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- 

 zoologists (Dobell, 1908). 



Metazoa. If we consider the mature egg, either 

 fertilized or parthenogenetic, as the starting point 

 of the germ-cell cycle in the Metazoa, we may 

 recognize seven or eight distinct periods as follows : 



1. The segregation of the primordial germ cells; 

 i.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 

 oogonia or spermatogonia to form a definite number 

 (Miastor and perhaps others) or indefinite number 

 (so far as we know) of oogonia or spermatogonia ; 



5. In some cases the differentiation of oogonia 

 into nurse cells and ultimate oogonia, and the 

 spermatogonia into Sertoli cells and ultimate sper- 

 matogonia ; 



6. The growth of the ultimate oogonia and sper- 



