OVARIAN ATRESIA AND PARTHENOGENESIS 47 



two polar bodies are formed in the ovary by 9 hours after 

 copulation. Our observations indicate that only the first 

 polar body is given off in the ovary and then the metaphase 

 plate of the second polar spindle is formed. Robinson (1918) 

 observed in the ferret, which also ovulates only after copu- 

 lation, that only the first polar body is given off in the ovary 

 some time after copulation. 



^ - i 



m 



^M 



Fig. 16. Ovum removed from a ripe follicle of a rabbit doe at two hours 

 after copulation. Note beginning of chromatin condensation. (From the 

 Journal of Experimental Medicine.) 



Before copulation occurs the mature ovum contains a 

 single large vesicular nucleus about 30 microns in diameter 

 (Figure 15; see also Plate III, Figs. 4 and 5). At two hours 

 after copulation signs of change are partially evident: some 

 of the ripe ova show the beginnings of tetrad formation in 

 the nucleus but the nuclear membrane is still intact (Fig- 

 ure 16). By four hours after copulation the tetrads of the 

 first polar spindle are formed and the nuclear membrane 

 is ordinarily dissolved (Figure 17). The metaphase plate 

 has a diameter of a little over 10 microns. The first polar 



