5th. It has not yet been shewn that one part of the seminal fluid is more necessary to impregnation than another. The seminal animalcules form a natural ingredient of the fluid secreted in the testicle at the time when it is excreted for the purposes of propagation ; they appear to be invariably present, but addi tional experiments are still wanting to prove them to he the active or essential agents of fecundation, much more the rudiments of the new being within the ovum.
6th. Like others of the operations of the animal economy, the action of fecundation is known principally in its effects; but it seems to be a question worthy of investigation whe ther, in the phenomena exhibited during fecun dation, or the laws by which this change is regulated, it be in any respect analogous in its nature to the operation of certain poisonous or contagious principles, as for example, the venereal virus, vaccine matter, the contagious principle of small-pox, measles, scarlatina, plague, fevers, &c. The inimitable Harvey thus expresses himself regarding the essential nature of fecundation in different parts of the forty-ninth Exercitation on the efficient cause of the chicken. " Although it be a known thing subscribed by all that the fcetus assumes its original and birth from the male and female, and consequently that the egge is produced by the cock and henne, and the chicken out of the egge, yet neither the schools of Physicians nor Aristotle's discerning brain have disclosed the manner how the cock and its seed doth mint and coine the chicken out of the egge." " This," he says," is agreed upon by universal consent; that all animals whatsoever, which arise from male and female, are generated by the coition of both sexes, and so begotten as it were per contagium aliquod, by a kind of con tagion." " Even also," he says, " by a breath or miasma," referring to the fecundation of the ova of fishes out of the body.
" The lac marls, male's milk, propagating or genital liquor, vitale virus, vital or quickening venom," are all names of the seminal fluid of the male. Again, " The efficient in an egge, by a plastical vertue (because the male did only touch, though he be now far from touching and have no extremity reached out to it) doth frame and set up a fcetus in its own species and resemblance.' " What is there in genera tion, that by a momentary touch (nay not touching at all, unlesse through the sides of many mediums) can orderly constitute the parts of the chicken by an epigenesis, and produce an univocal creature and its own like? and for no other reason but because it touched here tofore." " The qualities of both parents are observable in the offspring, or the paternal and maternal handy-work may be tracked and pointed out both in the body and soul." The first cause must therefore be of a mixed kind. " It is required of the primary efficient in the fabrick of the chicken, that he employ skill, providence, wisdom, goodness, and understanding far above the capacity of our rational souls."
7th. In respect to the part of the female generative system at which fecundation takes place, it appears most probable that in quadru peds and the human species this change occurs before the ovum reaches the uterus, or some way in the course of the Fallopian tubes ; perhaps most frequently in the sipper part of them. There is, however, probably some variation among animals and in different cir cumstances regarding this point. But while we state this as the conclusion most consistent with facts in the present state of our know ledge, we ought not to omit the mention of the more prominent facts by which it is opposed.
In some of the lower animals, fecundation seems to extend beyond the sphere of the ova which are ripe. In the Aphis (as was already mentioned at an early part of the paper) the production of young by the female goes on for several generations (eleven) without any sexual intercourse after that which gave rise to the first. In the Daphnia Longispina this is said also to be the case for twelve generations, and in the Monoculus pulex for fifteen. The queen-bee lays fruitful eggs during the whole year after being once impregnated ; and in the instance of the common fowl and some other birds, previously referred to more than once, if we reject the supposition of the seminal fluid remaining in action, it seems necessary to suppose that fecundation must occur in the ovary, since unripe ova are acted on by the fecundating medium at the same time with those which are arrived at maturity and are ready to descend into the oviduct.* Many physiologists also believe that the influence of the first impregnation extends to the products of subsequent ones. Thus Haller remarks that a mare which has bred with an ass and has had a mule foal, when it breeds next time with a horse, bears a foal having still some analogy with the ass. So also in the often cited instance of the mare which bred with a male Quagga, not only the immediate product, but three foals in subsequent breedings with an Arabian stallion, and these three even more than the first, partook of the peculiarities of the Quagga species.
Instances of the same kind are mentioned by Burdach as occurring in the sow and bitch; and it is affirmed that the human female also, when twice married, bears occasionally to the second husband children resembling the first, both in bodily structure and mental powers.
According to Hausmann, when a bitch has connexion with several dogs (and this is gene rally the case during the continuance of the heat, sometimes to the amount of twenty,) she usually bears two kinds of puppies at least, and the greater number of these resemble the dog with which she first had connexion.
We feel at a loss to decide what weight ought to be attached to these observations ; they appear to bear chiefly on the subjects which are discussed in the next part of this article.