REPRODUCTIVE ORGANS IN THE FEMALE. 311 



expelled, a great activity is at once manifest in the 

 ovum, whether it is fertilized or not. The nucleus, 

 which is near the margin of the ovum, divides in a 

 few hours, extruding what is termed the first polar 

 body. This is normal cell division, or mitosis. A 

 second division quickly follows, resulting in a second 

 polar body. Meanwhile the first polar body may 

 also divide. This second division results in a reduc- 

 tion of one-half the number of chromosomes, and 

 the nucleus thus reduced is called the female pro- 

 nucleus, which is now ready to unite with the male 

 pronucleus of the spermatozoon and complete the 

 process of fertilization. The phenomenon manifest 

 in the extrusion of the polar bodies is known as 

 maturation of the ovum, and seems to be an attempt 

 on the part of -the ovum to develop into a new indi- 

 vidual without the process of fertilization; that is, 

 parthenogenetically. If the ovum is not fertilized, it 

 shows no further activity and is lost. If the ovum 

 is fertilized it continues to divide regularly and in a 

 short time develops into the embryo. 



The developmental history of ova is full of interest. 

 They are very numerous and develop so very early 

 in embryonic life. During childhood they grow 

 large and accumulate a liberal storage of food, while 

 the sister epithelial cells that form the Graafian fol- 

 licle remain small and multiply rapidly to form the 

 ripe follicle. This latent condition extends over a 

 period of fifteen to forty years. When the ripe fol- 

 licle finally ruptures and the ovum is eliminated, a 

 rapid segmentation quickly follows resulting in the 

 extrusion of the polar bodies. This is followed by a 



