In all animals, with the exception of the simplest tribes already referred to, the descent from parent to offspring is through a product formed and perfected only by the concurrence of male and female organs ; but we are still at a loss to determine whether the unseen germinal bodies by which the Protozoa are reproduced are of the same or of a different nature. The structure of some of these ger minal bodies as described in the earlier part of this article (p. 7., &c.), bears a very great resemblance to that of true ova ; but 3 et the sexual distinction of the parent animals has not yet been discovered. The recent re searches of naturalists indeed show that our w, hole knowledge of the history of the Pro tozoa may be considered as only in its infancy. The discoveries as to the eneysted stage of existence among the Vorticellw and Gre garinm and others, the phenomena of conju gation observed in Greg,arina and Actin°. phrys, the entire knowledge lately gained of the form, structure, and habits of the Fora minifera, all point to important future dis coveries and modifications of our hitherto crude and imperfect views of these tribes of beings, and must make us refrain from at tempting at present to form any opinion or even conjecture as to the modes of their re production ; while at the same time the recent discoveries as to the existence of the sexual distinction in the simplest forms of plants encourage the hope that ere long the repro duction of the Protozoa may, in a similar intanner, be removed from the obscurity in which it now lies hidden. It does not appear necessary from these considerations that our definition should make any direct reference to animal bodies of the nature of which our knowledge is still so imperfect.
The result of development from a fecun dated ovum in all vertebrate and in a con siderable number of invertebrate animals, is the formation of an embryo which, by a pro cess of progressive growth, arrives at matu rity, and assumes the form, structure, and habits, either, as the case may be, of a her maphrodite animal, or of the parent of either sex. In a certain number of these instances, as in Batrachia, Insects, Crustacea, and others, growth is not altogether continuously pro gressive, but is subject to one or more breaks or changes as it were, which are rnarked by some change in the mode of life, or some difference in structure of the individual. To such marked changes in the course of the development or growth of an individual ani mal proceeding from a fecundated ovum, the name of Metamorphosis is given.
But from the facts narrated in the earlier part of this article, it appears that in a cer tain number of the invertebrate animals, such as those which have been referred to under the heads of Echinodermata, Polypina, Aca lepha, Tunicate Mollusca, Trematode and Cestoid Entozoa, Annelida and Insecta, a very different result may, either regularly and constantly in some, or only occasionally in others, attend the first development from the fecundated ovum. To this modification of
the developing and reproductive process the appellations of Alternate Generation or Meta genesis have been given, of which tertns the latter may perhaps be the most appropriate. The phenomena which have been described under this head are so very various, that it is difficult, if not impossible, to give a short and general statement of their nature. The dif ference between this and the better known form of direct generation may, however, be stated nearly as follows :—In the Metagenetic form of reproduction the individual formed by the development of the fecundated ovum is generally different in aspect, structure, and znode of life from the parent or parents by which the ova were produced ; this individual, or zod, though possessed, in many instances, of an organisation and of powers which fit it for the efficient perfonnance of many of the most important acts of independent animal existence, is yet wanting in the attribute of perfect anitnal maturity, viz., the sexual or gans and activity, and is consequently incapa ble by itself of an act of true generation, or, in other words, of the formation of fecundated ova, by which alone the species can be per manently reproduced. In such instances, then, it is only by the formation from these intermediate beings of others which are sexu ally perfect, that the generative act can be repeated. There are two phenornena re quiring to be distinguished in connection with the most common forms of this process ; the one the frequent multiplication of the im perfect intermediate beings, or zolds ; and the other the production either directly or by a succession of acts of development from the intermediate beings of those which are sexu ally perfect, or which resume the form be longing to the parents from which the fecnn dated ova were derived. It seems proper, therefore, to distinguish between an act of true sexual generation, and that by which new beings are formed from the intermediate individuals (or so-called nurses of Steen strop, or zoids of other authors); the first consisting invariably in development from a fecundated ovum ; the second being probably more analogous to a process of budding or gemmation from a parent stock. It must be confessed, however, that we have still much to learn regarding the phenomena of this pro cess, before we can form any general notion of its nature. The whole subject is replete with the deepest interest not only in connec tion with the history of reproduction, but in its influence, as stated in some parts of the preceding article, on the whole range of zoo logical classification and distinction. Our extended definition comprehends an allusion to these phenomena.