12 BRITISH FKKSH \VATEK KHIZOPODA. 



podium is clearly in the ectoplasm, and the rapidity of 

 its formation is increased by the peculiar " fountain 

 currents " characteristic of most pseudopodia. As 

 observed by Biitschli, an advancing stream of granules 

 flows through the centre, or axis, of the growing pseu- 

 dopodium, while near its apex return currents, " like 

 the falling drops of water in a fountain," surround the 

 central stream.* Rhumbler (1898) adduces the harden- 

 ing effect of water on protoplasm (first suggested by 

 Dr. Wallich) as explaining the formation of new ecto- 

 plasm, and the increase in surface of an advancing 

 pseudopodium. The outer ectoplasm has a firm con- 

 sistence, and as Rhumbler demonstrated by treatment 



n 



FIG. 5. Diagrammatic representation of a pseudopodium of Amaba 

 proteus. The central arrows represent the forward flow of the 

 endoplasm, the marginal ones the (apparent) " return currents." 



with diluted caustic potash in the case of Amwba 

 verrucosa may be isolated from the endoplasm. 

 Nevertheless, it is converted into streaming endoplasm 

 again. The conversion of ectoplasm into endoplasm, 

 and vice versa, in the activities of the Amwbu, is a 

 constantly-recurring phenomenon. 



But explanations of this nature, as Calkins says, 

 based upon purely physical laws of fluid substances, 

 seem inadequate to explain all types of pseudopodia, 

 the reticulate and long filamentous forms in particular. 

 " Up to the present time," this author remarks, " no 

 satisfactory and comprehensive explanation has been 

 made, and it should be recognized that the theories 

 advanced still remain only working hypotheses." 



* The " return currents " are more apparent than real. 



