314 THE MAMMALIAN EMBRYO. [CHAP. 



spheres which, largely enclose the inner spheres. The segmenta- 

 tion of both outer and inner spheres continues, and in the course 

 of it the outer spheres spread further and further over the inner, j 

 so that at the close of segmentation the inner spheres constitute a 

 central solid mass almost entirely surrounded by the outer 

 spheres. In a small circular area however the inner mass of 

 spheres remain for some time exposed at the surface (Fig. 95 A). 



The blastodermic vesicle. After its segmentation 

 the ovum passes into the uterus. The outer cells soon 

 grow over the blastopore and thus form a complete 

 superficial layer. A series of changes next take place 

 which result in the formation of what has been called 

 the hlastodermic vesicle. M 



These changes commence with the appearance of a 

 narrow cavity between the outer and inner layers, which 

 extends so as comjDletely to separate them except in the 

 region adjoining the original site of the blastopore (Fig. 

 95 B)\ The cavity so formed rapidly enlarges, and 

 with, it the ovum also ; so that this soon takes the form 

 of a thin walled vesicle with a large central cavity. 

 This vesicle is the blastodermic vesicle. The greater 

 part of its walls are formed of a single row of flattened 

 outer layer cells; while the inner mass of cells forms 

 a small lens-shaped mass attached to the' inner side of 

 the outer layer (Fig. 96). 



Although by this stage, which occurs in the rabbit 

 between seventy and ninety hours after impregnation, 

 the blastodermic vesicle has by no means attained its 

 greatest dimensions, it has nevertheless grown from 



1 Van Beneden regards it as probable that the blastopore is 

 situated somewhat escentrically iu relation to the area of attachment 

 of the inner mass to the outer layer. 



