THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 413 



follicle. The little elevation of the stratum granulosum in which the ovum 

 is embedded is known as the cumulus ovigerus or germ hill (see Fig. 18). 



The primary ovarian follicles at first lie rather near the surface of the ovary, 

 but as they enlarge and as the ovary enlarges they come to lie deeper. As the 

 follicle approaches maturity it increases greatly in size (5 ± mm.) and finally 

 extends through the entire thickness of the cortex, its theca touching the tunica 

 albuginea. 



In speaking of the development of the follicles, it must be remembered that 

 they develop slowly and do not reach maturity until near the age of puberty, and 

 furthermore that one, or very few at most, reach maturity at the same time. In 

 other words, when one follicle has reached maturity there are all intermediate 

 stages of development between this and the primitive follicles. When a follicle 

 reaches maturity it ruptures at the surface of the ovary and the ovum is set free 

 (p. 31). The ovum itself undergoes certain changes by which the somatic 

 number of chromosomes is reduced one-half (p. 17). It then unites with the 

 mature spermatozoon, which also contains one-half the somatic number of 

 chromosomes, and forms the starting point, so to speak, for a new individual. 

 At this point the processes by which an individual is carried through its life 

 period from its beginning as a fertilized ovum to the time when it produces the 

 next generation of mature sexual elements are ended. The developmental 

 cycle of one generation is complete. 



It has been estimated that approximately 36,000 primitive ova appear in 

 each human ovary. Since, as a rule, only one ovum escapes from the ovary at a 

 menstrual period or between two succeeding periods, it is obvious that the vast 

 majority of these never reach maturity. They probably degenerate, and, as a 

 matter of fact, atretic follicles may be found in an ovary at any time. 



Corpus Luteum. — After the rupture of the mature follicle at the surface of 

 the ovary and the escape of the ovum and liquor folliculi, blood from the rup- 

 tured vessels fills the interior of the follicle and forms a clot — the corpus hosmor- 

 rhagicum. The cells of the stratum granulosum proliferate and migrate into 

 the clot and gradually form a mass which replaces the blood. It is held by some 

 that the cells are derived from the theca folliculi. Whatever their origin, they 

 become infiltrated with a fatty substance known as lutein. Trabecule of 

 connective tissue grow into the mass of cells, carrying small blood vessels with 

 them. The (lutein) cells disintegrate and the products of disintegration are 

 probably carried off by the blood, and finally the entire corpus luteum is trans- 

 formed into a mass of connective tissue (Figs. 19, 20 and 21, and p. 32). 



Whether the escaped ovum is fertilized or not has an influence upon the 

 development of the corpus luteum. In case of fertilization, the corpus luteum 

 becomes quite large, increasing in size up to the fourth month of pregnancy, and 

 then degenerates. In case the ovum is not fertilized, the corpus luteum re- 



