ACCOUNT OF THE GERM-CELL CYCLE 35 



4. PERIOD OF MULTIPLICATION. Soon after the 

 germ cells aggregate to form more or less rounded 

 groups lying in the position of the definitive germ 

 glands mitotic division is resumed. At about this 

 time also, the sex of the individual can often be 

 determined by the shape of the germ-gland. Then 

 both the testes and the ovaries acquire envelopes 

 of the follicular cells, and frequently testicular cysts 

 and ovarian tubes or chambers develop. The ques- 

 tion of the origin of the follicular cells is still un- 

 settled, but the evidence in most cases seems to 

 favor the view that they are mesodermal. 



The multiplication of the germ cells by mitosis 

 continues rapidly from this time on. In only one 

 case, so far as I am aware, do we know the actual 

 number of germ cells produced by the primordial 

 germ cell ; this is in Miastor, where typically sixty- 

 four oogonia are formed (Fig. 26). As the germ 

 cells multiply they become smaller in size and the 

 substances present in the primordial germ cell 

 become divided among a large number of progeny. 

 Thus at the beginning of the growth period each 

 germ gland contains many oogonia or spermatogonia, 

 and each of these contains a small fraction of the 

 material present in the primordial germ cell, plus 

 whatever substances may have been assimilated 

 during the period of multiplication. 



5. THE ORIGIN OF NURSE CELLS AND SERTOLI 

 CELLS. Germ cells receive nourishment during the 

 growth period in many ways, e.g., from nurse cells, 

 follicle cells, or directly from the blood. The origin 



