468 



HISTOLOGY 



which probably prevents any more spermatozoa from entering. Figure 

 436 shows a spermatozoon in the ovum and another vainly attempting 

 to enter. Sometimes two or more do get in with abnormal results. 



FIG. 436. Asterias Forbesii. One sper- 

 matozoon entered into ovum; second 

 vainly attempting an entrance. X 1500. 

 (Drawn by H. E. JORDAN.) 



FIG. 437. Asterias Forbesii. Sperm 

 head, now becoming male pronu- 

 cleus, approaching the female pro- 

 nucleus. X 440. (Drawn by H. E. 

 JORDAN.) 



The use of some chemicals results in the postponing of the membrane 

 formation, and this almost always leads to such a polyspermic fertiliza- 

 tion. 



As the sperm head, with its middle piece attached behind, advances 

 toward the egg nucleus, it rotates so that the middle piece is in front. 

 The cytoplasm through which it has passed, spreads out behind it in a 

 widening "wake" of somewhat different staining power from its original 



condition. This is called the "en- 

 trance funnel." The sperm head now 

 begins to enlarge, and a centrosome, 

 arising from intimate connection with 

 the middle piece, acquires rays which 

 originate in and grow out into the cyto- 

 plasm (Fig. 437). These rays consti- 

 tute the future dynamic apparatus for 

 cell division. By the time the sperm 

 head has arrived next to the egg nu- 

 cleus, it has become enlarged and has 

 opened out its chromatin pattern to 

 form a sperm nucleus which is but 

 little smaller than the egg nucleus (Fig. 

 438). Asterias thus differs from some 

 other echinoderms, as Toxopneustes, in which the sperm head does not 

 open up much until it has joined the egg nucleus. The process of union is 



FIG. 438. Asterias Forbesii. Apposition 

 of the male and female pronuclei. X 440. 

 (Drawn by H. E. JORDAN.) 



