GEMMATION OF RADIATA AND ARTICULATA. 555 are in reality the representatives of the flower-buds of Plants, distinguished by their capability, not only of living and en during, but of obtaining their own nu triment, after their spontaneous detachment from the stock that bore them. Among the Medusa we occasionally meet with instances of propagation by buds that resemble the stock from which they proceed, and that are thrown off in due time so as to lead independent lives; but this kind of gemmation seems limited to the lower members of the group. In a large proportion of it, however, a very extraordinary kind of multiplication by gemmation takes place at an early period of development (§ 740).-In the highest Radiata, the class of Echinodermata, we take leave of multiplication by gemmation altogether; for although the bodies of these creatures possess a very extraordinary reproductive power, so that the result of very severe injuries may be repaired (§ 389), we do not find that they either spontaneously produce independent buds, or that they have the capacity for being multiplied by artificial division. Fig. 297.-POLYPES OF Undergoing fission; a, b, c, d, successive stages. 727. Among several of the lower Articulata, detached segments of the body appear to be capable of reproducing the whole; and there are some whose ordinary propagation is Fig. 298.-NEREIS PROLIFERA. accomplished by an exercise of this power. Thus in the Nais, an aquatic worm allied to the Earth-worm, the last joint of the body gradually extends and increases to the size of the rest of the animal; and a separation is made by a narrowing of the 556 GEMMATION OF ARTICULATA AND MOLLUSCA. preceding joint, which at last divides. Previously to its separation, however, the young one often shoots out another from its own last joint, in a similar manner; and three successions have thus been seen united. In some species of Nereis, the separation takes place nearer the middle of the body (fig. 298). In the greater number of cases, however, in which such a detachment of the posterior part of the body of Annelids takes place, the separated gemma does not contain the structure of the entire animal, but consists of little else than the generative apparatus, endowed with locomotive organs; so that this process of multiplication does not so much correspond with the ordinary propagation by buds, as with the peculiar development and throwing-off of generative buds to be presently described.—Among the higher Articulata, we do not meet with any instances of ordinary gemmation; but the non-sexual production, which is now known to take place not only in the Aphides (§ 746) but in many other Insects, as well as in Rotifera (WheelAnimalcules) in Entomostracous Crustacea (Water Fleas, &c.), and probably in some higher Crustacea, must be regarded as a peculiar form of the same process; the offspring being produced from eggs, which have the power of self-development without sexual fertilization, and which must therefore be accounted internal gemmæ. 728. In the Molluscous series, the power of multiplying by gemmation appears to be limited to the Tunicata (§ 114) and the Polyzoa (§ 115); being restricted in the first of these classes to a section of the group; whilst in the second, which closely follows the habit of Zoophytes, it seems to be universal. The bud arises in some instances directly from the body; but in other cases it is put forth by a stolon or creeping stem that connects all the bodies together (fig. 63). Among the Polyzoa the buds usually remain in connexion with the parent-stock, so as to form composite fabrics so closely resembling those of Zoophytes as to be commonly ranked with them. And the like happens also among the Compound Ascidians. But where gemmation takes place among the solitary Tunicata, the bud becomes detached, and maintains a perfectly independent existence. There is a very curious case of internal gemmation among the Salpa (a tribe of Tunicata which are not attached, but float over the waves); for the buds are developed, not from the exterior of the GEMMATION OF MOLLUSCA AND VERTEBRATA. 557 body, but from a kind of stolon within it ; and they differ from their parent-stock in having organs of attachment to each other, whereby they hold together in long Fig. 299.-AGGREGATE SALPE. Fig. 300.-SOLITARY SALPA. chains (fig. 299). These, in their turn, being furnished with true generative organs, give origin to the solitary Salpa (fig. 300) by ordinary sexual reproduction; whilst the solitary Salpa never comes to possess any sexual apparatus, but merely continues the race by gemmation from its internal stolon. 729. In the Vertebrated classes, as in the higher Mollusca, we lose all trace of propagation by gemmation as an ordinary method of multiplication. Yet there is evidence that this power is not altogether extinguished, even in Man. For we not unfrequently hear of "monstrosities by excess," that is, of cases in which the body possesses a superfluity of some of its parts; the simplest cases being those of double thumbs or of six fingers on each hand, and the gradation being so continuous from these to cases like that of the Siamese twins (in which there are two complete bodies united only by a cross band), as to make it evident that they are all referable to one common principle. And although it has been commonly believed that monsters with two heads and one body, or with two bodies and one head, or with supernumerary legs or arms, are results of the partial "fusion" of two distinct germs at an early period, yet there is now far stronger reason to believe that they proceed from a kind of attempt at multiplication by fission or gemmation, that is sometimes made by a single germ at a time when its grade of development corresponds with that of a Hydra or Planaria; which attempt, under peculiarly favourable circumstances, may proceed to the full length of production of two complete bodies. (See § 390.) Sexual Reproduction, or Generation. 730. We now have to notice the most important features of the proper Generative process, which differs from the 558 SEXUAL GENERATION:-SPERM-CELLS. preceding in exactly the same manner as the flowering and fruiting of Plants differ from their extension and propagationby leaf-buds. In all save the very lowest tribes of Animals, we meet at particular seasons with two peculiar sets of cells, termed sperm-cells and germ-cells; these are sometimes borne by the same individuals, which then correspond as regards their reproductive apparatus with the generality of Flowering Plants; but they are more commonly separated, as in diœcious Plants (VEGET. PHYSIOL., § 409); the individual bearing the "sperm-cells" being then designated as the male, and the individual bearing the "germ-cells" as the female. 731. The" sperm-cells" very closely resemble those contained within the antheridia of Cryptogamia (VEGET. PHYSIOL. § 399; BOTANY, §§ 737, 776). When mature, each cell is found to contain one or more spirally coiled filaments (fig. 301), which, when set free by the bursting of the cell, have an active spontaneous movement lasting for some time like ciliary action. These filaments were formerly regarded as true Animalcules; but since other examples of independent movement have been discovered in what are certainly nothing else than detached parts of the organism, and more especially since moving filaments of a precisely analogous character have been discovered in Plants, all idea of their independent animality has been laid aside, and they are now known as spermatozoids.-The use of their motor activity is obviously to bring them into contact with the germ-cells, when both have been set free from the interior of the bodies within which they were formed. When the sperm-cells are developed in a special or distinct organ, as happens in all save the lowest types of Animal structure, this organ usually more or less resembles the ordinary glands in structure (§§ 356, 357), and is termed the Testis. Fig. 301.-SPERMATOZOIDS: e, immature sperm-cells. 732. The "germ-cells" are not so clearly distinguished GERM-CELLS:-FERTILIZATION OF GERM. 559 from other cells by the nature of their contents, though they are usually recognisable by the peculiar nuclei they present; each cell is known as the germinal vesicle (fig. 302, d), whilst its nucleus (e) is designated the germinal spot.-The act of fertilization appears to consist in the contact of one or more spermatozoids with the exterior of the germinal vesicle; the spermatozoids, ceasing to move, undergo a sort of liquefaction; and the product of their dissolution, being received by absorption into the interior of the germinal vesicle, mingles with its contents, to form with them the basis of the new structure.When, as usually happens, the germ-cells are developed in a Fig. 302.-SECTION OF OVARIUM OF FOWL: a, fibrous substance of the ovary; b, yolk; c, yolk-bag; d, germinal vesicle; e, germinal spot. special and distinct organ, this organ, which is termed the Ovary, has very commonly among the lower animals a glandular character, the mature ova being discharged by the oviduct, just as the products of secretion pass-off through the ducts of their respective glands: but among the Vertebrata the Ovary has a much more solid texture, and the germ-cells, developed in the very midst of its fibrous tissue (fig. 302, a), have to find their way to its surface, and to burst forth from it; being then received into an oviduct, whose trumpet-shaped mouth embraces the ovary, so as to prevent the liberated germs from falling (as they would otherwise do) into the visceral cavity of the body. 733. With the "germ-cell" there is always associated in Animals, as in Flowering Plants, a store of nutriment that serves for the early development of the germ; this consists of a mixture of albuminous and oily matter, known as the yolk (fig. 302,6); and it is inclosed in a membranous envelope c, |