When we examine animals in the next grade, we find reproduction taking place by the con currence of sexes, or rather of two kinds of organs which we afterwards discover divided between different individuals, who are then said to be of opposite sexes. When the male and female organs are united in the same indi vidual it is denominated an hermaphrodite ani mal, and in some cases seems to suffice for its own impregnation ; more generally, however, hermaphrodite animals are not capable of per forming this act upon themselves, but require the concurrence of another individual of similar constitution: the two hermaphrodites meet and severally impregnate one another.
Among the more perfect classes of the ani mal kingdom the organs of reproduction are universally allotted to two different individuals, males and females, which consequently become in their dualism representatives of their species. Agreeing in this single feature, the modifica tions in the process of reproduction are never theless extremely numerous. In some cases the fecundating fluid of the male is only ap plied to the egg or germ of the female after its extrusion from her body, as happens among fishes, several reptiles, &c.; in others the male fluid is injected into the body of the female, and made to fecundate the germ still attached to its parent. This act is generally, though not invariably, accomplished by means of a penis, or male external organ, with which many birds and all the animals above them in the scale of animal creation are then provided.
With this contact or intermixture of bodies we have the following varieties in the after-parts of the process : the egg or germ now fecun dated is either forthwith expelled from the body, and it is only subsequently, under the in t, fluence of a certain temperature, and after the lapse of a certain time, that the young being bursts the shell and commences its independent existence ; this is the case among oviparous animals. Or otherwise : the fecundated egg makes its way so slowly through the passages that lead from the ovary outwards, that it is hatched before it can escape, so that the young one passes from the body of the mother imme diately. Animals in whom this happens are
justly said to be ovo-viviparous. In the third and last place, the fecundated ovum is imme diately loosened from the ovary, but instead of being laid, or extruded from the body immedi ately, it only passes along a canal to a certain distance from the ovary, where it meets with a reservoir or cavity (the uterus) to which it at taches itself, and within which it commences a series of evolutions, at the expense of the mother, preliminary to its final expulsion with instincts ready formed, and an organization so perfect as enables it to begin its separate ex istence. The classes in which this mode of reproduction obtains, and they are the highest of all, including quadrupeds and man, are en titled viviparous, so that in these, besides the connection . of the sexes and the fecundation of the germ, we have the phenomena of utero gestation and labour.
And here the proper work of reproduction ends; but the young are so generally born in some sort immature, that in the higher classes the connection between the offspring and pa rent does not cease immediately. In the class of mammalia, indeed, the connection is little less intimate during the earlier periods of extra uterine life than it was during the whole term of intrauterine existence ; the young being still depends upon its mother for the whole of its nourishment, and very generally for the supply of warmth it requires and the protection needful to it till able to provide for itself.
- Many of the particulars now merely glanced at, and numerous others, the mention of which has been omitted entirely, will be found de tailed, and their bearing and importance illus trated in the article on GENERATION, to which the reader is therefore referred.