MAMMALIA 



407 



Cleavage and Early Development in a Placenta! Mammal. It is 



not easy to compare the cleavage (Fig. 212) of the mammalian ovum 

 with that of any other form. It appears deceptively simple, but 

 we know that this apparent simplicity is a camouflage, for subse- 



FJG. 212. Cleavage of the ovum of the rabbit. A, Two-cell stage, 24 hours 

 after coitus, showing the two polar bodies separated. B, Four-cell stage, 25% 

 hours after coitus. C, Eight-cell stage, a, albuminous layer derived from the 

 wall of the oviduct; z, zona radiata. (From Kellicott, after Assheton.) 



quent events reveal that the apparent holoblastic cleavage gives 

 results that are similar to those resulting from a sauropsidan type 

 of meroblastic cleavage. It appears that the first two cleavages are 

 total and equal, just as in Amphioxus. After that the cleavages 



FIG. 213. Morula and early blastodermic vesicles of the rabbit. The zona 

 radiata and albuminous layer are not shown. A, Section through a morula 

 stage, 47 hours after coitus. B, Section through very young vesicle, 80 hours 

 after coitus. C, Section through more advanced vesicle, 83 hours after coitus; 

 taken from uterus, c, cavity of blastodermic vesicle; i, inner cell mass; w, wall 

 of the blastodermic vesicle (trophoblast). (From Kellicott, after Assheton.) 



are not easy to follow, since the cells seem to shift about and not to 

 retain their original positions. 



The blastula stage takes the form of a solid mass of cells, the 

 morula (Fig. 213, A), in which a peripheral layer of cells, the tropho- 



