REPRODUCTION, or the propagation of organized beings in the animal kingdom is accomplished by three different processes. The first of the three processes by which the multiplication of individuals takes place consists in the division of one organism into two, each of these, again, dividing into two others, and so on. This is termed reproduction by fission. The second mode of increase consists in the formation of a bud at some part of the body of the animal. This bud gradually approximates in form to that of the parent from which it springs; its pedicle or stem gradually disappears; and the liberated bud ultimately assumes a perfect form, resembling in all respects the parent from which it sprang (gemination). The third mode is far the most complicated. In it the new organism results from a series of changes occurring in an impregnated egg or ovum. For this process, distinct sexual organs; both male and female (which, how ever, may be associated in the seine individual, although in all the higher animals they occur in distinct individuals), are required; a female organ for the production of cells termed " germs," and a male organ for the production of certain cells termed " sperma tozoa." It is from the union (either within or without the body) and the inutile] action of these cells—the germ and the spermatozoon—that the impregnated ovum results. The new resulting body is altogether different from either of the cells which took part in its production. This is the ordinary form of reproduction in all the higher animals, and may be termed true generation, in contradistinction to the previous form of repro duction by multiplication. The terms digenesis and heterogenesis have been applied by recent physiological writers to designate the form of reproduction in which the contact of germs and sperrriatozoa gives rise to fecundation; while the terms monogenesis and komogenesis have been similarly applied to the eases iii which non-sexual reproduction takes place by fission or gemmation.
Fissiparous multiplication is best illustrated by a reference to the infusoria. It may be either longitudinal, as commonly occurs in vorticelkt ; or transverse, as occurs in stentor ; or indifferently longitudinal or transverse, as in ekilodon paramecium, etc. The
joints of tape-worms multiply in this manner, and when sufficiently developed become free. Among some of the annelids, or true worms. reproduction of this kind in a somewhat modified form is also observed. This was first noticed in a nais by the Dan ish naturalist Muller, by whom it was regarded as a rare and accidental occurrence. Themore recent researches of De Quatrefages and Milne-Edwards have, however, shown that the process is one of far more significance than Midler supposed. In the genus spills, De Quatrefages noticed the following appearances: When one of these worms is about to reproduce itself by fission, a number of rings become developed at its posterior extremity, and there is a notch or groove between the first of these rings and the part in front of it. The first ring soon becomes organized into a head provided with eyes and antenna'. The two annelids, parent and offspring, continue, however, to be united by the skin and intestine in such a manner that the latter animal lives solely upon the food swallowed by the former. During this period, each possesses independent life, for a struggle may often be observed between the two, each wishing to go its own way. After a lapse of a certain time, the body of the offspring becomes distended with ova fu some cases, and with spermatozoa in others, while neither of these structures is to be seen in the body of the primary animal. Complete division is at length effected„ and the offspring is free. In a few days, however, their bodies burst, from the distention caused by their contents. Ova and spermatozoa are thus diffused through the water, and fecundation thus takes place. In the genus myrianida (autolytus, accordino. 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 auimal was totally devoid. The youngest and smallest individual is the most remote front the tail.