348 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



Fig. 18.12. Photograph of a section of the ovary of a whale showing the typical 

 mammalian structure. In the largest follicles, the wall has split giving the appear- 

 ance of double sacs. In life, the minute ovum (not visible) is in the smaller sac 

 surrounded by fluid. There is a corpus luteum, the solid growth, at each end of the 

 ovary. When taken from the whale this ovary was about 14 inches long. (Courtesy, 

 The South Kensington Natural History Museum, London.) 



* 



ment of the latter. Without the embryo the reactions of the uterus are very 

 different; they are outlined in a later paragraph. 



Ovarian Hormones. The ovaries produce at least two hormones. Both are 

 secreted under the influence of the gonad-stimulating hormones (the follicle 

 stimulating hormone FSH and the luteinizing hormone LH) of the anterior 

 lobe of the pituitary gland and the luteotrophic hormone (LTH) (Fig. 18.13). 



Estrogen, the female counterpart of the testicular hormone, androgen, is 

 secreted by the follicle. Although the ovaries are the principal source of estro- 

 gen, it has also been extracted from the placenta, testes, cortices of the adrenal 

 glands, and even from certain plants. A second hormone, progesterone, is 

 secreted by the corpus luteum, also by the placenta and adrenal cortex. Pro- 

 gesterone, acting with estrogen, stimulates the uterine wall to receive and hold 

 the embryo; with estrogen it also stimulates the development and growth of 

 the mammary glands. Both hormones play important parts in the reproductive 

 cycles of the female, in the production of secondary sex characters, and in 

 sexual behavior. 



Female Reproductive Cycle 



In mammals generally the reproductive or estrous cycle includes the produc- 

 tion of one or more mature eggs and the preparations for the protection and 

 nourishment of one or more embryos. Fertilization of the eggs may not occur 



