276 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Fig. 15.18. Common grass frogs {Rana pipiens): one shows the natural light 

 brown and dark spots; the color of the nearly black one is the result of the power- 

 ful stimulation of the pigment cells by the injected intermedin, the hormone of 

 the pars intermedia of the pituitary. When colors are pale or hidden the pigment 

 granules are clumped in the center of the cell and its branches are invisible (Fig. 

 15.2). Injections of intermedin cause the granules to move into the branches and 

 the animal becomes deep brown or blackish. (Courtesy, Therapeutic Notes. 

 Detroit, Mich., Parke Davis & Co., April, 1935.) 



ductive structures are reduced, and the body often takes on fat. These changes 

 are caused by the absence of the hormone testosterone (CioHaoOo) thought 

 to be produced by cells (interstitial) that are packed in between the tubules 

 of the testes in which the sperm cells develop. 



Estrogen, a female sex hormone, is produced by follicular cells surrounding 

 the egg in the ovary, and is responsible for the estrus or heat in female 

 mammals. If the ovaries are removed from young females, they remain sex- 

 ually immature. On the other hand, if estrogen is injected into these castrated 

 females, the usual maturing is resumed. If it is injected into normal im- 

 mature females, the secondary sex structures and the estrous periods are 

 hastened into full development but the ovary is not affected and the develop- 

 ment of the eggs is not hurried. Another ovarian hormone, progesterone, is 

 produced by the corpus luteum formed from the cells of the Graafian follicle 

 which are left after an egg is shed from the ovary. Estrogen and progesterone, 

 working together, prepare the uterus for receiving the young embryo (Fig. 

 15.19). These two hormones also stimulate the enlargement of the mammary 

 glands in which the secretion of milk is later induced by the lactogenic hor- 

 mone of the pars anterior of the pituitary gland (figures and further discussion 

 of the sex organs, Chap. 18), 



