70 



VERTEBRATE REPRODUCTIVE CYCLES 



cease their growth, their blood vessels contract, and 

 many of their cells undergo resorption so that both 

 organs shrink rapidly to a minimum size. 



In the menstrual cycle of the monkeys, apes, and 

 man the same events take place in the same sequence. 

 At the end of a menstrual period the F.S.H. begins to 

 be secreted by the anterior pituitary with consequent 



? 

 Days 



4 6 d 10 12 1U 16 Id 20 22 2U 26 26 



Fig. 12. — Diagram illustrating the changes in foilicle 

 size wKich occur during a typical human menstrual 

 cycle. In monkeys and apes the average length of the 

 menstrual cycle varies from about i6 days in a 

 capuchin to about 45 days in a gorilla. 



oestrogen secretion and follicle growth. After ovulation 

 progesterone maintains the uterus in readiness for the 

 implantation of the embryo should fertilisation occur. 

 Finally, in the absence of fertilisation, the production of 

 progesterone slackens and the uterus, having neither 

 oestradiol nor progesterone to maintain it, undergoes 

 destructive changes. During this time the breakdown of 

 the uterine wall is so active that capillaries are ruptured 

 and a flow of blood escapes through the vagina. 



In a human cycle in which ovulation occurs half-way 

 between menstrual periods the phase of follicle grov^th 



