The origin of the germs, and the mode of their existence in the female, whether they are formed anew by the action of life, or are pre-existent, and inclosed within each other ; or whether they are disseminated, and require a concourse of circumstances to bring them into a situa tion favourable for their developement ; are questions, which, in the present state of our knowledge, it is utterly impossible for us to decide. These points have for a long time been agitated by physiologists; but the discussion seems now to be aban doned by universal consent.
The combination of the sexes, and the mode of fecundation, are subject to great variety. In some instances they are united in the same individual, and the animal impregnates itself. The acepha Ions mollusca and the echinus exemplify this structure. In others, although the sexes are united in each individual, an act of copulation is required, in which they both fecundate and are fecundated. This is the case with the gasteropodous mollusca, and several worms. In the re mainder of the animal kingdom the sexes belong to different individuals.
The fecundating liquor is always ap plied upon or about the germs. In many cases the ova are laid before they are touched by the semen ; as in some fishes of the bony division, and the cephalopo dons molluscs. Here, therefore, impreg nation is effected out of the body; as it is also in the frog and toad. But in the lat ter instances the male embraces the fe male, and discharges his semen in pro portion as she voids the eggs. In most animals the seminal liqour is introduced into the body of the female, and the ova are fecundated before they are discharg ed. This is the case in the mammalia, birds, most reptiles, and some fishes ; in the hermaphrodite gasteropodous mol lusca, in the crustacea, and insects. The act by which this is accomplished is termed copulation.
In all the last mentioned orders ova may be discharged without previous co pulation, as in the preceding ones. But they receive no further developement ; nor can they be fecundated when thus voided.
The effect of a single copulation va ries in its degree ; it usually fecundates one generation oily; but sometimes, as in poultry, several eggs are fecundated ; still, however, they only form one gene ration.
In a very few instances one act of co pulation fecundates several generations, which can propagate their species with out the aid of the male. In the plant
louse (aphis) this has been repeated eight times; and in some monoculi twelve or fifteen times.
When the germ is detached from the ovary, its mode of existence may be more or less complete. In most animals it is connected, by means of vessels, to an or mass, the absorption of which nourishes and developes it until the period of its birth. It derives nothing, therefore, from the body of the mother, from which it is separated by coverings varying in number and solidity. The germ, together with its mass of nourish ment, and the surrounding membranes, constitutes an egg or ovum ; and the ani mals which produce their young in this state are denominated oviparous.
In most of these the germ contained in the egg is not developed until that part has quitted the body of the mother, or has been laid: whether it be necessary that it should be afterwards fecundated, AS in many fishes, or required only the ap plication of artificial heat for its incuba tion, as in birds ; or that the natural heat of the climate is sufficient, as in reptiles, insects, &c. These are strictly oviparous animals.
The ovum, after being fecundated, and detached from the ovarium, remains, in some animals, within the body of the mo ther, until the contained germ be de veloped and hatched. These are false viviparous animals, or ovoviviparous. The viper and some fishes afford instan ces of this process.
Mammalia alone are truly viviparous animals. Their germ possesses no pro vision of nourishment, but grows by what it derives from the juices of the mother. For this purpose it is attached to the in ternal surface of the uterus, and some times by accident to other parts, by a kind, of root or infinite ramification of vessels called a placenta. It is not, therefore, completely separated from the mother by its coverings. It does not come into the world until it can enjoy an independent organic existence. The mammalia cannot, therefore, be said to possess an ovum, in the sense which we have assigned to that term.
From the above view of the subject, generation may be said to consist of four functions, differing in their importance, and in the number of animals to which they belong.