THE ORIGIN OF THE DEFINITIVE OVA 19 



but a one and one-half times compensatory hypertrophy 

 when careful comparison with a control series was made. 

 It is significant that in Emery's material about 50 per cent 

 of the rats were found at autopsy to have large ovarian 

 cysts. Similar cystic formations were observed in about 

 half of the semi-spayed females in Wang and Guttmacher's 

 (1927) series, and Wilhams (1909) reports that such cysts 

 are commonly found in ovarian fragments left after incom- 

 plete ovariectomy. 



Arai (19205) found definitely that the compensatory hyper- 

 trophy in the rat is due exclusively to an increase in the 

 number of large follicles and corpora lutea. Semi-spaying 

 before puberty when the formation of corpora lutea does not 

 normally occur led to a 40 per cent increase in ovarian weight, 

 whereas semi-spaying after puberty led to a 100 per cent 

 increase. Furthermore, by careful counts he established 

 that the total number of follicles in the ovary does not in- 

 crease after semi-spaying. In this Arai was confirmed by 

 Alien (1923) who found that in semi-spayed mice the num- 

 ber of ova differentiating from the germinal epithelium 

 during stages 2 and 3 (vide supra) was scarcely larger than, 

 normal whereas the average number of mature ova formed 

 was normal. The implication from these studies is that the 

 germinal epithelium produces a large more or less constant 

 number of young ova, that some extra-gonadal factor is 

 responsible for the ripening and maturation of a Hmited 

 number of follicles, and that the maturing crop of ova are 

 chiefly involved in the compensatory hypertrophy. It is 

 now well established that an enormous atresia of young 

 follicles occurs during the course of a single oestrus cycle. 

 Thus in swine 14 per cent of the visible follicles less than 

 3 mm. in diameter become mature (Allen, Kountz and 

 Francis, 1925) and in the rat of the ova less than 20 fx in 

 diameter only 0.8 per cent attain a diameter greater than 

 60 M (Arai, 1920a). This extensive destruction of young ova 

 and follicles is particularly striking in the dog and cat 

 (Evans and Swezy, 1931) where all the new eggs (except 



