OVARIAN ATRESIA AND PARTHENOGENESIS 51 



(1935) tested this possibility by injecting crystalline thyroxin 

 and thyroprotein into rabbit does on heat. In no instance 

 did ovulation occur but large doses of thyroxin did initiate 

 follicular atresia and a limited degree of o\aim maturation. 

 Again we see that atresia-inducing conditions also initiate 

 maturation. The common feature of atretic follicles and 

 preo\ailatory follicles is an isolation of the ovum from its 

 connections with the follicular epithelium. 



It is safe to conclude from the foregoing analysis that the 

 formation of the first polar body in the rabbit ovary (and in 

 the ferret's also) is dependent upon an increase of pituitary 

 hormones in the circulating blood. It happens that in all 

 spontaneously ovulating mammals except the dog the forma- 

 tion of the first polar body occurs in the ovary. Even in the 

 dog (Evans and Cole, 1931) certain signs of nuclear matura- 

 tion are observable in ovarian eggs. It is natural to infer 

 that in spontaneously ovulating animals the pituitary level 

 reached during oestrus is normally sufficient to induce ovula- 

 tion as well as polar body formation. 



Now it is notable that the atresia of ovarian eggs is often 

 initiated by the formation of a maturation spindle. We 

 have noted that Engle has designated the spindles of ova 

 destined to atrophy as '^pseudomaturation" although there 

 is no evidence that they are in fact typically unlike those 

 observed in normally maturing ova. Such spindles are ob- 

 served only in ova of full size. Measurements of spindle 

 containing ova in mouse ovaries give an average maximum 

 diameter of 70 microns, and mature ova with vesicular 

 nuclei had an average of 69 microns. The writer has also 

 made careful examination of a large number of rabbit ovaries 

 and has never observed typical spindles in immature eggs. 

 What ordinarily occurs is a complex fragmentation of the 

 chromatin (see Figure 19). That the spindles are the indices 

 of impending atresia is indicated by the observation that 

 when they are at a maximum the total follicular atresia is 

 also at a maximum (see Figures 13 and 14). A possible in- 

 terpretation of their presence may be that they occur as a 



