DEVELOPMENT AND PLACENTAT2ON. 



725 



" no larger than fern seed." They are formed from germinal 

 epithelium, the cells of which grow inwards in clustered 

 masses into the connective tissue or stroma of the ovary. 

 In each cluster one cell pre- 

 dominates over its neigh- 

 bours ; it becomes an ovum ; 

 the others invest and nourish 

 it, and are called follicle cells. 



In the middle of each clump 

 or Graafian follicle a cavity 

 is formed containing fluid, 

 and into this cavity the follicle 

 cells immediately surrounding 

 the ovum project, forming 

 what is called the discus pro- 

 ligerus (see Fig. 272, p. 514). 



When mature, the ovum 

 protrudes on the surface of 

 the ovary, and is liberated 

 by the bursting of the Graafian 

 follicle. Ovulation may occur 

 spontaneously as in man, 

 monkeys, horse, cattle, pig, 

 dogs ; or after sexual union 

 as in rabbit, guinea-pig, mouse, 

 and cat. An ingrowth of 

 epithelial cells surrounding 

 the follicle develops into a 

 glandular body called the 

 corpus luteum. Its secretion 

 is believed to be very import- 

 ant influencing the prepara- 

 tion of the uterus, the early 

 nutrition of the embryo, and 

 the multiplication of cells in 

 the milk glands. It seems 

 that the ovary, besides produc- 

 ing ova, is a gland whose in- 

 ternal secretion, passing into the blood, induces, directly or 

 indirectly, the phenomena of heat and menstruation, and 

 influences the uterus during pregnancy. 



FIG. 395. Segmentation of 

 rabbit's ovum. After Van 

 Beneden. 



e.c., External cells (epiblast) ; i.e., 

 internal cells (hypoblast); b.v., 

 blastodermic vesicle. 



