GENERAL EMBRYOLOGY. 



179 



the egg sends up a projection by which it can enter into 

 the interior of the yolk ; thereupon the egg becomes inac- 

 cessible to all the other spermatozoa, and the unsuccessful 

 ones perish. Only in case of pathological changes, or where 

 eggs are injured by long waiting, can it happen that two 

 or more spermatozoa enter. Over against the normal 

 simple fertilization or monospermy, we must place, as a 

 pathological phenomenon, multiple fertilization, dispermy 

 and polyspcrmy. There are means of protection against 

 this abnormal fertilization. One of these, though by no 



A B 



FlG. 90. Egg of Aster ias glacialis during fecundation. A, entrance of the spermatozoon, 

 B, the spermatozoon has entered : the yolk-membrane has formed. (After Fol.) 



means the only one, is the formation of the yolk-mem- 

 brane, an impermeable envelope which is suddenly secreted 

 from the surface of the egg, as soon as the spermatozoon 

 has accomplished the impregnation. Within the yolk- 

 membrane the body of the egg contracts into a smaller 

 volume by discharging some of the more fluid constituents, 

 so that, between the yolk-membrane and the surface of the 

 egg, a cavity is formed by which one can easily recognize 

 the smaller fertilized egg (Fig. 90, B). 



Essential Feature of Fertilization. After the sper- 

 matozoon has penetrated into the egg, the head and the 

 centrosome can still be recognized, but the tail becomes 

 fused with the yolk of the egg. In the cytoplasm of the 

 egg the achromatic end of the sperm-nucleus gives rise to 

 conspicuous rays, like those observed during division. 

 Preceded by these rays the sperm-nucleus travels towards 

 the egg-nucleus until it reaches it (Fig. 91); the sperm 

 and egg-nuclei unite and form a single nucleus, the cleavage 



