OVARIAN ATRESIA AND PARTHENOGENESIS 57 



do not act directly upon the ova (Pincus and Enzmann, 

 1935) by experiments in which ovarian ova with vesicular 

 nuclei were cultured in media containing various pituitary 

 extracts. The data of these experiments are summarized 

 in Table VII-A. They show that in both the extract- 

 containing media and the extract-free media maturation 

 proceeds at about the same rate. Furthermore thyroxin 

 which causes a certain degree of maturation when injected 

 in vivo (see page 51 above), causes in vitro no further degree 

 of development than thyroxin-free controls (Table VII-B). 

 The isolation of the ova from the normal follicular environ- 

 ment is sufficient to initiate activation. This implies that 

 in preovulatory follicles maturation is caused by either 

 (1) the mechanical separation of the ovum and its corona 

 or (2) the removal of an inhibiting influence. Mechanical 

 separation undoubtedly occurs (c/. Plate III, Figs. 8 and 

 9), but one cannot estimate the exact degree of isolation 

 necessary to initiate maturation, for it is certain (Pincus and 

 Enzmann, 1935) that maturation is initiated in ova still 

 having strands connecting them to the follicular epithelium. 

 In certain forms {e.g., man) the ova remain embedded in 

 the cumulus mass till just before ovulation and the corona 

 forms late. It is notable that Allen, Pratt, Newell and 

 Bland (19306) were able to obtain only one maturation stage 

 in some two hundred ova recovered from 3 to 20 mm. fol- 

 licles. The writer (unpublished data) has observed one 

 maturation occurring in a primate ovarian ovum, but when 

 primate ovarian ova are cultured in vitro considerable nu- 

 clear activity occurs. During the first stages of pituitary- 

 induced maturation in the rabbit a secretion of secondary 

 liquor folliculi is observed (Pincus and Enzmann, 1935). 

 This secretion may remove an activation-inhibiting influ- 

 ence. The maturation observed in ova of atretic follicles 

 may be due to a similar sort of secretion rather than to 

 simple isolation of the ovum from its follicular epithelium. 



On the basis of the foregoing considerations one might 

 conceivably encounter occasional evidences of activation 



