GENERAL EMBRYOLOGY. 



149 



of fertilization, for even here but one spermatozoon 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 which 

 contains the centrosome can still be recognized, as the chromatic 

 and. achromatic parts of the spermatozoon or sperm-nucleus (male 

 pronucleus), while the tail and the slight amount of protoplasm 

 disappear in the yolk. In the cytoplasm of the egg the centrosome- 

 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 (fig. 94) ;. 

 and fuses with it to form a single cleavage nucleus. Now the^ 

 centrosome divides into two, which migrate to opposite poles of 

 the nucleus, while the cleavage nucleus changes to a cleavage 

 spindle, which divides and thus initiates the embryonic develop- 

 ment, the successive divisions being known as the cleavage or seg- 

 mentation of the egg. Since not until this point is fertilization 

 complete, we arrive at the fundamentally important proposition 

 that the essential feature of fertilization consists in the union of 

 egg and sperm nuclei. 



FIG. 94. Stages in the fertilization of the egg of the sea-urchin. (After O. Hertwig.y 

 The sperm-nucleus (x/f ) with its rays is near the surface in one egg, in the other 

 near the egg-nucleus (ek). 



Part Played by the Two Nuclei in Fertilization. In many 

 cases an abbreviation of development may take place, the stage of 

 the cleavage nucleus being omitted, and the egg and sperm nuclei, 

 without previously uniting, pass directly into the cleavage spindle. 

 This fact in no wise alters the above-mentioned proposition, but 

 yet it is important, because it shows more plainly in what way the 

 two nuclei participate in the formation of the cleavage spindle. 

 It shows that of the chromosomes which form the equatorial plate 

 of the nucleus, exactly one half are furnished by the egg-nucleus, 



