454 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the number of chromosomes is halved, that the number in the two mature 

 germ-cells is the same, and that this number is one-halt' that of the chromo- 

 somes of the somatic cells. It is wholly probable that these facts are uni- 

 versal 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. 

 Many biologists follow Hertwig and Strasburger 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 chro- 

 matin in cells would become in a very few generations inconveniently great. 

 The most striking feature of maturation, however, is the halving of the num- 

 ber of chromosomes. The significance of this is not clear. Nevertheless it 

 is evident that maturation is a preparation of each germ-cell for union with 

 its mate. 1 



The Ovary ; Ovulation. — The ovaries (Fig. 222, 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 consists of fine connective tissue with numerous connective-tissue 

 cells. The ova are developed in the interior within cavities called, from 

 their discoverer, Graafian follicles (Gf), from primitive ova that are modified 

 cells of the germinal epithelium of the embryo. It has been calculated 

 that a single human ovary at the age of seventeen years contains 17,600 

 primitive ova, 2 but that not more than 400 of these arrive at maturity. 3 

 Each Graafian follicle is lined by an epithelial layer several cells thick, 

 the membrane granulosa, and is filled with a clear, serous, viscid liquid, the 

 liquor foUicvM. 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 epithelium 

 immediately surrounding the ovum is the discus proligervs. Within the 

 discus the ovum grows and becomes surrounded by the zona pellucida. In 

 the process of growth the Graafian follicle approaches the surface of the ovary, 

 and finally comes to form a minute rounded vesicular projection covered only 

 by the ovarian epithelium. When fully ready for discharge, the wall of the 

 follicle becomes ruptured, probably by the increasing pressure of the contained 

 liquid, and the ovum with its zona peUueida and a portion or all of the discus 

 proligerus, now called the corona radiata, is cast out upon the surface of the 



1 For a critical discussion of maturation, see Wilson : The Cell in Development and Inheril- 

 1900, 2d ed., Now York. 



2 Heyse ArcMv fiir Gynakologie, 1897, liii. 8. 321. 

 3 Henle: Handbuch dcr Anatomie, 1873. 



