Reproduction by Fission and Gemmation. Agamogenesis.
In the lowest forms of life, the only mode of generation at present known is the division of the body into two or more parts, each of which then grows to the size and assumes the form of its parent, and repeats the process of multiplication. This method of multiplication by fission is properly called generation, because the parts which are separated are severally competent to give rise to individual organisms of the same nature as that from which they arose.
In many of the lowest organisms the process is modified so far, instead of the parent dividing into two equal parts, only a small portion of its substance is detached, as a bud which develops into the likeness of its parent. This is generation by gemmation. Generation by fission and by gemmation are not confined to the simplest forms of life, however. On the contrary, both modes of multiplication are common not only among, plants, but among animals of considerable complexity.
The multiplication of flowering plants by bulbs, that of annelids by fission, and that of polypes by budding, are well-known examples of these modes or reproduction. In all these cases, the bud or the segment consists of a multitude of more or less metamorphosed cells. But, in other instances, a single cell detached from a mass of such undifferential cells contained in the parental organism is the foundation of the new organism, and it is hard to say whether such a detached cell may be more fitly called a bud or a segment -- whether the process is more akin to fission or to gemmation.
In all these cases the development of the new being from the detached germ takes place without the influence of other living matter. Common as the process is in plants and in the lower animals, it becomes rare among the higher animals. In these, the reproduction of the whole organism from a part, in the way indicated above, ceases. At most, we find that the cells at the end of an amputated portion of the organism are capable of reproducing the lost part; and, in the very highest animals, even this power vanishes in the adult; and, in most parts of the body, though the undifferentiated cells are capable of multiplication, their progeny grow, not into whole organisms like that of which they form a part, but into elements of the tissues.
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