OF THE EMBRYO.
We shall conclude with a brief notice of the mode or modes of reproduction in the different classes of animals, beginning with the lowest.
In the subkingdom PROTOZOA, reproduction takes place by all three modes, viz., by fission, gemination, and impregnated ova; but fission is here the predominating form; and it is only in the infusoria that there is undoubted evidence of true generation by ova and spermatozoa. It is worthy of notice, that in the infusoria, propagation is effected in no less than four different ways—viz., by the three processes already described in this article, and by a process known as " encystation." See INFUSORIA.
In the subkingdom CCELENTERATA, it is found that both the hydrozon and the antho Z0a multiply by gemination, by a true reproductive process, and in a few genera by fission.
In the ECHINODERWATA, fission has been observed in one class, the holothuroidea, which, moreover, have distinct sexual organs combined in the same individual. In the other classes—the echinoidea, asteroidea, and crinoidca—the sexes are separate, and gen eration only takes place by the union of germs or ova and spermatozoa.
In the ANNELIDA. true generation takes place, although, as has been already shown, multiplication sometimes takes place by fission. In the lower mollusca or molluseoids, multiplication takes place by gemmation and by true generation; whilst in the higher mollusca, multiplication only takes place by true generation.
In the ARTICULATA—inseCts. crustaceans, etc.—distinct generative organs are always present, aud, excepting in one class of crustaceans—the cirrhopoda—the sexes are dis tinct.
In the VERTEBRATA, we meet with the highest and most complex development of the generative function. In them, with a doubtful exception in the ease of one or two genera of fishes, the sexes are always distinct.
The osseous and cartilaginous fishes present important differences in their reproductive organs and in their modes of reproduction. In the osseous fishes, the essential female organ—the ovary, or roe—consists of a large membranous bag, usually in two lobes. but sometimes single. When distended with ova, this organ fills the greater part of the abdominal cavity, and its lining membrane is arranged in folds, wherein the ova are formed and retained until sufficiently ripe for expulsion. They then escape into the ovarian cavity, and are expelled in almost incredible numbers through a special opening immediately behind the anus and in front of the urinary canal. As a general rule, the ova of fishes are impregnated after their expulsion; and in order that the impregna tion of a sufficient number of eggs may be secured, the male secretion of fishes—the fluid containing the spermatozoa—is very abundant; the nude secreting gland, which in fishes is termed "the milt" or " soft roe," being equal in bulk to the ovary of the female. In a few instances, however, the young are hatched in the ovary, and grew to a consid erable size before they are born, and in these cases—as, for example, in the viviparous blenny—impregnation must take place internally. Iu the cartilaginous fishes—as the
sharks and rays—the generative organs are of a higher type. The eggs are always impregnated within the body of the female, the male having special organs by which true sexual congress is effected, and the ovaries form two large racemous bunches, placed on either side of the spine. The eggs are large in size, and comparatively small in num ber; and as each egg escapes from the ovary, it is seized by a true oviduct, ivhich fur nishes it with additional protective coverings. About the middle of this tube " there is a thick glandular mass, destined to secrete a horny shell, in which the yelk and white of the egg become incased. The egg, when completed, has somewhat the shape of a pil low-case, with the four corners lengthened out into long tendril-like cords, whereby the egg is entangled among the sea-weed at the bottom of the ocean. A brittle egg-shell would soon be destroyed by the beating of the waves; hence the necessity for the cor neous nature of the envelope; and yet how is the feeble embryo to escape from such a tough and leather-like cradle? This has likewise been provided for. The egg remains permanently open at one extremity; the slightest pressure from within, therefore, sepa rates the valvular lips of the opening, and no sooner has the little shark thus extricated itself from its confinement, than the two sides close so accurately that the fissure is imperceptible."—It. Jones's General Outline of the Animal Kingdom, 1841, p. 534.
In the amphibia or batrachia, the sexes are more closely associated than in the osseous fishes, the ova being generally impregnated by the male as they escape from the abdom inal cavity of the female. The mode of reproduction of one amphibian, the Surinam toad, is remarkable and anomalous. See PIPA.
In the true reptiles, the male sexual organs become more perfect, instruments being given to facilitate the impregnation of the female during that congress of the sexes which now becomes essential to fecundation.
In birds, the generative organs present a close analogy to those of the higher reptiles. There is only a single ovary (the left) that has a bunch-like or ravenous appearance; the right, with its oviduct, being always atrophied or rudimentary—a remarkable violation of symmetry, resembling that which occurs in tlae lungs of serpents. As prolonged uterogestation would be incompatible with flight, incubation here attains its highest per fection.
In mammals, a new organ for the first time appears, from which that important class derives its name. In most of them (see MAnsiALIA and PLACENTA), a temporary organ, termed the placenta, is also formed, by which the fetus is nourished during uter ine existence.
For further details on the subject of this article, the reader is referred to De Quatre fages's Rambles of a Naturalist, and to his Metamorphoses of plan and the Lower Animals ; Dr. Allen Thomson's article "Ovum" in the Cyclopadia of Anatomy and Physiology; Dr. Carpenter's Comparative Physiology; and to Kolliker's Entwickelungsgesehichte des Menschen and der holteren Thier:e.