ABSTRACT
It was suggested in Chapter VII that the main difference between the menstrual cycle of the primate and the œstrous cycle of the lower mammal lies in the distribution of their respective follicular phases—phases during which the mating hormone, cestrin, is effective. The follicular phase of the lower mammal ends virtually with the cellular destruction that characterizes œstrus, and the endometrium of pseudo-pregnancy is built up, under the influence of luteal hormones, from an endometrium that has undergone degeneration at the time of ovulation. In the primate, on the other hand, the effects of the active secretion of œstrus are manifest throughout the cycle, although its main effects, as judged by the changes in the sexual skin, occur before ovulation. The growth of pseudo-pregnancy that follows ovulation is superimposed upon a persisting pro-œstrous endometrium. Interpreted in this way, the difference in sexual behaviour between primates and lower mammals reflects their different sexual physiological mechanisms. The matings of the lower mammal are confined to short œstrous periods, towards the end of the follicular phase. The matings of the primate, as is explained below, are diffused over the entire cycle, but vary in frequency according to the varying degrees of the activity of the follicular hormone.
