Reproduction. and fecundation thus takes place. In the genus myrianida (autolytus, according to Grube's classification), Milne-Edwards has seen no less than six new individuals (instead of a single one, as in syllis), formed in gradual succession, one before the other, between the two terminal segments of the original body. Each of these new individuals, as it arrived at maturity, and acquired the external form (in reduced "dimensions) of the parents, was found to be possessed of reproductive organs, of which the original animal was totally devoid. The youngest and smallest individual is the most remote from the tail. In these instances, multiplication by division occurs as a natural process, but there are many cases in which artificial division gives rise to multiplication. Bonnet having found that a certain kind of small worm, when cut in two, reproduced a tail at the cut extremity of the cephalic half, and formed a head upon the caudal half, increased the number of sections, and finally succeeded in dividing one worm into twenty-six parts, almost all of which acquired a head and tail, and thus became distinct individuals. Cor responding results may be obtained by dividing a planaria or actinia into many seg ments. Reproduction by gemmation is a phenomenon of very frequent occurrence in the lower departments of the animal kingdom. In the lowest of the animal subkingdoms, the PROTOZOA, it occurs in the rhizopoda-viz., in the foraminifera; in the spongia, being probably the most common form of reproduction in sponges; and in the infusoria, as, for example, in corticella. In the CŒLENTERATA, it is of almost general occurrence in the classes hydrozoa and actinozoa; and in the MOLLUSCOIDS it occurs in polyzoa and in tunicata. If some hydras are kept for a few days in a glass of their native water, knotlike excrescences will be seen on their bodies. These are the buds or gemma, which rapidly enlarge, and each by degrees assumes the appearance of a young hydra, tenta cles appearing about the mouth, just as in the original animal. For some time, a portion of the food (minute infusoria, entomostraca, etc.), caught and digested by the parent, passes into the body of the offspring; but when the tentacles are sufficiently developed, the young polype catches food for itself, and when it is sufficiently matured to commence an independent existence, the connecting pedicle gives way, and the young animal is free and independent. It must be distinctly understood, that the fact of an organism reproducing itself by fission or gemmation does not by any means exclude the possibility that it may also be reproduced by fecundated ova. That this is the case, is indeed shown in the instance of the worm myriana, and a very large number of corroborative cases might be readily given. The Hydra increases by ova, but if cut up, each piece becomes a perfect animal. In true generation, two special organs are required-a female organ for producing the germ-cell or ovum, and a male organ for producing the sperm-cell or spermatozoon; and each form of generative apparatus consists of two parts, of which one is a formative organ-in the female, termed an ovarium, or ovary, and in the male, a testis-in which the reproductive cells are formed, and which is essential; and an efferent duct, by which the products of secretion are carried off. The male and female organs may exist in separate individuals, or they may co-exist in the same individual, giving rise to the condition known as hermaphroditism (q.v.). The former condition is termed bisexual or diœcious, and the latter unisexual or monacious. For a general description of the changes which take place in the impregnated egg, the reader is referred to the article DEVELOPMENT 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 usually by fission, occasionally by gemmation; but no satisfactory instances of true sexual propagation by means of ova and spermatozoa are known, those cases described in certain infusoria not having been confirmed by subsequent observers. It is worthy of notice, that in the infusoria, propagation is effected in no less than three different ways-viz., by the two processes already described in this article, and by a process known as "encystation." See INFUSORIA. In the subkingdom CŒLENTERATA, it is found that both the hydrozoa and the anthozoa multiply by gemmation, by a true reproductive process, and in a few genera by fission. In the ECHINODERMATA, 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 crinoidea-the sexes are separate, and generation 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 molluscoids, 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, and, 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 case of one or two genera of fishes, the sexes are always distinct. Reptiles. 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 impregnation of a sufficient number of eggs may be secured, the male secretion of fishes-the fluid containing the spermatozoa-is very abundant; the male 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 grow 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. In 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 number; and as each egg escapes from the ovary, it is seized by a true oviduct, which furnishes 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 pillow-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 corneous 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, separates 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."-R. 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 some amphibians, as 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 racemous appearance; the right, with its oviduct, being always atrophied or rudimentary-a remarkable violation of symmetry, resembling that which occurs in the lungs of serpents. As prolonged uterogestation would be incompatible with flight, incubation here attains its highest perfection. In mammals, a new organ for the first time appears, from which that important class derives its name. In most of them (see MAMMALIA and PLACENTA), a temporary organ, termed the placenta, is also formed, by which the fetus is nourished during uterine existence. For further details on the subject of this article, the reader is referred to De Quatrefages's Rambles of a Naturalist, and to his Metamorphoses of Man and the Lower Animals; Dr. Allen Thomson's article "Ovum" in the Cyclopædia of Anatomy and Physiology; Dr. Carpenter's Comparative Physiology; and to Kölliker's Entwickelungsgeschichte des Men schen und der höheren Thiere. REPRODUCTION IN PLANTS. See PLANT, VEGETABLE PHYSIOLOGY, and FECUNDATM TION. REPTILES (Lat. repo, I creep), constitute a class of the subkingdom vertebrata, lying between the classes of amphibians and birds. They may be briefly characterized as being cold-blooded, having a heart composed of only three cavities-viz., two auricles and a single ventricle, and as breathing by lungs throughout the whole period of their existence; in which respect they differ from the amphibians, which some zoologists asso ciate with them, and which, in the early part of their existence, are furnished with gills for aquatic respiration. They are divided into the following orders: 1. Ophidia, or ser pents; 2. Sauria, or lizards; 3. Loricata, or crocodiles; and 4. Chelonia, or tortoises; so that in so far as external form is concerned, the members of this class present a far greater diversity than is observed amongst the members of the other classes of vertebrates. With the exception of the tortoises, the reptiles in general are of an elongated form, the body being often nearly cylindrical, and usually terminating in a very long tail. In A considerable number (as the serpents and some of the lizards) no traces of limbs are apparent; in some (as certain lizards), the limbs are rudimentary; while in the remainder |