148 GENERAL PRINCIPLES OF ZOOLOGY. 



Fertilization. — The process of fertilization in the narrower 

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

 Usnally the egg is surrounderl by a gelatinous envelope, the 

 chorion, to the surface of wliicli 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 aiid resisting, there exists in it very 

 often a special arrangement, the 'iincrojjijlar apparatus, rendering- 

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

 canal e.xtending through the chorion, as in tlie eggs of fishes, or a 

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



Monospermy and Polyspermy. — Many spermatozoa may pass 

 through the gelatinous envelope, or through the microp^de 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 iirocess 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 sjiermatozoa 

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

 pathological phenomenon. There are means of protection against 



A n 



Fig. 93.— Egg of ^l.stej'ms (/(nckiiis lUiring fecundation. (After Fol.) ,1, entrance of the 

 spermatozoon; J5, the s,termatozoon has entered : the yolk-membrane has formed. 



this abnormal fertilization. One, though by uo 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 smne of the more fluid constituents, 

 so that between the yolk-memliranc and the surface of the egg a 

 cavitv is formed easily recognized in smaller fertilized eggs 

 (fig. '%\, B). 



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

 si)ermatozoa may normally enter. But this does not alter the conception 



