MATURATION AND IMPREGNATION OF THE OVUM. 125 



Firx. 12. Fig. 13. 



Fig. 12 and 13. — Small portion of the ovum of Asterias glacialis. The 

 spermatozoa are shown 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). 



raorphosis of the tail itself (Fig. 14). This body vanishes in its 

 turn. 



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 

 spermatozoon into the ovum. It shows the prominence of the ovum 

 through which the spermatazoon has entered. A vitelline membrane 

 with a crater-like opening has become distinctly formed. From living 

 ovum (copied from Fol). 



The head of the spermotozoon 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 formation the proto- 

 plasm in its neighbourhood assumes a radiate arrangement (Fig. 

 15). At whatever point of the egg the spermatozoon may have 



VOL. XVIII. NEW SER. I 



