Chap. 18 REPRODUCTION 



Table 18.1 



Sequence of Events in the Human Cycles of Menstruation 



AND Pregnancy 



(See also Figs. 18.14 and 18.15) 



353 



Comparison of Female Reproductive Cycles 



In all mammals, the most important feature of the cycle is ovulation. In most 

 mammals, this is accompanied by marked sexual excitement. Mating is con- 

 nected v.'ith the ovulation period and limited to it. In the human species and 

 higher primates, there are usually few or no outward symptoms of ovulation. 

 Mating occurs at any time and without reference to ovulation. 



In most mammals, if no egg is fertilized, the preparation of the uterus for 

 the embryo subsides gradually without rupture of blood vessels or glands. In 

 the human species and certain primates, under the same conditions, the prep- 

 arations in the uterus are drastically destroyed with the rupture of blood ves- 

 sels and the sloughing off of much of the uterine lining. The physiological 

 advantages of menstruation are not evident. Nonmenstruating animals expe- 

 rience essentially the same reproductive cycles, and the uterine life of the 

 embryo is quite as complete as in menstruating ones. 



