Credible observations even exist, indicating that among many primitive peoples, in whom at the time of puberty no social laws hinder the limitless exercise of the reproductive functions, this capacity on the part of woman to conceive at any time has no existence, and that the reproductive capacity of such human beings is, like that of the lower animals, confined to a certain season of the year. Thus, _G. Schlesinger_ reports of the Ainus of the island of Yezo, “A friend of mine in Sapporo believes himself to have observed that the Ainus have a certain definite rutting period, and that in them, as in many of the lower animals, the process of reproducing the species occurs only at a certain season of the year.” An identical statement is current concerning the Indians of Western America. The mucous membrane of the uterus undergoes during menstruation important changes, and a question much disputed is, whether in the course of menstruation the whole of the uterine mucous membrane is removed, or a part only, whether it is shed in its entire thickness, or is at least deprived of its epithelium. According to the observations made by _Leopold_ on dead bodies, the mucous membrane of the uterus becomes swollen shortly before the commencement of the menstrual discharge, until, partly in consequence of cellular proliferation, partly in consequence of œdematous infiltration, and partly in consequence of enlargement of the lymph-spaces, it attains a thickness of 6 to 7 millimetres (¼ of an inch). The superficial capillaries are notably enlarged, and an effusion of blood-elements continues for several days, without the occurrence of any fatty degeneration in the tissues. The epithelium and the most superficial cell-layers of the mucous membrane are, however, undermined and shed. No complete destruction of the mucous membrane occurs, however, and fatty degeneration forms no part of the menstrual process as such. _Möricke_, who examined portions of the uterine mucous membrane removed with the curette during menstruation from living women, found the superficial layers of the mucous membrane to be intact, and he regards the shedding of the epithelium described by other authorities as cadaveric phenomenon. _Sinéty_, who also found the uterine mucous membrane intact during menstruation, adheres to the same view. _Von Kahlden_ concludes, as a result of investigations made post mortem, that during menstruation the greater part of the mucous membrane, not the superficial epithelium only, but the stroma itself down to its deepest layers, is shed. According to _von Tassenbroek_ and _Mendes le Leon_, however, the most superficial layers only are shed during menstruation.