ISO 



GENERAL PRINCIPLES OF ZOOLOGY. 



nucleus, which rapidly forms a nuclear spindle (cleavage: 

 spindle), thereby giving the impulse to the beginning of 

 embryonic development, the cleavage of the egg (segmen- 

 tation of the egg). Since, not until this point is fertiliza- 



FIG. 91. Stages in the fertilization of the egg of the sea-urchin. fAfter O. Hertwig.) The 

 sperm-nucleus (sk) with its rays in one egg is near the surface, in the other near the egg- 

 nucleus (ek). 



tion completed, we here find the important fundamental 

 proposition that the essential feature of fertilisation consists 

 in the union of egg and sperm nuclei. 



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 



FIG. 92. Fertilization of A scar is megalocephala. (After Boveri.) A, the ends (centro- 

 somes) of the spindle formed ; , the spindle completed ; j/, sperm-nucleus with its chro- 

 mosomes ; ei, egg-nucleus ; /, polar bodies. 



alters the above-mentioned proposition, but yet it is impor- 

 tant, 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, i.e., the chromatic ele- 



