242 



GENERAL ZOOLOGY 



across the perivitelline space until it touches the vitelline mem- 

 brane. 



The next step is the retraction of the cone, as a result of which 

 the sperm head is pulled into the egg, leaving the middle piece 

 and tail outside. As the sperm head is drawn through the hole 

 in the vitelline membrane, it is stretched into an elongated 

 shape (Fig. 149), and as it is pulled away from the egg membrane 



. .^:::-:^M: l^2$?'M ^■'4 ) - ) ^2 





e f 



Fig. 149. — a, spermatozoon of Nereis, h, head; m, middle piece; p, perforator- 

 ium; t, tail, b-f, penetration of the spermatozoon into the egg. b, 37 minutes 

 after insemination. The perforatorium is imbedded in the fertilization or 

 entrance cone (closely stippled) which has withdrawn from the surface mem- 

 brane. The tail of the spermatozoon is not shown, c, d, e, stages from eggs 

 killed 48 minutes after insemination, showing the cone sinking into the egg 

 and drawing the sperm head with it. f, 54 minutes after insemination, the 

 sperm head is entirely within the egg, leaving the middle piece (m) outside. 

 (After F. R. Lillie.) 



deeper into the interior of the egg, it gradually becomes vesicular 

 (Fig. 150). In the meantime the first and second polar bodies 

 are formed and the primary oocyte is converted into an ootid. 

 The nucleus of the ootid and the nucleus derived from the head of 

 the sperm, the sperm nucleus, unite to form the fertilization 

 nucleus. A spindle then forms, the nuclear membrane dissolves, 

 and the egg is ready to undergo the first cleavage. The chromo- 

 somes of the first cleavage spindle consist of those contributed 

 by the egg nucleus and of those contributed by the sperm. 



