_Nägele_ already mentioned the view, that inasmuch as immediately after the first appearance of menstruation a woman has become capable of reproducing the species, each process of menstruation must be regarded as a renewal of the exhausted faculty for conception. _Pflüger’s_ teaching has been opposed by _Sigismund_, who, whilst admitting the periodicity of ovulation and menstruation, yet regards the two processes, in the uterus the formation of the menstrual decidua, in the ovary the rupture of the graafian follicle, as independent of one another, even though they occur simultaneously. Should fertilization occur, the ovum implants itself in the prepared soil; should fertilization fail to occur, the menstrual hæmorrhage ensues. Thus, the occurrence of menstruation indicates that fertilization of the ovum has failed to occur. On this theory, then, the ovum that is fertilized belongs to the first period missed, whereas _Pflüger_ assumes that when pregnancy occurs, it is always the ovum belonging to the time of the previous menstruation—the last actual menstrual discharge—that is fertilized. _Löwenhardt_, in his work on the _Diagnosis and Duration of Pregnancy_, advances the same views as _Sigismund_. The fertilized ovum, in his opinion also, is that of the first period missed; and since at the time at which he believes fertilization to occur the ovum is certainly still in the ovary, fertilization, on this theory, must always take place in the ovary itself, and the fertilized ovum cannot begin its intra-uterine life till a month has elapsed after fertilization. _Reichert_, _Kundrat_, _Engelmann_, and _Williams_, basing their views on anatomical data, are of opinion that ovulation recurs periodically, and that the extrusion of the ovum occurs not before but after the commencement of menstruation. According to _Hensen_, the observed facts support the view that the follicles burst as a rule toward the end of menstruation; anticipation or postponement of the opening of the follicle (conception before or after menstruation) would, however, appear not to be impossible. _Leopold_, who assumes that menstruation may occur without ovulation and ovulation without menstruation, maintains on anatomical grounds that the rupture of the graafian follicle occurs chiefly during menstruation, under the influence of the swelling due to menstrual congestion. Menstruation with ovulation he believes to be a common occurrence, menstruation without ovulation, an unusual occurrence. Further, it is certain that, at the time when the periodic bleeding is due, ovulation may occur, even though the menstrual discharge fails to make its appearance (ovulation without menstruation). _Chazan_ and _Gläveke_ also adhere to the generally accepted view that ovulation is a periodic process, usually but not necessarily synchronous with menstruation.