FERTILIZATION OF THE OVUM 



791 



the first polar body itself divides into two. A second polar body is 

 formed in the same way by karyokinesis of the daughter-nucleus remain- 

 ing in the ovum. These two polar bodies finally disappear. The 



PB 



Fig. 217. Karyokinesis (Boveri). 



A, two germ-nuclei in the egg of the gastropod Pterotrachea ; E, egg-nucleus ; S, sperm-nucleus, 

 each nucleus containing sixteen chromosomes; PB, polar bodies the centrosome has divided into 

 two to form an amphiaster. B, later stage, showing the fully-developed amphiaster ; PB, polar bodies. 



process just described is called maturation of the ovum, and it occurs in 

 mammals before the ovum leaves the Graafian follicle. 



Fertilization of the Ovum. The head of a spermatozoid, containing 

 chromosomes, penetrates the vitelline membrane and comes in contact 



