O VITIATION 333 



Maturation. But previous to its discharge from the Graafian 

 follicle, the ovum undergoes certain changes a ripening process 

 whereby it is made ready to receive and be impregnated by 

 the spermatozoon. This maturation consists in the discharge 

 from the cell proper of a part of its nucleus and a part of its pro- 

 toplasm. The nucleus moves toward the periphery, and the 

 perinuclear membrane is lost. As the nucleus approaches the 

 surface of the egg it undergoes karyokinesis, and a part of it, 

 together with a little surrounding protoplasm, is extruded and 



FIG. 95. The fertilized ovum, or blastophere. (Kirkes.) 



finds itself in the perivitelline space. This is the first polar body. 

 A second polar body is likewise later discharged by karyokinetic 

 division. (See Fig. 95.) 



The object of this extrusion and the final fate of the polar 

 bodies are matters of speculation. That portion of the nucleus 

 which remains after the polar bodies have been thrown off finds 

 its way back to the center of the ovum. It soon develops a cover- 

 ing membrane, and is now the female pronudeus, ready for union 

 with the male pronucleus. It is about the time of the completion 

 of this process that the follicle ruptures and the discharge of the 

 ovum ovulation occurs. 



Ovulation. It is supposed that from puberty to the meno- 

 pause one (or more?) ovum is discharged at tolerably regular 



