118 



THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



table, pp. 121-124). On the other hand, other students have described the 

 origin of the germ cells from mesodermal tissue — some during the early period 

 of embryonic development, while others suggest that the primordial germ cells 

 arise from peritoneal (mesodermal) tissue at a much later time. 



In more recent years much discussion has been aroused relative to the 

 origin of the definitive germ cells in mammals, particularly in the female. 

 According to one view the definitive germ cells which differentiate into the 

 mature gametes of the ovary arise from the germinal epithelium (peritoneal 

 covering) of the ovary during each estrous cycle (figs. 39A, 63, 64). For 

 example, Evans and Swezy ('31) reached the conclusion that all germ cells 

 in the ovaries of the cat and dog between the various reproductive periods 

 degenerate excepting those which take part in the ovulatory phenomena. 

 Accordingly, the new germ cells for each cycle arise from the germinal epi- 

 thelium. A similar belief of a periodic proliferation of new germ cells by the 



POLE 



PLASM 



Fig. 62. Early development of the fly, Miastor. (A) Miaslor metraloas. (B) Miastor 

 ainerkana. In (A) the division figures I and III (II not shown) are undergoing chromatin 

 diminution, while nucleus IV divides as usual. In (B) one segregated germ is shown at 

 the pole of the egg. This cell will give origin to the germ cells. Other division figures 

 experiencing chromatin diminution. 



