HORMONES AND REPRODUCTION 69 



During this period of rapid follicle growth, which in 

 the mouse ovary lasts for about 3 days and in the human 

 ovary for about 2 weeks, the rate of secretion of oes- 

 tradiol rises to a maximum. This causes the growth and 

 distension of the vagina and uterus, and, as described in 

 the next chapter, it also commonly changes the whole 

 behaviour of the animal so that it becomes ready and 

 eager to mate. 



As the rate of oestrogen secretion reaches a maximum 

 it reacts on the pituitary to depress the rate of production 

 of F.S.H., and, so it is said, to increase the rate of 

 secretion of I.C.S.H. Thus about the time of ovulation 

 the supply of F.S.H. and of oestrogen ig diminishing, and 

 the I.C.S.H. is then mainly responsible for the luteiniza- 

 tion of the cells of the ruptured Graafian follicle to form 

 the corpus luteum. In most species the corpus luteum 

 immediately begins to secrete progesterone, but the 

 rodents are peculiar in that the corpus luteum does not 

 become functional unless pregnancy ensues. 



Progesterone exerts a variety of influences at this time. 

 In the first place it stimulates the final build-up of the 

 uterine lining ready for the implantation of the embryo 

 should fertilisation occur. In the second, it inhibits any 

 further production of F.S.H. by the anterior pituitary 

 and so it may prevent the growth of any more ovarian 

 follicles. Thirdly it is also said to facilitate the excretion 

 of any remaining oestrogen via the kidneys. If the animal 

 becomes pregnant the activity of the corpus luteum is 

 usually prolonged, and the uterus continues to grow in 

 size to accommodate the enlarging embryo. 



In the absence of pregnancy, however, the activity of 

 the corpus luteum quickly wanes so that, with neither 

 oestradiol nor progesterone present in sufficient quanti- 

 ties to inhibit it, the anterior pituitary can once more 

 begin to secrete its follicle-stimulating-hormone to 

 begin another cycle. At this moment, when the con- 

 centrations of oestradiol, progesterone, and the pituitary 

 hormones are all at a minimum, the uterus and vagina 



