Spallanzani, with a view to investigate the powers of a vapour supposed to rise from the seminal fluid, exposed a quantity of the ripe nnimpregnated spawn of the frog for some time in the same vessel with a quantity of seminal fluid, the latter being placed at the bottom of the vessel, the ova at the top, and never was any fecundation produced ;—an experiment, it is true, from which no more than negative evidence can be derived, but upon the whole more worthy of trust as being subject to fewer fallacies than those of Mondat.
The instances in which it has been alleged that impregnation has taken place in the human female without there being any possibility of the seminal fluid itself passing inwards in the female genital passages, are of a very doubtful nature, and liable to so many sources of fallacy, that we feel little disposed to admit them as grounds of proof of the agency of an aura seminalis. In some of the cases in which it has been found, either in the course of pregnancy or at the time of child-birth, that the female passages are obstructed, there is reason to be lieve that the closure has been produced sub sequent to the occurrence of conception ; and the same may be said of those cases of ovarian gestations in which an obliteration of the Fal lopian tubes has been observed. In the greater number of such cases, it may also be observed, the malformation of the parts has consisted in the much contracted state of the external orifice or some other part of the passage, rather than in their absolute closure, so that there was merely a difficulty and not an impossibility of the entrance of seminal fluid. But in opposi tion to such vague and ill-ascertained observa tions, a variety of circumstances, which it is not necessary to particularize, might he adduced, tending to show how very easily in the human female as well as in other animals all mecha nical ubstructions to the entrance of the seminal fluid into the uterus tend to prevent conception.
Gcncral conclusions respating fecundation. in conclusion we would remark, 1st, that while we readily admit a very small quantity indeed of the seminal fluid to be sufficient to produce fecundation, we think that what has previously been stated warrants the conclusion, that material contact of a certain quantity, however small, of the seminal fluid with the ovum is necessary to give rise to its fecunda tion, and, consequently, that the hypothesis of an aura is untenable. And for the same reasons it follows that there are no just grounds for holding the opinion either that fecundation consists in a sympathetic action of a nervous kind, or that it is brought about by the absorp Lion of the semen into the circulatory or lymphatic vessels of the generative system.
2d. It is sufficiently obvious that in quadru peds there is no exact proportion between the quantity of seminal substance or fluid received by the female or emitted by the male, and its effect in producing fecundation,—a circum stance which points out a distinction which ought always to be borne in mind between that vital change on the female genital system and the whole economy and ovum, and the simple physical re-action which may take place be tween the semen and ovum themselves.
3d. We may regard venereal excitement of the genital organs and impregnation of an ovum as different phenomena, for though they usually occur together, there are instances in which they take place quite independently.
4th. The action of impregnation is to be regarded as sui gcneris, or quite peculiar among the vital processes. It is not capable of being imitated by any other substance than the seminal fluid, and neither experiment nor ob servation enables us to form the most distant conjecture what the nature of that action may be, which, from the influence of the male pro duct, confers upon the ovum a new and independent life, and enables to give birth to a new individual the mass of organic matter in the egg, which, without the change of fecun dation, would prove altogether barren and undergo no other changes than those of similar dead matters. The action, however, is in some respects reciprocal, and we cannot determine what part either of the two agents concerned performs in the change of fecundation : we know only this, that unless the seminal fluid be of the suitable composition it is ineffectual, and that ova are susceptible of its influence only when in that period of their evolution when they are ripe. Nor can we with certainty fix on what part of the egg the influence of the male semen more immediately operates. Since the foetus grows from the centre of the germinal layer, it has been commonly supposed that this is the part of the egg which is most imme diately affected by fecundation, but we know nothing of this; and it might be held, on the other hand, that the effect of fecundation ope rates on the rest of the contents of the egg in enabling them to be assimilated round the germinal centre or rallying point of the de velopment of the new being.