In all truly viviparous, in most ovo-vivi parous, and also in many oviparous animals, the ova are fecundated within the parent's body ; and we find provided for the purpose of introducing the seminal fluid into the female genital organs, a more or less complicated ap paratus in both male and female, by which the union of the sexes is brought about. In the greater number of strictly oviparous ani mals, and particularly those that are aquatic, as osseous fishes and Batrachian Reptiles, fecundation is operated externally to the pa rent body ; that is, there is no union of the sexual organs of the male and female, but the ova laid by the female are covered with a certain quantity of seminal fluid shed by the male.* Utero-gestation in the Mammalia is termi nated by parturition or the birth of the young; while in the bird or oviparous animal bath con sists in the exclusion of the young from the egg. At this period in Mammalia, the organic con nection between mother and offspring is dis solved, and in both viviparous and oviparous animals the birth is accompanied by important structural changes, which fit the offspring for independent life and aerial or aquatic respira tion. The young of Mammalia after birth, though they cease to be organically connected with the mother, continue to derive a certain quantity of support from her, feeding on the milk secreted by the mamma?. But in all other classes of animals, the young are at birth capable of feeding on external aliment.
Varieties in respect to utero-gestation and the developement of the young.—There is con siderable variety among different animals in the degree of perfection at which the young have arrived at the period of birth. Thus, Insects, Batrachian Reptiles, and some other animals leave the egg at a very early period; differing widely from their parents in struc ture and functions, they live for a time in a masked or larva condition, and undergo after wards various changes or so-called metamor phoses before attaining the mature condition. Fishes leave the egg while their structure is yet very incomplete; and even in the higher animals we observe varieties in this respect : thus, some birds, more especially those build ing on trees, are unfledged, blind, and help less when the shell is broken; and some quadrupeds, among which may be mentioned the Rodentia, Feline, and Canine species, are at birth blind and weak, and with little power of supporting their natural high tem perature.* The most remarkable instance of variety of the kind now alluded to, how ever, occurs in the Kangaroo and other Mar supial animals, the generation of which de serves more particular mention in this place as it exhibits a considerable deviation from the more ordinary reproductive process in Mam malia, and is attended with some important modifications in the structure of the generative organs.
In the Mammalia generally, it has already been stated that there is an intimate organic connection between the foetus and mother, by means of which the former is supplied with the materials of its growth. The intimacy of this union (as we shall explain more fully else where) varies much in different tribes of ani mals. It is greatest in the placenta of the human female, and from this there may be traced a series of animals in which it becomes more and more loose. According to recent researches in Comparative Anatomy, there is also observable in the descending series of these animals a nearer and nearer approach in the general structure of the body, and in the conformation of the generative organs to the oviparous type. This approach to the ovipa
rous structure is most strongly marked in the Marsupiata, as the Opossum, Kangaroo, &c. and in the Monotrcmata, as the Ornithorynchus and Echidna.f Marsupiate generation.—The foetus of the marsupiate animal leaves the uterine system of the mother or is born at an early period of its formation, while it is yet of a very small size, and its organs are comparatively imperfectly formed. On being born it is introduced by the mother into the pouch or marsupium formed by a reduplication of the integuments of the lower part of the belly, and a short time after it gets there, the foetus is found attached by its mouth to one of the nipples of the mamma, which are concealed within the mar supium. The young of the marsupiate animal there receives its food by the mouth and is nourished by digestion at an early period of its advancement; and, although its external form and organization very much resemble that of the foetus of other mammiferous animals at a similar stage of their advancement when they are still confined to the uterus of the mother, their internal organization undergoes at the time of their exclusion the same remarkable changes which occur at birth, and which are connected with aerial respiration and the independence of the vital functions.* Illonotranatous gencration.—The generation of the Ornithorynehus and other monotrema toils animals deserves also to be noticed here as differing in some respects from that of other Mammalia; but, unfortunately, this subject, which has been involved in obscurity ever since the first discovery of these remarkable animals, notwithstanding that several important facts have been recently ascertained, cannot be con sidered as completely understood. The Orni tliorynehus and Echidna were long regarded as holding an intermediate place in respect to their organization between Mammalia and Birds. The existence of mammary glands was denied by the first dissectors of these animals; and from this circumstance principally, toge ther with the analogy in general structure al ready alluded to, the Ornithorynchus was be lieved by many to be oviparous. The recent investigations of Owen have proved the ex istence of mammary glands as well as the suckling of the young, while they at the same time shew that the generative organs and the ova within the ovaries partake in a great degree of the oviparous structure.t In this approach to the oviparous type, however, it has been satisfactorily shewn that the Ornithorynchus resembles the class of Reptiles rather than that of Birds. The ova of this animal have not, how ever, been found in any of its haunts; and, although no one has yet had an opportunity of dissecting the gravid uterus, naturalists are now inclined to hold the opinion that it bears its young alive. Should this be fully proved to be the ease, the Ornitborynehus may with justice be considered as an example among niammiferons animals of the ovo-viviparous form of generation, most analogous to that occurring in the Slow-worm or Adder ; the ovum being, at the time of its descent into the oviduct, of proportionally large size, and there being no proper placenta or intimate organic union between the mother and foetus.