ABSTRACT
Old world monkeys and apes, like man, differ from most lower mammals in experiencing a continuous series of monthly menstrual cycles in the absence of pregnancy. The difference is not one of kind, for the menstrual cycle, like the œstrous cycle, consists of a rhythmical series of morphological changes in the ovaries and accessory reproductive organs. It is, however, distinguished by a phase of endometrial destruction, lasting about four days, which results in uterine haemorrhage and which is more pronounced and extensive than the cyclical endometrial degeneration of lower mammals.
