6th. Both Spallanzani and Prevostand Dumas have attempted to estimate the quantity of seminal fluid required for the fecundation of a certain number of ova, and the latter observers, pursuing their favourite idea to the utmost, have even endeavoured to calculate the number of animalcules which are necessary for the fructi ficatinu of one or more ova. In Spallanzani's experiments two grains of the semen of the toad fecundated one hundred and thirteen ova. Five grains of semen were mixed with eighteen ounces of water ; the point of a needle dipped in this was made to touch an egg for an instant and produced fecundation. The proportion here might be estimated as semen 1, to egg 1,064,000,000. The addition of a larger quantity of semen, or its remaining longer in contact with the egg, did not, accord ing to Spallanzani, render the fecundation more complete than the instantaneous contact of the wetted needle's point. Prevost and Dumas state that they found the number of ova fecun dated by a given quantity of seminal fluid is always below that of the animalcule' which they estimated that fluid to contain; and by a sim ple process of calculation it was easy to find how many animalcule: served each ovum. A certain quantity of seminal fluid, for example, containing 225 animalcules, served to fecun date 61 only out of 380 ova, to which it was added, so that each ovum required about 3i animalcule' for its fecundation, or making allowance for a few of the animalcules which went astray into other ova, it may be stated as three in round numbers. It will be long before the vital processes can be traced with the arith metical precision displayed in this calculation. Unfortunately for the calculations and even the observations upon which they are founded, one of the authors at a subsequent period published the theory which appears to have prompted them to revive an old and fanciful notion that the animalcule forms the rudiment of the new being. The animalcule, according to this hy pothesis, makes its way through the stiff jelly surrounding the yolk, gains the centre of the germinal membrane, and esconces itself there in the very centre of that germinal membrane, laying thus the foundation of the primitive streak or the brain and spinal marrow of the foetus: its position (which is always the same) being no doubt determined by the laws of po larity depending upon the electromagnetic properties with which, according to equally fanciful theorists, the rudiments of the new being in the egg are endowed.
Hitherto cold-blooded and oviparous animals only have been alluded to ; but there are not wanting facts which render it highly probable that in viviparous animals also, contact of se minal fluid with the ovum is the essential part of the fecundating process. Thus, Spallanzani confined a bitch for fourteen days before the arrival of heat, and for twenty-six days after it, so that, during that time, it could have had no connexion with any dog, and at the time of the heat he injected by means of a syringe a quantity of the dog's seminal fluid into the vagina. The bitch brought forth three young exactly at the usual length of time from the period of heat; — an experiment on artificial fecundation, which may in some sort be said to have been performed in the human species is the well-known instance in which John Hun ter recommended to a man affected with hy pospadiac malformation of the urethra, which rendered intromission of the seminal fluid impossible, the injection, by means of a sy ringe, of the seminal fluid into the vagina,— an operation which, it is related, was attended with complete success.
While these facts on the one hand tend to shew that no parts of the genital organs and no other agents are concerned in fecundation excepting the seminal fluid and the ova, and on the other hand afford the only positive evidence that can be obtained, that actual con tact of the one with the other is necessary to induce the change, they have appeared unsa tisfactory to some physiologists,who cling to the opinion that, in quadrupeds and birds at least, contact is not necessary, and that fecundation may he effected either by some hidden sym pathy (or concurrent action taking place in remote parts) between the external and internal organs of the female, or that this change may be operated by some imponderable influence which emanates from the seminal substance, to which the vague name of aura seminalis has been given.
Course of the seminaliluid within thefemale organs.—In pursuing our examination of the alleged evidence upon which these and similar hypotheses are founded, it will be necessary to consider in this place another question, respecting which it is difficult in the present state of the inquiry to form a decided opinion, viz. whether, on the supposition of the seminal fluid and ova coming into actual contact, the course of the seminal fluid within the female organs of generation can in any instances be traced, and in what part of these organs it may be supposed to meet with the ova and operate their fecundation.
In the first place the examples of ovarian conceptions, or rather gestations, have been ad duced by some as a proof that fecundation necessarily takes place in the ovaries them selves. But from what was said in a former part of this paper, it will be seen that such a belief is founded on an erroneous view of the nature of these misplaced gestations, as well as of the phenomena which occur in the ovary after conception. There is no reason to believe, we may repeat, that ova found developed in the neighbourhood of the ovary have retained their situation within the Graafian vesicle. On the contrary, they must in all probability have been first discharged from the ovary upon the rupture of the vesicle, and their places occu pied by corpora lutea; and they may have been fecundated either while loose in the cavity of the peritoneum, or when they have descended some way in the Fallopian tubes, and meeting with the seminal fluid in the course of that tube, have been returned to the vicinity of the ovary by some inverted or un natural action of the parts.* Although, therefore, no very decided opinion respecting the place at which fecundation occurs can be formed from the observation of what are termed ovarian and tubular gesta tions, we are inclined to think that they shew that this process may take place, in some in stances at least, in the upper parts of the Fal lopian tube, or even in the infundibulum.