A new epoch of scientific research into the nature of menstruation began with _de Graaf’s_ discovery of the ovarian follicles (1672). This discovery did not, indeed, bring ovulation and menstruation into immediate relationship, but it certainly paved the way for the opinion expressed by _Sintemma_, a countryman of _de Graaf_, that the ova, even in virgins, leave the ovary spontaneously, and by their contact with the capillary terminations of the bloodvessels give rise to the menstrual bleeding (1728). As a result of anatomical investigations, _Négrier_, in 1840, was the first to establish the thesis that in women suffering from congenital absence of the ovaries, menstruation never occurs; that after the loss of the ovaries, menstruation always ceases; that during pregnancy and lactation and during the climacteric period, ovulation ceases; and that a relation of temporal succession obtains between ovulation and menstruation. This close relation between the two processes was maintained also by _Gendrin_ at about the same date. Later, _Girdwood_, by post mortem research, proved that the number of scars in the ovary coincides with the number of previous menstruations. _Brierre de Boismont_, in his exhaustive work on _Menstruation_, lays stress on the view that the periodically recurring ovulation furnishes the impulse for the menstrual flow. First among German investigators, _Bischoff_ upheld the opinion that maturation and discharge of ova are spontaneous processes occurring independently of sexual intercourse, and compared heat or rut in other animals to menstruation in women—a view shared by _Pouchet_ and _Coste_. Ovulation occurs simultaneously with the menstrual flow, and the follicles burst toward the end of menstruation. _Pflüger_, in his important work on the significance and cause of menstruation, has demonstrated the causal connection between menstruation and ovulation. The bleeding and the discharge of the ova are according to him joint effects of a common cause. It is not the bursting of the follicle, but the ripening of the follicle, that gives rise to the menstrual congestion. The pressure of the growing follicle on the surrounding ovarian tissue gives rise to a continued stimulation of the ovarian nerves; the summation of these stimuli, which after the lapse of a certain time attain always a certain degree of intensity, results in a reflex from the spinal cord taking the form of great congestion of the genital organs; this congestion leads, on the one hand, to hæmorrhage from the uterine mucous membrane, and, on the other hand, and as a rule simultaneously, to the bursting of the ovarian follicle. The swelling and granulation of the uterine mucous membrane at every menstrual period signifies nothing else than the commencement of the formation of the decidua.