378 ROBERT W. HEGNER 



2. Association of one or more cleavage nuclei with part or all 

 of the Keimbahn-determinants to form one or more primordial 

 germ cells; 



3. The apparently equal distribution of the Keimbahn- 

 determinants between the daughter germ cells at each mitotic 

 division (Sagitta possibly excepted); 



4. The disappearance of the Keimbahn-determinants in the 

 oogonia and spermatogonia; 



5. The reappearance of the Keimbahn-determinants in the 

 oocyte or mature egg. 



In the general history of the germ cells there may be two 

 periods of differentiation : 



1. The segregation of the primordial germ cells during cleavage 

 stages. 



2. The differentiation of nurse cells and ultimate oogonia, or 

 Sertoli cells and ultimate spermatogonia in the germ glands. 

 This second differentiation does not occur in Miastor and certain 

 other animals, and even when it does occur it is doubtful whether 

 the nurse cells and Sertoli cells should be considered as true 

 somatic cells or simply as abortive oogonia or spermatogonia 

 which have been unsuccessful in the struggle for development. 

 A casual examination is Hable to delude one into thinking that 

 the differentiations mentioned above are widely separated in 

 the germ cell cycle, but a little closer study shows that they 

 really occur during a relatively short period in the entire history. 

 For example, in certain insects, where nurse cells arise from 

 oogonia, this process takes place just before the growth period 

 during which the Keimbahn-determinants became localized in 

 preparation for the primary cellular differentiation. 



It is evident from the general outline as stated above that 

 the most important period in the germ cell cycle is that extending 

 from the formation of the ultimate oogonia and spermatogonia 

 to the complete segregation of the primordial germ cells. Our 

 knowledge of events during the latter part of this period is 

 comparatively great, whereas we know practically nothing about 

 the early stages involving the genesis of the Keimbahn-deter- 

 minants and their localization in the oocyte or mature egg. 



