CI1CO3 in the structure of the other parts of the body.
In Hermaphrodite animals there are two modes in which fecundation takes place. In some of the Acephala, and in the Holothuriw, the union of the sexual organs necessary for fecundation takes place in a single individual ; while in others, as Helix and Lymneus among the Gasteropoda, copulation, or the union of two individuals, is required, and there is mutual impregnation, the female organ of each animal being fecundated by the male of the other,— a mode of impregnation which also exists in the common Earth-worm, Leech, and some other animals. Occasionally we find that three or more individuals engage in this sort of mu tual fecundation, being arranged in a chain or circle.* (See IIERNI A Pll ROD1TE.) Diacious reproduction, or with distinct in dividuals of different sexes. Oviparous and viviparous generation.—In those animals again in which the position of the sexual organs on separate individuals renders copulation neces sary, the mode of production of the new animal from the egg seems to be the most prominent circumstance according to which the reproduc tive process is modified. Thus, while in a certain number of them the young are born alive, in others they are hatched from eggs laid by the female parent. This constitutes the difference between Viviparous and Oviparous animals; to the first of which classes Mam malia belong, to the second Birds, and most Reptiles and Fishes. A short comparison of the more important steps of the generative process in the Mammiferous animal and the Bird will most readily explain the difference between viviparous and oviparous generation.t In both these classes of animals ova are formed from the ovary, and in both the ova are fecundated within the body of the female parent. The process by which the egg is sepa rated from the place of its formation, and the changes it undergoes in being perfected after this separation, are the same in both : hutafter the fecundation and completion of the egg, it is differently placed in the two classes of animals; for in birds the egg passes through the oviduct and leaves the body of the female parent, to be hatched into life under the influence of favour able external agents; while in the mammiferous quadruped, the egg remains within the uterus of the female generative organs, becomes at tached to it, and has there formed from it the young animal, which does not quit the body of the parent until it is capable of independent life. The egg of the bird leaves the body of
the mother provided with a considerable quan tity of organic matter, by which alone, under the influence of heat and air, the embryo is nourished during incubation. The egg of the mammiferous animal is extremely small com pared to the size of the young animal at birth, and the foetus consequently draws a continual supply of the materials of its nourishment from the uterus of the mother, with which it is more or less intimately connected. The residence of the child or young animal in the body of the mother during its formation and growth is termed pregnancy, or utero-gestation.* Ovoviviparous generation.—There are other animals, however, besides Mammalia, which bear their young alive, as is the case in many cartilaginous and a few osseous fishes, in several Batrachia, Sauria, and Ophidia, and also in some Gasteropodous Mollusca, Insects, Annelida, and Entozoa. But there is an im portant difference to be pointed out between the viviparous form of generation occurring in these animals and that which belongs to the Mammalia. For the female generative organs of the above-mentioned animals, as well as the eggs they produce, resemble much more closely in their structure those of oviparous than those of strictly viviparous animals. As, in the animals now under consideration. the egg is proportionally large, the fcetus grows principally by the assimilation of materials procured from it, and there is not that intimate connection of structure nor interchange of substance between the mother and foetus which occurs in Mammalia. The term ovo-viviparous is applied to the variety of reproduction now under consideration; as expressing that in it, although the foetus is produced fully formed and alive, the ovum is merely hatched within the parent body. We find, accordingly, that this form of generation is liable to vary, and occasionally to run into the truly oviparous kind. Thus, some animals which bear live young at one season lay eggs at another season of the year, as occurs in some Insects ; and in others, as Lacerta agilis, the ova remain within the mother's body for a part only of the time employed in the development of the young; the process of hatching beginning and going on for a longer or shorter time within the female parent, and being completed as in the bird without.