Although a few cases are on record which indicate that menstruation in woman, as in the monkey,1may occur independently of ovulation, it is held that, in general, ovulation occurs at a fairly definite time in each period, producing a corpus luteum which regulates the time of onset of the succeeding menstruation. Evidence for this has been obtained from patients at operation or from autopsies.2The actual day is given as between the eighth and the seventeenth, the exact time varying with the author. Woman differs from the female in other species examined, except the macaque, in allowing coitus at any time during the cycle, and the question accordingly arises whether conception may occur at any time. This would appear to be unlikely if ovulation occurs at a definite point in the cycle, for it is improbable that the ovum preserves the power of fertilization for long after