FERTILIZATION 



169 



Molluscs). Thus the region which is to receive the sperma- 

 tozoon is already determined, and frequently the cytoplasm is 

 considerably modified, in the region just beneath the micropyle, 

 into a special substance concerned in the receipt of the sperm; 

 this is known as the entrance disc (Fig. 86). 



FIG. 86. Entrance of the spermatozoon in the fertilization of the Annulate, 

 Nereis limbata. After Lillie. A. Spermatozoon. B. Perforatorium has pene- 

 trated egg membrane; entrance cone well developed. Fifteen minutes after 

 insemination. C. Thirty-seven minutes after insemination. D. Entrance cone 

 sinking in and drawing the head of the spermatozoon after it. Forty-eight and 

 one-half minutes after insemination. E. Head drawn in still further. Forty- 

 eight and one-half minutes after insemination. F. Entrance completed. First 

 maturation division in anaphase. Fifty-four minutes after insemination. The 

 middle piece, as well as the tail, remains outside, c, head cap; e, entrance cone; 

 h, head of spermatozoon (nucleus); ra, middle piece; p, perforatorium ; v, vitelline 

 membrane; I, first polar division figure. 



With but comparatively few exceptions, only one sperm cell 

 normally enters a single egg (monospermy) . This sperm is the 

 first one to reach the egg or micropyle, and there are various 

 methods of excluding additional sperm, and thus of preventing 



