632 



VERTEBRATE LIFE AND ORGANIZATION 



Figure 30.13. The menstrual cycle in the human female. The solid lines indicate 

 the course of events if the egg is not fertilized; the dotted lines indicate the course 

 of events when pregnancy occurs. The actions of the hormones of the pituitary and 

 ovary in regulating the cycle are indicated by arrows. (Villee: Biology.) 



mixture of FSH and LH from the pituitary. Ovulation occurs about 

 fifteen days after tfie beginning of the previous period of menstruation. 

 The corpus luteum develops and under the stimulation of LH, secretes 

 progesterone. Progesterone, together with estradiol, promotes further 

 growth of the endometrium. The endometrial glands grow further and 

 become secretory and the blood vessels become long and coiled. Pro- 

 gesterone decreases the activity of the uterine muscles and brings the 

 uterus into a condition so that the developing embryo formed from the 

 fertilized egg can become implanted and develop. Progesterone inhibits 

 the development of other follicles. If fertilization and implantation do 

 not occur the corpus luteum begins to regress, it secretes less progester- 

 one, and the endometrium, no longer provided with sufficient progester- 

 one to be maintained, begins to slough. Thus menstruation ensues, 

 completing the cycle. 



If pregnancy occurs the corpus luteum remains and continues se- 

 creting progesterone, which is necessary for the continuation of preg- 

 nancy. Removal of the ovary or of the corpus luteum results in the 

 termination of pregnancy. In some animals the placenta produces enough 

 progesterone so that loss of the corpus luteum does not result in abor- 

 tion. Progesterone also stimulates the growth of the glands and ducts of 

 the breasts during the latter months of pregnancy, and prepares them 



