86 THE PROTOZOA 



and Rhumbler recognize that the longer the action of water is con- 

 tinued upon the ectoplasm, the greater the stiffening; hence, Rhumbler 

 argues, the new ectoplasm forming at the edge of the advancing 

 pseudopodium is less resisting than elsewhere and the forward flow 

 continues in one direction until the surface tension is equalized. New 

 material for the advancing pseudopodium must be supplied from 

 endoplasm, and this in turn from the posterior ectoplasm, so the 

 assumption is made by Rhumbler that there is a continual change 

 of Amoeba s protoplasm from ectoplasm into endoplasm, and from 

 endoplasm into ectoplasm. 



Explanations of this nature, 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 no satisfactory and comprehensive explanation has been made, 

 and it should be recognized that the theories advanced still remain 

 only working hypotheses. Hofer ('89) and Verworn ('91), and 

 many others have demonstrated that an enucleated amoeboid mass 

 soon comes to rest and assumes a spherical form. After a few days, 

 movement recommences, and is interpreted by Hofer as an expression 

 of the changes in surface tension. Such observations make it" prob- 

 able that the chemical activity, which is constantly operating between 

 the numerous substances which make up the protoplasm, plays an 

 important part in pseudopodia-formation, and with our present imper- 

 fect knowledge of these intra-cellular reactions, it is premature to 

 settle upon any one cause, however suggestive and attractive it may 

 appear, of this widely varied phenomenon. 



In many cases, especially among the Heliozoa, pseudopodia-motion 

 approximates flagella-motion. In many of the shelled Rhizopoda 

 (e.g. Arcella, or some species of Difflugid), the hyaline pseudopodia 

 sway backward and forward like thick, slow-moving flagella, while in 

 some Heliozoa (e.g. Artodiscns} this motion is much more energetic, 

 causing the organism to dance about like a monad. The resemblance 

 is more noteworthy and interesting from a theoretical point of view, 

 because both the flagellum of Mastigophora and the axial filament of 

 Heliozoa arise in the same manner in the endoplasm, and both are 

 apparently connected with a "division centre," a central granule, 

 which is analogous to the centrosome of metazoan cells. 1 



C. THE NUCLEUS 



Nuclei are almost as varied in the different forms of Sarcodina as 

 are the different types of the animals as a whole. In some cases, 

 there is no well-defined nucleus, the chromatin being scattered in the 



l Cf. p. 271. 



