GENERAL EMBRYOLOGY 135 



the protoplasm (fig. 96, A) by means of which it enters the egg. The 

 egg is now impervious to all others. Only in pathological eggs can two or 

 more spermatozoa enter and then multiple impregnation {di- or polyspermy) 

 occurs. There are means of protection against this abnormal fertilization ; 

 one, though not 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 entered. Within the yolk-membrane 



Fig. 96. — Egg of Asterias glacialls during fecundation (after Fol). A, entrance 

 of the spermatozoon; B, tlie spermatozoon lias entered; tlie yolk-membrane has 

 formed. 



the body of the egg contracts by discharging some of the more fluid con- 

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

 a cavity is formed, easily recognized in smaller fertilized eggs (fig. 96, B). 



In the large yoIk-Iaden eggs of many insects and vertebrates several sper- 

 matozoa may normally enter; but only one fuses with the egg-nucleus, the others 

 degenerating sooner or later. 



Essential Feature of Fertilization. — After the spermatozoon has 

 penetrated into the egg, the head and the middle piece containing the 

 centrosome can still be recognized, as the chromatic and achromatic 

 parts of the spermatozoon or sperm-nucleus (male pronncleus), while the 

 tail and the slight amount of protoplasm disappear iir the yolk. The centro- 

 some of the sperm-nucleus gives rise to rays in the cytoplasm of the egg, 

 like those observed during division. Preceded by these rays the sperm- 

 nucleus travels towards the egg-nucleus untU it reaches (fig. 97), and 

 fuses with it to form a single cleavage nucleus. The centrosome now 

 divides into two daughter centrosomes, wliich migrate to opposite poles 

 of the cleavage nucleus and control its division. The cleavage nucleus 

 changes to a cleavage spindle, which divides and thus initiates the em- 

 bryonic development, the successive divisions being known as the cleavage 

 or segmentation of the egg. Since not until this point is fertilization 

 complete, we arrive at the fundamentally important proposition that the 

 essential J'eatvre of fertilisation consists in the union of egg and sperm nuclei. 



