12 BRITISH FRESHWATER RHIZOPODA. 
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 Bittschli, 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 
Fig. 5,— Diagrammatic representation of a pseudopodium of Ameba 
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 Amba 
verrucusa—may be isolated from the endoplasm. 
Nevertheless, it is converted into streaming endoplasm 
again. The conversion of ectoplasm into endoplasm, 
and vice versd, in the activities of the Amabu, 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 he recognized that the theories 
advanced still remain only working hypotheses.” 
* The “return currents ” are more apparent than real. 
