524 



SEGMENTATION OF OVUM. 



bisected ; b, cells of tunica granulosa have disappeared, and the yolk is 

 in four segments ; c, continued advance in segmentation ; d, the zona 

 has become thicker, and the segmentation more complete ; e, ovum burst 

 by. compression : some of the segments have escaped ; each shows a 

 bright spot or vesicle. 



Fig. 255. 



Segmentation of ovum. 



Fig. 256, cleavage of the yolk after fecundation : #, an ovum of As- 

 caris nigrovenosa, the yolk of which is divided into two equal portions ; 

 the upper portion contains a cell with a large nucleus, the lower a sim- 

 ilar cell with two small nuclei ; &, ovum subdivided into four portions ; 

 c, the subdivision has reached sixteen, each possessing a mono-nucleated 

 cell ; d, ovum of Ascaris acuminata, showing the stages of subdivision, 

 the portions becoming very small ; 6, the portions preparing to be mould- 

 ed into the young worm, (#, &, c, Kolliker ; d, e, Bagge.) 



Fig. 256. 



Segmentation of ovum. 



As the ovum is about to enter the uterus, each portion which has arisen 

 from the segmentation of the yolk has become a perfect cell. This cell 

 formation having been accomplished at the surface of the yolk first, the 

 cells there begin to coalesce into a membrane, with an aspect like that 

 of hexagonal pavement epithelium, and, as the change passes toward the 

 centre, the cells, as they form, come toward the membrane and thicken 

 it, leaving a clear liquid within. In this manner a secondary vesicle 

 forms within the zona pellucida : it is the blastodermic vesicle : it is the 



