534 MATURATION AND IMPREGNATION OF THE OVUM. 



the portion of the spindle remaining in the egg becomes con- 

 verted into two or three clear vesicles (Fig. 10) which soon 

 unite to form a single nucleus, the female pronucleus (Fig. n). 



FIG. 10. Portion of the ovum of Asterias glacialis after the formation of the second 

 polar cell, shewing the part of the spindle remaining in the ovum becoming 

 converted into two clear vesicles. Picric acid preparation (copied from Fol). 



FIG. u. Ovum of Asterias glacialis with the two polar bodies and the female 

 pronucleus surrounded by radial striae, as seen in the living egg (copied from Fol). 



The two polar cells appear to be situated between two membranes, 

 the outer of which is very delicate and only distinct where it 

 covers the polar cells, while the inner one is thicker and becomes, 

 after impregnation, more distinct and then forms what Fol speaks 

 of as the vitelline membrane. It is clear, as Hertwig has pointed 

 out, that the polar bodies originate by a regular cell division and 

 have the value of cells. 



