540 MATURATION AND IMPREGNATION OF THE OVUM. 



FIG. 12. 



FIGS. 12 and 13. Small portion of the ovum of Asterias glacialis. The spermatozoa 

 are shewn enveloped in the mucilaginous coat. In Fig. 12 a prominence is 

 rising from the surface of the egg towards the nearest spermatozoon ; and in Fig. 

 13 the spermatozoon and prominence have met. From living ovum (copied from 

 Fol). 



At the moment of contact between the spermatozoon and the 

 egg the outermost layer of the protoplasm of the latter raises 

 itself as distinct membrane, which separates from the egg and 

 prevents the entrance of any more spermatozoa. At the point 

 where the spermatozoon entered a crater-like opening is left in 

 the membrane (Fig. 14). 



FIG. 14. Portion of the ovum of Asterias glacialis after the entrance of a spermato- 

 zoon into the ovum. It shows the prominence of the ovum through which the 

 spermatozoon has entered. A vitelline membrane with a crater-like opening has 

 become distinctly formed. From living ovum (copied from Fol). 



The head of the spermatozoon when in the egg forms a 

 nucleus for which the name male pronucleus may be conveniently 

 adopted. It grows in size by absorbing, it is said, material from 

 the ovum, though this may be doubted, and around it is formed 

 a clear space free from yolk-spherules. Shortly after its forma- 



