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 
saits \ question upon 
an which author- 
ot See g ities differ. In 
certain cases 
it seems pos- 
Fic. 5.—Reproduction in Arcella vulgaris. A. For- sible to distin- 
mation of secondary nuclei. Ch = chromidia; 
nm =secondary nuclei; N=primary nucleus. guish between 
(From Hertwig, 1899.) B.Twogametes. (From ‘ 
Elpatiewsky, 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 (NV); 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 (7), each of which is cut off, with 
