1 8o 



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. ('After O. Hertwlg.) 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 impoiiiant fundamental 

 proposition that the essential feature of fertilization consists 

 in tlic 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 



B 



FIG. Q2. Fertilization ot Asian's megalocephaln. (After Boveri.) A, the ends (centro- 

 somes) of the spindle formed ; B, the spindle completed ; sf, 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- 



