The precise period at which conception in the human subject occurs in most cases cannot, for obvious reasons, be determined, Nit when ever conception can be traced to a single op portunity, the process of impregnation, or the fertilisation of the ovum by contact with the spermatozoa, may be assumed to take place within a few hours after the act of insemina tion ; for the spermatic fluid rapidly traverses the generative canal, while here spermatozoa cease to have motion within thirty hours at latest frorn the time of emission.
From various niethods of computation it is supposed that in a large majority of cases con ception occurs during the first half of a men strual interval, and most commonly during the first week. In sixteen instances noted by Raciborski conception occurred as late as the tenth day after menstruation in only one case.* The number of instances in which con ception can be ascertained, or may be fairly assumed, to have taken place in the latter half of a menstrual interval is comparatively small. Nevertheless impregnation may unquestionably occur during this time, and even within a day or two of the next menstrual flow, which is then usually diminished in duration and quan tity, or is reduced to a mere show.
Now if we endeavour to explain these facts, relating to the times of conception, by the aid of an ovular theory of menstruation, the ques tion may be brought within very narrow limits. One of two postulates may be assumed. An ovum emitted at or soon after a menstrual period either remains susceptible of impreg nation through the whole of the succeeding interval, or it loses that susceptibility, and perhaps perishes before the recurrence of the next menstrual flow.
The first hypothesis would sufficiently account for impregnation taking place at any part of a menstrual interval ; but it has little or no evidence for its support. Nothing, in deed, is known regarding the length of time during which the buman ovum remains sus ceptible of impregnation after it has escaped from the ovary. The period of susceptibility in the mammalia generally is variable. In the bitch, as already' stated (p. 606.), the ovum, after quitting the ovary, is supposed to re main in the tube during six or eight days. Its passage is probably quite completed in ten days. In the guinea-pig the period is much shorter, as the ovum enters the uterus at the end of the third day. In the rabbit also the pe riod does not extend beyond the beginning of the fourth day. But by the time that the ovum
reaches the uterus, or sometimes even the lower end of the oviduct, in most of the rnammalia yet observed, the cestrus is past, and with it also the opportunity for impregnation. The evidence therefore obtainable from the main malia fails to support the conjecture, that in man an ovum detached during menstruation can remain susceptible of impregnation through the whole of a !menstrual interval, consisting of twenty-three or more days, although the period of this susceptibility may be longer in man than in the other examples cited.
But if this first hypothesis fails, the second appears inevitable, viz., that an ovum emitted during menstruation loses its susceptibility of impregnation before the termination of the succeeding menstrual interval. M. Pouchet supposes, that in the human subject the dura tion of this susceptibility does not exceed four teen days. Consequently if, according to the strict formula of the latter physiologist, ova are emitted only at or shortly after the menstrual periods, there must remain a portion of each menstrual interval, during which every woman is physically incapable of conception. And this alternative M. Pouchet* does not hesitate to adopt.
But since this conclusion is incompatible with the facts already stated regarding the occasional, though probably rare occurrence of conception during the latter portion of a men strual interval, and especially towards its con clusion, M. Coste, who shares with many others a belief in these facts, has proposed an explanation which constitutes a very con siderable modification of the ovular theory of menstruation. To account for impregnation at a later period than usual of a menstrual in terval, M. Coste supposes that a ripe or dis tended Graafian follicle, having failed in reach ing the point of rupture, may remain stationary, as it sometimes does in mammals and that the influence of the male is sufficient to de termine the dehiscence of a follicle in such a state. And in order to anticipate the obvious objection, that if the emission of an ovum fi-om the ovary is the cause or occasion of menstrua tion, the latter phenomenon ought to be re peated whenever the former event occurs ; and consequently in the case now under conside ration NI. Coste sugezests that the same cause which provokes the discharge of the ovum in this case, also occasions fecundation, Ns hich arrests the menstrual flux before this has dine to manifest itself.