THE ORIGIN OF THE DEFINITIVE OVA 



11 



(1920) observed typical meiotic prophases in nests of young 

 oocytes in the adult ovaries of Galago. On the basis of 

 these observations and the presence of typical oocytes in 

 certain undescribed adult ovaries of Loris (material of 

 Prof. J. P. Hill and Dr. A. Subba Ran), Brambell (1930) 

 inclines to the belief that these primate oocytes derive from 

 primitive oogonia, not the germinal epithelium. 



Fig. 3. A stage 4 ovum (see text) in the mouse. Note 

 complete layer of follicle cells. (From the American Journal 

 of Anato7ny.) 



In rodents, however, such typical meiotic prophases have 

 never been described. Here the observations of Swezy 

 (1929a) and also of Evans and Swezy (1931), are very much 

 to the point and apparently resolve the mystery. Swezy 

 found the classical meiotic stages in the oocytes of rat 

 embryos and young rats up to five days post partum (Plate I, 

 Figs. 1-5), but she noted definite degeneration of all these 

 ova by the loth day post partum. By the 10th day 

 definitely atypical synizesis and pachytene stages occur 

 (Plate I, Figs. 6-13) and in 15 day old rats (Plate I, Figs. 14- 

 16) synizesis stages are rare or missing, the pachytene mod- 

 ified to a chromatin aggregation much less sharp than in 

 typical stages, and the diplonema chromosomes also less 

 distinct. On twenty day old rats (Plate II, Figs. 17-22) 

 nuclear growth of oocytes involves essentially similarly mod- 

 ifications, and in the adult the new ova derived from the ger- 



