A Gemmation And Fission. Gemmation, or budding, consists in the production of a bud, or buds, generally from the exterior, but sometimes from the interior, of the body of an animal, which buds are developed into independent beings, which may or may not remain permanently attached to the parent organism. Fission differs from gemmation solely in the fact that the new structures in the former case are produced by a division of the body of the original organism into separate parts, which may remain in connection, or may undergo detachment.
The simplest form of gemmation, perhaps, is seen in the power possessed by certain animals of reproducing parts of their bodies which they may have lost. Thus, the Crustacea possess the power of reproducing a lost limb, by means of a bud which is gradually developed till it assumes the form and takes the place of the missing member. In these cases, however, the process is not in any way generative, and the product of gemmation can in no sense be spoken of as a distinct being (or zooid).
An excellent example, however, of true gemmation is exhibited in such an organism as the common sea-mat (Flustra), which is a composite organism composed of a multitude of similar beings, each of which inhabits a little chamber, or cell; the whole forming a structure not unlike a sea-weed in appearance. This colony is produced by gemmation from a single primitive being ("polypide "), which throws out buds, each of which repeats the process, apparently almost indefinitely. All the buds remain in contact and connected with one another, but each is, nevertheless, a distinct and independent being, tion never occurs, but the plant is fertilised by the intervention of insects. Thus, in many plants the stamens and pistil arrive at maturity at different times, whilst in others the stamens and stigma are placed at different heights in the flower, and do not always occupy the same position even in a single species.
Capable of performing all the functions of life. In this case, therefore, each one of the innumerable buds becomes an independent being, similar to, though not detached from, the organism which gave it birth. This is an instance of what is called "continuous gemmation."
In other cases - as in the common fresh-water polype or Hydra - the buds which are thrown out by the primitive organism become developed into creatures exactly resembling the parent, but, instead of remaining permanently attached, and thus giving rise to a compound organism, they are detached to lead an entirely independent existence. This is a simple instance of what is termed "discontinuous gemmation."
The method and results of fission may be regarded as essentially the same as in the case of gemmation. The products of the division of the body of the primitive organism may either remain undetached, when they will give rise to a composite structure (as in many corals), or they may be thrown off and live an independent existence (as in some of the Hydrozoa).
We are now in a position to understand what is meant, strictly speaking, by the term "individual." In zoological language, an individual is defined as "equal to the total result of the development of a single ovum." Amongst the higher animals there is no difficulty about this, for each ovum gives rise to no more than one single being, which is incapable of repeating itself in any other way than by the production of another ovum; so that an individual is a single animal. It is most important, however, to comprehend that this is not necessarily or always the case. In such an organism as the sea-mat, the ovum gives rise to a primitive polypide, which repeats itself by a process of continuous gemmation until an entire colony is produced, each member of which is independent of its fellows, and is capable of producing ova. In such a case, therefore, the term "individual" must be applied to the entire colony, since this is the result of the development of a single ovum. The separate beings which compose the colony are technically called zooids. In like manner the Hydra, which produces fresh and independent Hydrae by discontinuous gemmation, is not an "individual," but a zooid. Here the zooids are not permanently united to one another, and the "individual" Hydra consists really of the primitive Hydra, plus all the detached Hydrae to which it gave rise. In this case, therefore, the "individual" is composed of a number of disconnected and wholly independent beings, all of which are the result of the development of a single ovum. It is to be remembered that both the parent zooid and the "produced zooids " are capable of giving rise to fresh Hydrae by a true generative process. It must also be borne in mind that this production of fresh zooids by a process of gemmation is not so essentially different from the true sexual process of reproduction as might at first sight appear, since the ovum itself may be regarded merely as a highly specialised bud. In the Hydra, in fact, where the ovum is produced as an external process of the wall of the body, this likeness is extremely striking. The ovarian bud, however, differs from the true gemmae or buds in its inability to develop itself into an independent organism, unless previously brought into contact with another special generative element. The only exceptions to this statement are in the rare cases of true "parthenogenesis," to be subsequently alluded to.