1 86 



FKRTn.IZATIOy OF THE OfT.)/ 



orii;in of which still rcnKiins one ol 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 j^ole of the es;g, and the entire polar mitotic 

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





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. 



