2l6 



THE SEGMENTATION. 



Van Beneden the blastoporc, though it is highly improbable that 

 it in any way corresponds with the blastopore of other vertebrate 



ova 1 . 



R 



FIG. 134. OPTICAL SECTIONS OF A RAKBIT'S OVUM AT TWO STAGES CLOSELY 

 FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.) 



ep. epiblast ; hy. primary hypoblast ; bp. Van Beneden's blastopore. 

 The shading of the epiblast and hypoblast is diagrammatic. 



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 blas- 

 todermic vescicle. To Ed. van Beneden we owe the fullest 

 account of these changes ; to Hensen 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 ex- 

 tends so as completely to separate these two layers except in 

 the region adjoining the original site of the blastopore (fig. 134 

 B) a . 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 blastodermic 



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

 mencement 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. 



* Van Beneden 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. 



