148 



GENERAL PRINCIPLES OF ZOOLOGY. 



Fertilization. The process of fertilization in the narrower 

 sense begins with the entrance of the spermatozoon into the egg. 

 Usually the egg is surrounded by a gelatinous envelope, the 

 chorion, to the surface of which the spermatozoa adhere, and 

 through which they bore until they reach the surface of the egg 

 (fig. 93). But since the chorion, particularly in eggs which" are 

 laid in the air, may be hard and resisting, there exists in it very 

 often a special arrangement, the micropylar apparatus, rendering 

 possible the entrance of the spermatozoon; this may be a single 

 canal extending through the chorion, as in the eggs of fishes, or a 

 group of such canals, as in those of almost all insects. 



Monospermy and Polyspermy, Many spermatozoa may pass 

 through the gelatinous envelope, or through the micropyle canal, 

 but under normal conditions only one serves for fertilization. 

 The spermatozoon which is in the slightest degree ahead of the 

 others is met by a process of the protoplasm by means of which it 

 enters the egg. The egg is now impervious to all others. Only 

 in the case of pathological eggs can two or more spermatozoa 

 enter and then multiple impregnation (di- or polyspermy) occurs, a 

 pathological phenomenon. There are means of protection against 



Fio. 93. Egg of Aster ins ; aclalis during fecundation. (After Fo;.) A, entrance of the 

 spermatozoon; _B, the s .ermatozoon has entered : the yolk-membrane has formed. 



this abnormal fertilization. One, though by no means the only 

 one, is the formation of the yolk-membrane, 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 easily recognized in smaller fertilized eggs 

 (fig. 93, B). 



In the large yolk-laden eggs of many insects and vertebrates several 

 spermatozoa may normally enter. But this does not alter the conception 



