MAMMALIA. 



179 



Beheden the blastopore, though it is highly improbable that it in 

 any way corresponds with the blastopore of other vertebrate ova l . 



After its segmentation the ovum passes into the uterus. The 

 epiblast 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 blastodermic vesicle. 

 To Ed. van Beneden we owe the fullest account of these changes ; 

 to Hen sen and Kolliker however we are also indebted for valuable 

 observations, especially on the later stages in the development of this 

 vesicle. 



The succeeding changes commence with the appearance of a 

 narrow cavity between the 

 epiblast and hypoblast, 

 which extends so as com- 

 pletely to separate these 

 two layers except in the 

 region adjoining the ori- 

 ginal site of the blastopore 

 (fig. 134 B) 2 . The cavity 

 so formed rapidly enlarges, 

 and with it the ovum also ; 

 which soon takes the form 

 of a thin - walled vesicle 

 with a large central cavity. 

 This vesicle is the blasto- 

 dermic vesicle. The greater 

 part of its walls are formed 

 of a single row of flattened 

 epiblast cells; while the 

 hypoblast cells form a small 

 lens-shaped mass attached 

 to the inner side of the 

 epiblast cells (fig. 135). 



Tn the Vespertilionidse 

 Van. Beneden and Julin have 



shewn that the ovum undergoes at the close of segmentation changes of 

 a more or less similar nature to those in. the rabbit ; the blastopore would 

 however appear to be wider, and to persist even after the cavity of the 

 blastodermic vesicle has commenced to be developed. 



Although by this stage, which occurs in the rabbit between 

 seventy and ninety hours after impregnation, the blastodermic vesicle 



1 It is stated by Bischoff that shortly after impregnation, and before the commence- 

 ment of the segmentation, the ova of the rabbit and guinea-pig are covered with cilia 

 and exhibit the phenomenon of rotation. This has not been noticed by other 

 observers. 



2 Van Beueden regards it as probable that the blastopore is situated somewhat 

 excentrically in relation to the area of attachment of the hypoblastic mass to the 

 epiblast. 



122 



FIG. 135. KABBIT'S OVUM BETWEEN 70 90 



HOURS AFTER IMPREGNATION. (After E. Van 



Beueden.) 



bv. cavity of blastodermic vesicle (yolk-sack); 

 ep. epiblast; Jnj. primitive hypoblast; Zp. mu- 

 cous envelope (zona pellucida). 



