242 The Vertebrate Organ Systems 



The interior portion of the ovary is formed of connective tissue and is 

 known as the medulla, while the outer portion is the cortex. Within 

 the medulla are many interstitial cells. (Fig. 78.) 



At intervals throughout the reproductive life of the individual, 

 single cells enlarge at the expense of adjacent ones to become ova 

 (Fig. 79). Each future ovum, with its surrounding nutritive cells, 

 constitutes a Graafian jollide. Gradually this follicle fills with fluid 

 and finally the ovum bursts forth and moves into the oviduct. If the 

 ovum is fertilized, implantation and development occur in the uterus. 

 After the ovum is released, the ruptured follicle is gradually filled with 

 tissues from the lining cells and with the ingrowth of blood vessels. 

 This new structure is somewhat yellowish in appearance and is known 

 as the corpus luteum. If fertilization of the ovum does not occur, the 

 corpus luteum lasts only a short time and then disappears. If fertiliza- 

 tion does occur the corpus luteum persists for some time. 



The Function of the Female Hormones. — Two distinct hor- 

 mones have been demonstrated to be produced by the ovary. The first 

 of these, estrogen, is produced by the follicle. It is important in the 

 development of the secondary sexual characters and behavior. The 

 second important hormone is that produced by the corpus luteum. 

 It is known as progesterone, and its importance is twofold : first it 

 cooperates with estrogen in influencing secondary sexual characters, 

 and, second, it prepares the female for lactation and pregnancy. 



If pregnancy does occur, the placenta (the attachment organ of 

 the embryo to the uterus) becomes important as a source of endo- 

 crines. Three distinct hormones have been demonstrated to be pro- 

 duced : chorionic gonadotropin, estrogen, and progesterone. Chorionic 

 gonadotropin is perhops responsible for the suppression of further 

 ovulation during pregnancy while the estrogen and progesterone pro- 

 duced probably replace or augment those of the ovary. Another hor- 

 mone, relaxin, has also been demonstrated in the urine of pregnant ani- 

 mals. It apparently is important in relaxing the ligament of the pubic 

 symphyses to facilitate parturition. Just where it is produced is not 

 entirely clear, but possibly it is from the walls of the pregnant uterus. 



The Composition of the Female Hormones. — Like testosterone 

 and corticosterone, estrogen and progesterone are members of the 

 class of compounds known as the steroids. Relaxin, so far as can be 

 determined, is not a member of this class of compounds. 



