HUMAN REPRODUCTION 



247 



the same changes occur in the uterine wall. If implantation fails to happen, most 

 of the thickened uterine wall is sloughed off with bleeding, the cycle begins anew, 

 and the uterus again makes ready for reception of an embryo. Since the bleeding 

 comes at intervals of about 28 days, it is called menstruation (Latin, menstruus, 

 "monthly"), and the special primate type of estrus cycle, which occurs only in 

 mammals of that order, is called the menstrual cycle. 



The pregnancy cycle. If a fertilized egg becomes implanted in the uterus, 

 the menstrual cycle is interrupted and the 9-months pregnancy cycle intervenes. 



Fig. 16.3. Diagram of the changes occuring in the human menstrual cycle (left portion of 

 figure) and in the first 8 weeks of t"he pregnancy cycle (right portion of figure). A, hormonal 

 changes. These are only approximate and are not intended to show exact quantitative rela- 

 tionships. B, ovarian changes. C, changes in the thickness and blood supply of the uterine 

 wall. (From Amber son and Smith, Outline of Physiology, by permission The Williams & 

 Wilkins Company.) 



The uterine lining remains thickened throughout this period, and various other 

 changes occur in the body related to nutrition of the developing embryo and in 

 preparation for its birth and postnatal care. Like those of the menstrual cycle, 

 these changes are produced in response to a number of hormones which constitute 

 an interrelated and balanced system of chemical control. 



The ovarian hormones. During the first week following menstruation the 

 ovum and the follicle in which it lies develop and ripen. The follicle becomes larger, 

 and its central cavity fills with a fluid — the follicular fluid — which contains the 

 hormones called estrogens. 1 These are a group of chemical compounds related 

 to one another and to the androgens of the male and, like the latter, have certain 



1 Also called theelin or estrone. 



