Is material contact of the semen and ovum necessary for fecundation?—No one has ever discovered any of the seminal fluid within the egg : the most minute observation does not de tect any appearance of this. A question then naturally presents itself in reference to the sub ject of our present inquiry, viz, whether it is necessary that a certain quantity of the sub stance of the seminal fluid should be brought into actual contact with the egg in order to cause its fecundation ? and if so, in what man ner and in what part of the female organs such contact is brought about? Were we to look no further than to the manner in which fecundation is effected in many of the inferior animals, we might be in duced at once to form the opinion that the mere contact of a certain quantity of the seminal fluid with their ova, in whatever way brought about, is all that is necessary for producing their fecun dation. Thus, in the greater number of fishes the milt or seminal fluid of the male is shed over the spawn of the female after it is laid in water, without there being any nearer sexual intercourse; the fecundation is external to the body of the female, and we thus know with certainty that the ova and the seminal fluid are the only parts immediately concerned in the process. fhe same is the case in the common frog, in which there is copulation ; for in that animal, although the male and female remain united firmly together during a longer period than any other kind of animal, yet this union is not a means of producing fecundation, but rather of promoting the discharge of unim pregnated ova from the generative system of the female. There is in fact no true sexual union : the spawn is laid by the female un fecundated, and the male (separating then in general from the female) sprinkles seminal fluid on the ova floating in water.
External and artificial mode of fecundation just now mentioned sug Bested to Spallanzani the ingenious expe riment of artificial fecundation, which he first performed, and which furnished the most con vincing proof that could be obtained, that, in such animals as the frog, sexual union is not essential to fecundation, and that, when the ova are ripe and the seminal fluid of the suit able quality, the mere contact of the male and female products is sufficient to confer fertility upon the ova.
Spallanzani opened very many female frogs at the time of propagation, but before they had laid any spawn, and consequently before im pregnation could have occurred, and he satis fied himself that the ripest ova extracted from the oviduct, andplaced in water, gradually passed into putrefaction without undergoing any of the changes of development; while some of the same ova, upon which he had sprinkled some of the seminal fluid taken from the body of the male, and placed in similar vessels of water, had tadpoles formed from them in the same manner exactly as those which were fe cundated by the male frog The same experiments were performed by Spallanzani on toads and newts with exactly the same result.
Spallanzani, in order to avoid every fallacy, allowed the female to remain in union with the male, and to lay her spawn in the natural way, preventing only the access of any of the seminal fluid of the male to the ova, by tying up the hinder part of the male's body in oiled silk, and these ova were alike barren, unless he added to them some of the seminal fluid in the artificial mode.
Treviranust mentions the performance of the experiment of artificial fecundation in Fishes, viz. on the spawn of the Salmon, Trout, and
Carp, by Duhamel and by Jacobi. Jacobi's experiments were repeated by Dr. Walker of Edinburgh ; and very recently it has again been performed on the spawn of the Tench and Bleak by Itusconi of Pavia. (Cyprinus Tinca and Alburnus.) The very complete series of experiments of Messrs. Prevost and Dumas§ on the frog afford the most satisfactory confirmation of those of the Abbe Spallanzani.
The following appear to be the more im portant results deducible from these two sets of experiments.
1st. That a very small quantity indeed of the seminal matter is requisite for the fecundation of the ovum.
2d. That dilution of the seminal fluid with water within certain limits does not impede, but rather is fiivourable to its operation.
3d. That the absorbent power of the albumi nous or gelatinous matter which surrounds the black yolk is highly useful in bringing the seminal substance in contact with the yolk, where it is obvious its effect must be produced.
This albuminous covering, corresponding to the white of the bird's egg, possesses the remark able property of absorbing water, somewhat like gum tragaeanth, in a determinate quantity, and thus increases greatly in bulk after being laid in water. In the experiments referred to, the absorption of the water by the jelly was fully demonstrated by the immersion of the ova in coloured water, and it was found also that the experiment of artificial fecundation suc ceeded best when the ova had not been im mersed in water for any considerable time pre vious to the addition of the seminal fluid. The fecundation was less certain the longer the ova were allowed to remain in water before the ad dition of the semen ; and it was sliewn that this did not depend simply on the length of time of the separation of the ova from the body of the parent, by the fact that ova taken from the oviduct and kept without moisture re tained their susceptibility of being fecundated for a much longer period, as sixteen or twenty hours.
4th. That the seminal fluid of the frog retains its fecundating power for about thirty hours after it has left the body of the male.
5th. Attempts were made by both the expe rimenters above quoted to ascertain, by way of experiment, whether the seminal animalcules are indispensable to fecundation. Spallanzani came to the conclusion that the seminal fluid did not lose its peculiar powers although de prived of its animalcules, or although the ani malcules were dead ; but it must he admitted that the means employed by that observer to ascertain the presence or absence of the seminal animalcule were inferior to those we possess in more recent times. Messrs. Prevost and Dumas, who, it has already been remarked, consider the animalcules as the most important part of the seminal fluid in reference to its fecundating properties, state tbat they found in their experiments, that that part of the seminal fluid which had been subjected to a very careful filtration, and which had thus been wholly de prived of its animalcules, had lost all fecunda ting, power, while the substance which remained in the filter, and which was rich in animalcules when diluted with water, possessed the same powers of fecundation as the pure seminal fluid. We think this experiment requires repetition and some modifications, for other ingredients, besides the animalcules of the seminal fluid, might be retained on the filter.