HUMAN REPRODUCTION 



249 



the ovarian activity. It appears that the estrogens inhibit the release of FSH; at 

 higher concentrations they stimulate the cells which produce ICSH, and at still 

 higher levels they inhibit ICSH formation and cause disappearance of the corpus 

 luteum. In the menstrual cycle the second (postovulation) peak of estrogen in 

 the blood prevents release of FSH and ICSH. The resultant breakdown of the 

 corpus luteum causes estrogen to fall to a very low level, and since this hormone 

 is necessary to sustain the thickened uterine walls, the latter slough off and 

 menstrual bleeding occurs. With the disappearance of estrogen from the blood, 

 FSH is released once more, and another follicle starts to mature. 



Alternatives: (a) If fertilization has not occurred 



(b) If fertilization has occurred 



14th to 280th. 



No follicle formation or 

 ovulation during preg- 

 nancy 



Remains during 

 first 7 months 

 of pregnancy 



Fertilized ovum imbeds 

 itself in uterine wall 

 and grows 



1 Modified from The Machinery of the Body, by A. J. Carlson and Victor Johnson, 

 courtesy of the University of Chicago Press. All figures are approximate and vary 

 from individual to individual. 



