THE ORIGIN OF THE DEFINITIVE OVA 



23 



decreased when the hypophysis is secreting active maturity 

 hormone as in non-pregnant females. The maturity hormone 

 is concerned with the ripening of large follicles, ovulation 

 and corpus luteum formation. During pregnancy and pseu- 

 dopregnancy maturity hormone is secreted only in sub- 

 threshold amount, as evidenced by cyclic ovarian changes 



100 



200 



300 



700 



800 



900 1.000 



400 500 600 



AGE - DAYS 

 Fig. 6. Showing the total number of ova as well as the number of ova of 

 different sizes in the albino rat at different ages (condensed). (From the 

 American Journal of Anatomy.) 



in the ovary during pregnancy (Swezy and Evans, 1930), 

 so that the hypophysectomized level is attained. During 

 the normal non-pregnant ovogenetic cycle that portion 

 marked by the presence in the ovary of ripe follicles and 

 fresh corpora lutea is always associated with a minimum of 

 small, newly formed ova. The pituitary secretions, then, 

 are concerned with promotion of o\ailation and luteinization 

 and presumably inhibit ovogenesis to a certain extent. The 

 factor controlling ovogenesis is unknown although the effects 

 of thyroidectomy indicate that the thyroid may promote 

 ovogenesis to a certain extent. It should be pointed out, 

 however, that the thyroidectomized rats were much the eld- 

 est of the lot and Arai (1920a) has demonstrated a small 



