This circumstance is rendered more striking in animals in whom this interval is longest, as in the roe-deer, where the cestrus returns only once annually, and in whom the capacity for procreation is limited to a few weeks, for the reason stated by Bischoff, that then the ovary contains ripe ova and the testes ripe semen, and at no other time.
l3ut in the human female, whatever views may be entertained regarding the connexion of a separate act of ovulation with each menstrual period, it is certain that here a marked astrus is wanting, and that although the capacity for impregnation is apparently greatest about the times of menstruation, yet, notwithstanding the assertions of those who maintain that there is a perpetual recur rence of temporary incapacity for procrea tion, there is no period at which the healthy human female can be shown to be positively incapable of conception during any part of menstrual life.
It may, however, be asked whether the oc casional occurrence of impregnation during an intermenstrual period, at a date more distant than usual from the last menstrual act can be explained consistently with a strict interpretation of the law that menstruation and ovulation are contemporaneous acts.
This appears to be reconcilable with the circumstance that although these acts, so far as observation has yet gone, are very fre quently and perhaps usually coincident, yet exceptionally an ovum may be emitted during an intermenstrual period, the ripening and not the time or the act of emission of the ovum being probably the essential feature, or that the ovum, supposing it to have been emitted from the ovary at the time of men struation, may possibly remain in the tube susceptible of' impregnation longer in the human female than in the mammalia gener ally, or may even be impregnated after reach ing the uterus.*
That the Fallopian tube in the human subject is, occasionally at least, the seat of iinpregnation, is demonstrated by the occur rence of the tubal form of extra-uterine ges tation ; while the numerous examples already quoted of other mammalia render it highly probable, by analogical reasoning, that this is the normal seat of that function in man.
That the first encounter of the generative elements may also take place either in the uterus or upon or even within the ovary, is plainly possible. That it occurs sometimes at or near the ovary is evidenced by the varieties of extra-uterine gestation termed ovarian and ovario-tubal. It is even possible that, in some of these, insemination may have been so coincident with the spontaneous opening of the Graafian follicle, that the spermatozoa, penetrating further than usual, may have reached the ovary at that precise n2oment when a passage had been prepared for the ovum, and some may have actually passed into the follicle and have impregnated the ovum there. No argument certainly can be opposed to this on the ground of physical impossibility 1- ; while, on the other hand, it is also conceivable that impregnation may be delayed until after the ovum has entered the uterus, as in the case just suggested of a fertilising coitus occurring later than usual after the nienstrual period ; but I am not aware of any good anatomical or physiological reason for regarding the uterus, as by pre ference, the seat of normal impregnation ; while such a view is opposed to those nu merous observations upon the mainmalian ovum generally, which show, that before the ovum quits the oviduct, the deNelopmental changes in it are already advanced many stages, while, by the time that it -arrives at the uterus, the opportunity for impregnation bas already passed away for that occasion.