But it is in the gelatinous Polypes that we meet with the most perfect forms of gemmi ferous propagation : of this the Hydra viridis, or fresh-water polype, afford san interesting illus tration, and from the facility with which it may be procured and examined by glasses of very ordinary powers, it is well calculated to illus trate the mode of generation which we are at present considering. The body of this simple polype is transparent, and under the microscope appears to be entirely made up of translucent granules, without any trace of internal appara tus appropriated to reproduction. The gem mules by which it is propagated sprout from some part of the surface, appearing first as mere gelatinous excrescences, but gradually enlarging they assume the form of their parent, acquiring similar filamentary tentacles and a gastric cavity of the same simple structure. As long as the junction between the polype and its off spring continues, both seem to enjoy a commu nity of being, the food caught by the original one being destined for the nourishment of both ; hut at length, the newly-formed animal having at tained a certain bulk, and become capable of employing its own tentacles for the prehension of food, detaches itself with an effort, and as sumes an independent existence. This mode of multiplication is exceedingly rapid, a few hours sufficing for the perfect developement of the young creature; and not unfrequently even before its separation, another gemmulc may be observed emerging from the newly-formed polype, soon to exhibit the same form and ex ercise the same functions as the parent from which it sprouts.
The propagation of some of the lithophytous polypes resembles that of the hydru, the young being produced from buds or gemmules, which sprout from the living investment of their cal careous skeleton. Such are the Fungia, in which the young are at first peduneulated, and fixed to the laminae upon the upper surface of the mass from which they spring ; in this state they might readily be mistaken for solitary Caryophyllia, but in time they separate from the parent stock, and loosing the pedicle which originally supported them, they assume the form of their species.
Ovilerous generation.—In the third, and by far the most numerous division of the animal kingdom, the young are derived from ova or eggs, in which the germ of the future being is evolved, and from which the young animals escape in a more or less perfect state.
It will be seen that the ovum which gives birth to all the higher animals differs essentially from the gemma furnished by the gemmiferous classes ; in the gemmiferous type the bud or offshoot of the parent appears by a kind of vegetative evolution to assume the proportions and functions of the original from which it sprang. The ovum we would define as a nidus, containing not only the germ of the future animal, but a sufficient quantity of nu tritious matter, serving as a pabulum to the embryo during its earliest state of existence, and supplying the materials for its growth until sufficiently mature to derive them from other sources. We have already shewn that in the
fissiparous and gemmiferous animals there is no necessity for any special generative appa ratus, but in the oviferous classes we find, for the most part, a distinct system, more or less complicated in structure, in which the repro ductive ova are developed and matured. It must be confessed, however, that in the present state of our knowledge upon this subject we are not prepared to state how far the existence of a generative system is exclusively confined to the ovigerous type. We are well aware that many authors describe generative canals to exist in several of the polypiferous tribes, although the reproductive germs produced from them resemble in their ciliated organs of pro gression and mode of development the gem mules of less elaborately organized polypes ; yet, on the other hand, as we have abundant evidence to prove that such polypes as have the ovigerous canals most distinctly formed, as the ActiniT for instance, produce their young per fectly organized and evidently developed from true ova, we are content, in the present state of our knowledge upon this subject, to regard the presence of generative canals as co-existont with ovigerous generation, and shall leave future observations to determine more accu rately the mode of reproduction in the corti ciferous polypes, which is a subject at present involved in much contradiction and obscurity.
Taking this view of the subject, we find, upon cursorily glancing over the ovigerous classes of animals, that important modifications of structure in the generative system render further classification necessary. In the lower forms, ovigerous organs only have been dis covered, in which the ova are secreted, and when matured, escape from the body, fit in every respect for the production of a new animal. In other instances, in addition to the apparatus immediately appropriated to the de velopement of the ova, we find a supemdded portion destined to furnish a secretion which is essential to their fertility, forming an apparatus of impregnation. Sometimes the impregnating organs are found in every individual, appended to the ovigerous parts, rendering each creature sufficient for the impregnation of its own ova : in other instances, although each animal pos sesses both ovigerous and impregnating or pns, the cooperation of two individuals or more is necessary to fertility ; and in other cases again, the apparatus which furnishes the ova, and that destined to the production of the im pregnating fluid, are found in distinct indivi duals, distinguished by the appellations of male and female : we shall accordingly di vide all oviparous animals into the following groups:— 1. Such as are provided with ovigerous organs only.