26 GERM-CELL CYCLE IN ANIMALS 



are recognizably concerned with the performance 

 of certain definite functions. The fundamental 

 difference, then, between the one-celled and the 

 many-celled animals is that the differentiated struc- 

 tures in the former are not separated from one 

 another by cell walls as in multicellular organisms. 



Whether all PROTOZOA possess a body which can be 

 considered as specialized and set aside for reproduc- 

 tion purposes, 

 as the germ- 

 plasm theory 

 requires, is a 



n " ^ question upon 



which author- 

 ities differ. In 

 certain cases 

 it seems pos- 



Fio.5. Reproduction in Arcella vulgaris. A. For- c iUl,i f n rlictin 



mation of secondary nuclei. . Ch = chromidia; " 



n = secondary nuclei; N = primary nucleus, guish between 

 (From Hertwig, 1899.} B, Two gametes. (From . -. . 



Eipatiewsky, 1907.) germinal and 



somatic proto- 

 plasm without any difficulty. The life history of the 

 fresh water rhizopod, Arcella vulgaris (Fig. 5), will 

 serve to illustrate this (Hertwig, 1899 ; Elpatiewsky, 

 1907; Swarczewsky, 1908; Calkins, 1911). The 

 single nucleus of the young Arcella divides to form 

 two primary nuclei (N) ; chromatin from these mi- 

 grates out and forms a layer near the periphery (Ch) 

 the " chromidial net " of Hertwig. This chromatin 

 substance in the mature individual produces hundreds 

 of secondary nuclei (n)', each of "which is cut off, with 



