluid as Hyman's 

 theory seems to imply, a metabolic gradient would seem to be 

 precluded. 



In the ectoplasm however there exists a time gradient ; that at 

 the base of a pseudopod is older than that near the tip, and ob- 

 servation generally tends to confirm the view that the older it is 

 the firmer it becomes. This gradient in the amount or extent of 

 gelation corresponds with the disintegration gradient of cyanide 

 along a forming pseudopod. That is, the rate of disintegration is 

 proportional to the age of the ectoplasm. There is however no 

 good evidence that the age of ectoplasm corresponds to the 

 rate of metabolism, so that the younger the ectoplasm is the 

 higher will be the metabolic rate in it. The following statement 

 seems to bear this out : "As soon as the pseudopodiiim extends 

 into the water its surfaces gelatinizes because of contact with the 

 water" (Hyman, '17, p. 89). Gelation is, according to Hyman, 

 a passive process and therefore not distinctively metabolic. She 

 continues: "It is necessary therefore for the continuous pro- 

 duction of a pseudopodium, that the metabolic change which is the 

 cause of the liquefaction should continue to occur at the pseudo- 

 podial tip. There is thus produced the metabolic gradient along 

 the pseudopodium which I have described . . ." 



But if the metabolic gradient is bound up with the process of 

 liquefaction, it is difficult to see how there can be a gradient along 

 the pseudopod, for liquefaction takes place only at the tip, ac- 

 cording to her own statement. As a matter of fact, however, 

 gelation is constantly occurring at the tip of the pseudopod and 

 to a less degree back along the sides of the pseudopod. Lique- 

 faction occurs only at the posterior end of the ameba in orderly 

 movement. 



We must conclude therefore that while Hyman's data are of 

 the first importance in contributing to the structure and behavior 

 of the ameba, her contention that a metabolic gradient is demon- 

 strated in the ameba is not convincing. 



From this short account of the main theories that have been 

 advanced to explain ameboid movement it appears that of the 

 modern theories the only one that has been capable of adjusting 



