OOGENESIS IN THE WHITE MOUSE 301 



along, and most probably this is adhered to in this instance. 

 The primitive germ cells, centrally located, are the first to start 

 on their course of degeneration, a process which later involves 

 many of the definitive oocytes, probably beginning with those 

 more deeply situated. This degeneration of definitive egg- 

 cells, which in all likelihood sets in before sexual maturity, is 

 continued through the whole sexual life of the individual, as is, 

 of course, too well known to need emphasis. Since, then, a 

 large number of definitive oocytes degenerate, and since these 

 are situated more superficially than the primitive germ cells, 

 of which an extremely large number certainly degenerate, it is 

 not unreasonable to conclude that all these primitive oocytes 

 degenerate and are resorbed, and that the definitive ova are all 

 of a later, that is, post natal, origin from the germinal epithelium. 



SUMMARY AND CONCLUSIONS 



1. In the development of the ovary of the mouse there are two 

 proliferations of cells from the germinal epithelium. The first, 

 occurring before birth, gives rise to germ cells, the 'primitive 

 germ cells,' all of which degenerate and are resorbed; the second, 

 extending from birth or a few days after nearly to sexual maturity, 

 forms the definitive ova. 



2. The evidence here presented, while not conclusive, indi- 

 cates the possibility that the rete ovarii is formed by in-growths 

 from the mesonephros. 



3. A periovarian capsule (bursa ovarica), which develops as a 

 fold of the peritoneum, encloses the ovary more or less completely. 

 The oviduct penetrates this capsule, which is a closed sac with 

 no evident opening into the peritoneal cavity in about 50 per 

 cent of the cases examined. 



4. The follicle cells of the primitive oocytes arise from 'in- 

 different cells' which, also originating from the germinal epithe- 

 lium, grow down into, or are left behind in, the ovary along with 

 the germ cells. In the course of development, they come to 

 surround the egg-cells, forming thus the primitive follicles. 



5. The follicles of the definitive oocytes arise also from the 

 germinal epithelium. While the egg-cell is developing in the 



