GENERATION, ORGANS OF, (Com parative Anatomy).—Few subjects connected with physiology have been investigated more assiduously than that of the generation of ani mals ; and in none, perhaps, has the poverty of our knowledge of the operations of nature been more conspicuously exemplified. In studying many functions of the animal economy, the laws of chemistry and mechanics have been suc cessfully appealed to by the philosopher, and their application to the operations of the animal frame satisfactorily substantiated ; but in at tempting to explain the wonderful process by which organized bodies are perpetuated, all the resources of modern science have been found totally inadequate to the task, and we are still left to record facts and observations concerning the structure of the organs appropriated to the propagation of animals, without being in any degree able to connect them with the results so continually offered to our contemplation. In taking a general survey of the animal kingdom, we are at once struck with the infinite variety of forms which it presents, adapted to an end less diversity of circumstances, and might expect to meet with a corresponding dissimilarity in the organization of the generative apparatus pecu liar to each : no such dissimilarity, however, exists in nature, the modes of reproduction conform to a few grand types, and the increasing complexity of parts, apparent as we ascend to higher classes, which it will be our business to trace in this article, will be seen to depend rather upon modifications in the arrangement of secondary structures than upon any deviation from the fundamental organization of the more immediate agents.
Without entering upon any discussion con cerning the theories which have from time to time been advocated relative to spontaneous generation, we shall divide all animals as re lates to the generative function into three great classes, grouping together such as are 1st. Fissiparous, in which the propagation of the species is effected by the spontaneous divi sion of one individual into two or more, pre cisely resembling the original being.
2nd. Genniparous, in which the young sprout as it were from the substance of the parent.
3rd. Oviparous, producing their offspring from ova or germs developed in special organs adapted to their formation.
Of these modes of reproduction the two first are confined to the lowest or acrite division of the animal kingdom, whilst the third or ovipa rous type is common to all other classes.
Fissiparous generation is the simplest possi ble, and presupposes a corresponding simpli city of structure in the animals which propa gate in this manner. It is principally confined
to the Polygastric animalcules, most of which are multiplied by the spontaneous separation of an individual into two portions precisely resem bling each other, and capable of performing all the functions which originally belonged to the undivided creature. Some of the larger species of Trichoda are well calculated to exhibit to the microscopic observer the steps by which this process is accomplished : the animalcule, prior to its division, is seen to become slightly elongated, and a transparent line is gradually distinguishable, indicating the course of the in tended fissure ; at each extremity of this line a contraction of the body is speedily observable, and the lateral indentations become deeper and deeper, till at length a perfect separation is effected. The direction in which this divi sion occurs is not always the same even in the same species ; thus, instead of traversing the shorter axis of the body it not unfrequently assumes a longitudinal or oblique direction, and from this cause it is not unusual to find the newly divided creatures differing materially in appearance from their adult or rather conjoined form ; for in this process the old animalcules literally become converted into young ones.
In some of the more complex forms of Poly gastrica the fissiparous mode of generation exhi bits modifications which are extremely curious. In the beautiful Vorticella, whose bell-shaped bodies are supported on long and exquisitely irritable stems, the division commences at the large ciliated extremity of the animalcule, from which point it gradually extends in a longitudi nal direction towards the insertion of the stem, dividing the body into two equal portions, one or both of which becoming speedily detached from the pedicle, might easily in this state be mistaken for creatures of a different genus, and have in fact been described as such by many authors. The new animalcule, when thus deprived of its pedicle, is seen to be fur nished with cilia at the opposite extremity to that on which they were previously found, while from the other end, originally the mouth, a new foot-stalk becomes gradually developed, and the creature assumes the shape proper to its species. If one of the bells remain attached to the pedi cle, it continues to perform the same movements as before the separation of the new animalcule ; but if both become detached, the foot-stalk perishes.