892 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



belonging to the ovarian ovum, are resolved again into scattered chromatic 

 substance. It develops a membrane and becomes again a resting nucleus. It 

 is known henceforth as the egg-nucleus, or female pronucleus, and it awaits the 

 coming of the male. Its centrosome gradually degenerates and disappears. 



Thus the curious process of maturation of the ovum is different in detail 

 from that of maturation of the spermatozoon. In the latter the primary 

 spermatocyte divides into four functional spermatozoa ; in the former the pri- 

 mary ovocyte divides into two functionless polar bodies (or, by subdivision of 

 the first, three, which have been called abortive eggs) and one functional ovum. 

 It is entirely probable, however, that the essence of the process is exactly the 

 same in the two cases, and lies in the reduction of the chromatic substance of 

 the nucleus. Van Beneden found in Ascaris that in the maturation of the 

 ovum, as in that of the spermatozoon already referred to, the number of chro- 

 mosomes is halved and that the number in the two germ-cells is the same. 

 This has since been proved abundantly in other forms, as well as the further 

 associated fact that the mature germ-cells contain each one-half the number 

 of chromosomes that are characteristic of the somatic cells ; it is wholly prob- 

 able that these facts are universal in sexual reproduction. Each mature germ- 

 cell, therefore, while in reality a cell, is, when compared with the somatic cells, 

 incomplete. The subsequent union of the two in fertilization restores the 

 chromosomes to their normal number. Inasmuch as the chromatin is probably 

 the all-important constituent of the germ-cells, the bearer of the paternal and 

 the maternal inherited characteristics, the phenomena of maturation are of 

 great interest. Most biologists follow Hertwig and Weismann in regarding 

 maturation as an adaptation for the prevention of the constant increase in 

 quantity of the hereditary substance that would otherwise take place with 

 every union of ovum and spermatozoon. Without a reducing process the 

 quantity of chromatin in cells would become in a very few generations incon- 

 veniently great. Maturation is a preparation of each germ-cell for union with 

 its mate. 



The Ovary ; Ovulation. The ovaries (Fig. 309, o) are often spoken of as 

 glands, but they are not glands according to the ordinary histological and 

 physiological use of the term. They are solid organs with a structure peculiar 

 to themselves, and their function is the production of ova. Their stroma con- 

 sists of fine connective tissue with numerous connective-tissue cells. The ova 

 are developed in the interior within cavities called, from their discoverer, 

 Graafian follicles (G.f), from primitive ova that are modified cells of the germinal 

 epithelium of the embryo. It has been calculated that the two human ovaries 

 at the age of eighteen years contain an average of 72,000 primitive ova, but 

 that not more than four hundred of these arrive at maturity. Each Graafian 

 follicle is lined by an epithelial layer several cells thick, the membrana 

 granulosa, and is filled with clear viscid fluid, the liquor folliculi, which con- 

 tains albuminoid matter. Imbedded in the epithelium upon one side is 

 usually a single ovum, completely surrounded by the cells and forming a 

 prominent hillock which projects well into the cavity of the follicle. The 



