186 



FERTILIZATION OF THE OVUM 



origin of which still remains one of the most difficult, as it is one of 

 the most interesting, problems relating to fertilization. 



After the formation of the polar bodies, the egg-nucleus is recon- 

 stituted near the upper pole of the egg, and the entire polar mitotic 

 apparatus disappears. In the meantime a new astral system (sperm- 



- PB 



B 



Fig- 93- Fertilization of the egg of the gasteropod, Pterotrachea. \ BOVERI.] 

 A. The egg-nucleus (E] and sperm-nucleus (S) approaching after formation of the polar 

 bodies; the latter shown above (P. B.) ; each germ-nucleus contains sixteen chromosomes; the 

 sperm-amphiaster fully developed. B. The mitotic figure for the first cleavage nearly established; 

 the nuclear membranes have disappeared, leaving the maternal group of chromosomes above the 

 spindle, the paternal below it. 



