312 



THE MAMMALIAN EMBRYO. 



[CHAP. 



that the peripheral nucleus was derived from the sper- 

 matic element. 



Segmentation. The process of segmentation oc- 

 cupies in the rabbit about 72 hours; but the time of 

 this and all other stages of development varies con- 

 siderably in different animals. 



The details of segmentation in the rabbit are differ- 

 ently described by various observers ; but at the close of 

 segmentation the ovum appears undoubtedly to be 

 composed of an outer layer of cubical hyaline cells, 

 almost entirely surrounding an inner mass of highly 

 granular rounded or polygonal cells. 



Fio. 95. 



Optical Sections op a Rabbit's Ovum at two Stages 



closely followina upon the segmentation. 



(After E. van Beneden.) 



ffp. outer layer ; hy. inner mass ; hp. Van Beneden's blastopore. 



The shading of the outer and inner layers is diagrammatic. 



In a small circular area however the inner mass of 

 cells remains exposed at the surface (Fig. 95, A). This 



