In temperate climates most forms of _Hydra_ breed at the approach of winter, but starvation undoubtedly induces a precocious sexual activity, and the same is probably the case as regards other unfavourable conditions such as lack of oxygen in the water and either too high or too low a temperature.
Downing states that in N. America (Chicago) _H. vulgaris_ breeds in spring and sometimes as late as December; in Calcutta it has only been found breeding in February and March. Except during the breeding-season sexual organs are absent; they do not appear in the same position on the column in all species.
The spermaries take the form of small mound-shaped projections on the surface of the column. Each consists of a mass of sperm-mother cells, in which the spermatozoa originate in large numbers. The spermatozoa resemble those of other animals, each possessing a head, which is shaped like an acorn, and a long vibratile tail by means of which it moves through the water. In the cells of the spermary the spermatozoa are closely packed together, with their heads pointing outwards towards the summit of the mound through which they finally make their way into the water. The aperture is formed by their own movements. Downing (Zool. Jahrb. (Anat.) xxi, p. 379, 1905) and other authors have studied the origin of the spermatozoa in great detail.
[Illustration: Fig. 28.--Eggs of _Hydra_ (magnified).
A=egg of _H. vulgaris_ (after Chun). B=vertical section through egg of _H. oligactis_, form A (after Brauer). C=vertical section through egg of _H. oligactis_, form B (after Brauer).]
The ovaries consist of rounded masses of cells lying at the base of the ectoderm. One of these cells, the future egg, grows more rapidly than the others, some or all of which it finally absorbs by means of lobose pseudopodia extruded from its margin. It then makes its way by amoeboid movements between the cells of the ectoderm until it reaches the surface. In _H. vulgaris_ (Mem. Asiat. Soc. Beng. i, p. 350, 1906) the egg is first visible with the aid of a lens as a minute star-shaped body of an intense white colour lying at the base of the ectoderm cells. It increases in size rapidly, gradually draws in its pseudopodia (the rays of the star) and makes its way through the ectoderm to the exterior. The process occupies not more than two hours. The issuing ovum does not destroy the ectoderm cells as it passes out, but squeezes them together round the aperture it makes. Owing to the pressure it exerts upon them, they become much elongated and form a cup, in which the embryo rests on the surface of the parent. By the time that the egg has become globular, organic connection has ceased to exist. The embryo is held in position partly by means of the cup of elongated ectoderm cells and partly by a delicate film of mucus secreted by the parent. The most recent account of the oogenesis ("ovogenesis") is by Downing (Zool. Jahrb. (Anat.) xxvii, p. 295, 1909).