The question of the regeneration of lost parts in _Hydra_ cannot well be separated from that of reproduction by fission. Over a hundred and fifty years ago Trembley found that if a polyp were cut into several pieces, each piece produced those structures necessary to render it a perfect polyp. He also believed that he had induced a polyp that had been turned inside out to adapt itself to circumstances and to reverse the functions and structure of the two cellular layers of its body. In this, however, he was probably mistaken, for there can be little doubt that his polyp turned right side out while not under his immediate observation. Many investigators have repeated some of his other experiments with success in Europe, but the Calcutta _Hydra_ is too delicate an animal to survive vivisection and invariably dies if lacerated. It appears that, even in favourable circumstances, for a fresh polyp to be formed by artificial fission it is necessary for the piece to contain cells of both cell-layers.
The egg of _Hydra_ is said to be fertilized as it lies at the base of the ectoderm, through which the fertilizing spermatozoon bores its way. As soon as the egg has emerged from the cells of its parent it begins to split up in such a manner as to form a hollow mass of comparatively large equal cells. Smaller cells are separated off from these and soon fill the central cavity. Before segmentation begins a delicate film of mucus is secreted over the egg, and within this film the larger cells secrete first a thick chitinous or horny egg-shell and within it a delicate membrane. Development in some cases is delayed for a considerable period, but sooner or later, by repeated division of the cells, an oval hollow embryo is formed and escapes into the water by the disintegration of the egg-shell and the subsequent rupture of the inner membrane. Tentacles soon sprout out from one end of the embryo's body and a mouth is formed; the column becomes more slender and attaches itself by the aboral pole to some solid object.
_Hydra_ seems to have few natural enemies. Martin (Q. J. Micr. Sci. London, lii, p. 261, 1908) has, however, described how the minute worm _Microstoma lineare_ attacks _Hydra "rubra"_ in Scottish lochs, while the larva of a midge devours _H. vulgaris_ in considerable numbers in Calcutta tanks (p. 156).