IMPREGNATION OF THE OYTM. 



It will be convenient again to take Asterias glaeialis as the type. 

 The part of the germinal vesicle which remains in the egg, after the 

 formation of the second polar cell, becomes converted into a number 

 of small vesicles (fig. 29), which aggregate themselves into a single 

 clear nucleus, which gradually travels toward the centre of the egg 

 and around which, as a centre, the protoplasm becomes radiately 

 striated (fig. 30). This nucleus is known as the female pronucleus. 



" O" <- ! V^'O^O 0_". . '"Q . a 



" 



13. 



FIG. 31. SMALL PORTIONS OF THE OVUM OF ASTERIAS GLACIALIS. THE SPERMATOZOA 



ARE SHEWN ENVELOPED IN THE MUCILAGINOUS COAT. IN A. A PROMINENCE IS RISING 

 FROM THE SURFACE OF THE EGG TOWARDS THE NEAREST SPERMATOZOON ', AND IN B. 

 THE SPERMATOZOON AND PROMINENCE HAVE MET. (Copied frOIll Fol.) 



By the action of reagents a nucleolus maybe shewn in it. In Asterias 

 glaeialis the most favourable period for fecunda- 

 tion is about an hour after the formation of the 

 female pronucleus. 1 f at this time the spermatozoa 

 are allowed to come in contact with the egg, their 

 heads soon become enveloped in the investing 

 mucilaginous coat. A prominence, pointing to- 

 wards the nearest spermatozoon, now arises from 

 the superficial layer of protoplasm of the egg, and 

 grows till it comes in contact with the sperma- 

 tozoon (fig. 31, A and B). Under normal cir- 

 cumstances the spermatozoon which meets the 

 prominence is the only one concerned in the 

 fertilization, and it makes its way into the egg 

 by passing through the prominence. The tail of 

 the spermatozoon, no longer motile, remains visible 

 for some time after the head has bored its way 

 in, but its place is soon taken by a pale conical 

 body, which is, however, probably in part a product 

 of the metamorphosis of the tail itself (fig. 32). 

 It ev.entually becomes absorbed into the body of 

 the ovum. 



At the moment of contact between the spermatozoon and the egg 

 the outermost layer of the protoplasm of the latter raises itself as 

 a distinct membrane, which separates from the egg and prevents the 



B. K. 5 



FIG.SJ. PORTION OF 

 THE OVUM OF ASTEHIAS 



TOZOON INTO THE OVUM. 

 IT SHEWS THE PROMI- 



WH*CHTHE 

 SPERMATOZOON HAS EN- 

 TERED. A 



HAS BECOME DISTINCTLY 

 FORMED. (CopieJfrom 



"C 1 1 1 



