334 REPRODUCTION 



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 con- 

 sists in the discharge from the cell proper of a part of its 

 nucleus and a part of its protoplasm. 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, to- 

 gether with a little surrounding protoplasm, is extruded and 



FIG. 95. The 'fertilized ovum, or blastophere. (Kirkcs.) 



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 nu- 

 cleus which remains after the polar bodies have been thrown 

 off finds its way back to the center of the ovum. It soon de- 

 velops a covering membrane, and is now the female pronu- 

 cleus, 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 regu- 



