92 



THE EGGS OF MAMMALS 



TABLE XVI 



The Mean Number of Blastomeres per Ovum at Various Times after 

 Copulation in Large and Small Rabbits and in Certain Hybrids 

 BETWEEN Them. (From Castle and Gregory, 1929, and Gregory and 

 Castle, 1931) 



A = large race. 



B = small race. 



AB = Fi hybrid. 



BD = seven-eights small (D = AB XB). 



F = three-quarters large (AB X A). 



actual = number of blastomeres observed. 



prospective = number of blastomeres observed plus the number of mitoses. 



dermic vesicle forms at the end of the 3d day (Plate VII, 

 Figs. 18-20), and the embryonic disc by the 168th hour 

 after coitus. Castle and Gregory therefore attribute large 

 size to an inherent mitotic intensity independent of dif- 

 ferentiation potentials. 



The ova of the rabbit begin their differentiation early 

 in comparison with the eggs of other species. Thus Gregory 

 (1930) detected the beginning of the formation of the inner 

 cell mass just after the 16-cell stage at about 47 hours after 



