THE EMBRYOLOGY OF THE OPOSSUM -~) 



times, apparently, the ova are discharged more than 12 hours 

 before copulation, and on these occasions the mating is in- 

 fertile. 



The sperm twins always separate before fertilization. This 

 is indicated by the fact that the sperm found entrapped in 

 the albumen of the egg are always single. The actual entrance 

 of the spermatozoon into the ovum has not been observed in 

 any marsupial. 



In summary: Occasionally precocious ovulation results in 

 sterile mating. Occasionally, perhaps, delayed ovulation re- 

 sults in a post-oestrous period. Most often ovulation occurs 

 within a few hours before or after copulation, and fertilization 

 by single spermatozoa takes place in the upper part of the 

 fallopian tubes within a few hours after ovulation. 



The tubal ovum. From about the twelfth hour to the 

 thirty-sixth hour after coitus one may expect normally to 

 find the ova in the tubes. It is possible that immediately 

 after dehiscence of the follicle the granulosa cells are ad- 

 herent to the zona pellucida for a short time, but, if so, they 

 are very soon shed and the ovum is left covered only by the 

 zona. In some way (perhaps by a change in osmotic pressure 

 in the surrounding fluid) the volume of the egg is slightly 

 reduced at this time (Hartman, '19). Its new dimensions are 

 0.122 mm. for the longest diameter, and 0.104 mm. for the 

 shortest (fig. 6). 



After the formation of the first polar body in the ovary 

 the chromosomes apparently do not enter a resting stage 

 as they do at the corresponding stage of spermatogenesis 

 (vide, p. 17), but immediately arrange themselves on the 

 spindle for the second maturation division. This is the con- 

 dition in the early tubal ovum (fig. 6). If fertilization does 

 not take place, the second maturation division does not com- 

 plete itself, though the X-diad, which is sometimes precocious, 

 may divide. The other chromosomes remain in a metaphase 

 distribution during the entire descent of the tubes (24 hours). 



