28 GERM-CELL CYCLE IN ANIMALS 
ticity has been suggested as more appropriate, and 
the two kinds of chromatin involved have been called 
idiochromatin, which is reproductive in function, 
and trophochromatin, which is vegetative in function. 
The hypothesis has not gained many adherents and 
is considered of doubtful value by eminent proto- 
zodlogists (Dobell, 1908). 
Merazoa. If we consider the mature egg, either 
fertilized or parthenogenetic, as the starting point 
of the germ-cell cycle in the MetTazoa, we may 
recognize seven or eight distinct periods as follows : 
1. The segregation of the primordial germ cells; 
z.e., the formation of one or more primordial germ 
cells during the segmentation of the egg; 
2. Early multiplication of the primordial germ 
cells; | 
3. A long period of “rest”? characterized by cessa- 
tion of cell division, either active or passive change 
of position, separation of the germ cells into two 
groups which become the definitive germ glands, 
accompanied by the general growth of the embryo 
until the larval stage is almost attained ; 
4. Multiplication by mitosis of the primitive 
odgonia or spermatogonia to form a definite number 
(Miastor and perhaps others) or indefinite number 
(so far as we know) of odgonia or spermatogonia ; 
5. In some cases the differentiation of odgonia 
into nurse cells and ultimate odgonia, and the 
spermatogonia into Sertoli cells and ultimate sper- 
matogonia ; 
6. The growth of the ultimate odgonia and sper- 
