In these instances, multiplication by division occurs as a natural process, but there are many cases in which artificial division gives rise to multiplication. Bonnet having found that a certain kind of small worm, when cut in two, reproduced a tail at the cut extremity of the cephalic half, and formed a head upon the caudal half, increased the number of sections, and finally succeeded in dividing one worm into twenty-six parts, almost all of which acquired a head and tail, and thus became distinct individuals. Cor responding results may be obtained by dividing a planaria or actiuia into many seg ments.
Reproduction by gemmation is a phenomenon of very frequent occurrence in the lower departments of the animal kingdom. In the lowest of the animal subkingdoms, the PROTOZOA. it occurs in the rhizopoda—viz., in the foraminifera; in the sponghe, being probably the most common form of reproduction in sponges; and in the infusoriu, as, for example, in xorticella. In the CCELENTERATA. it is of almost general occurrence in the classes hydrozoa and aetinozoa; and in the MoLfRscOtns it occurs in polyzou and in tunicata. If some hydras are kept for a few days in a glass of their native water, knot like excrescences will be seen on their bodies. These are the buds or goramm, which rapidly enlarge, and each by degrees assumes the appearance of a young hydra, tenta cles appearing about the month; just as in the original animal. For sonic time, a por tion of the food (minute infusoria, entomostraca, etc.), caught and digested by the parent, passes into the body of the offspring; but when the tentacles are sufficiently developed, the young polype catches food for itself, and when it is sufficiently matured to commence an independent existence, the connecting pedicle gives way, and the young animal is free and independent.
It must be distinctly understood, that the fact of an organism reproducing itself by fission or gemmation does not by any means exclude the possibility that it may also be reproduced by fecundated ova. That this is the case, i3 indeed shown in the instance of the worm myriana, and a very large number of corroborative cases might be readily given.
In true generation, two special organs are required—a female organ for producing the germ-cell or ovum, and a male organ for producing the sperm-cell or spermatozoon; and each form of generative apparatus consists of two parts, of which one is a formative organ—in the female, termed an otarium, or ovary, and in the male; a testis—in which the reproductive cells are formed, and which is essential; and an efferent duct, by which the products of secretion are carried off. The male and female organs may exist in sep arate individuals, or they may co-exist ir. the same individual, giving rise to the condition known as hermaphroditism (q.v.). The former condition is termed bisexual or dueeious, and the latter unisexual or momeciaus. For a general description of the changes which take place in the impregnated egg, the reader is referred to the article DEVELOPMENT