_Kehrer_, who in general supports the view that the _modus coeundi_ and an active attitude on the part of the female have an important influence on the occurrence of conception, assumes that independent contractions of the cervix occur, whereby is expelled the delicate plug of mucus that fills the cervical canal and offers an obstacle to the passage of the spermatozoa. He believes that the duration of the act of intercourse, the mechanical relations between the penis and the vagina, the activity of the uterine muscle, the secretory activity of the utero-vaginal mucosa during the act, and the posture of the female _post coitum_, are all important factors in the occurrence of conception. Thus, he believes that if during intercourse there is a failure of the uterine contractions, which should expel the plug of cervical mucus, the semen flows away without effecting fertilization; if an unsuitable posture is assumed during intercourse the woman remains sterile, but can be fertilized without difficulty by coitus effected in the proper manner. _Haussmann_ has shown, that in the same woman, and in similar conditions, spermatozoa will on one occasion be found in the cervical canal, and on another occasion will not be found there; and he has further shown, that in some women we fail to find spermatozoa in the cervical canal in circumstances in which, in other women, we regularly find them in that situation. Far as we may be from a complete knowledge of the conditions upon which conception depends, this at least is certain, that the passage of spermatozoa through the os uteri externum is a sine qua non of fertilization. Indeed, it would seem that we must accept as true the assumption of _Meyerhofer_, that fertilization is possible only if the semen passes at once into the cervical canal, mingles, that is, at once with the alkaline cervical mucus—unless, indeed, the coitus takes place during the catamenial flow, when the blood has neutralized the acid reaction in the vagina, or takes place when some morbid condition has had the same result. The theory of _Johann Müller_, regarding the piston-like action of the penis during coitus, by which the semen is actually forced through the cervix, must be rejected; equally unsound is _Holst’s_ assumption that during intercourse the semen is ejaculated through the enlarged cervical canal directly into the cavity of the uterus. It would appear, however, to be a necessary condition of fertilization, that the semen should be ejaculated into the uppermost segment of the vagina, so that the fluid comes into actual contact with the os uteri externum; it may be that the alleged aspiratory force of the uterus then comes into play, by means of which the semen is sucked into the cavity of that organ; it may be, on the other hand, that _Beigel_ is right in his theory of the existence of a _receptaculum seminis_, formed by the anterior and posterior lips of the cervix uteri