[Some aspects of the gametogenesis and fertilization ofHydra].

1. The gametogenesis ofHydra attenuata undHydra circumcincta has been examined by light- and electron-microscopical methods. 2. In most cases one cell of each group of oogonia II develops into an oocyte, which engulfs the neighbouring oogonia II by phagocytosis. The oocytes fuse to form a polyenergetic plasmodium, in which one of the nuclei becomes the germinal vesicle, while the others degenerate. The ooplasm contains a great number of necrotic cells, so called "shrunkencells", which we suggest participate to some extent in the formation of the ooplasm. 3. Once the egg has reached the spherical form, it is fertile for only 7 hours. 4. During spermatogenesis cell division may not occur in one or both reduction divisions, so that 2 or 4 spermatozoa are differentiated within a common membrane. The spermatozoa ofHydra attenuata are of primitive structure without acrosomes. 5. The ejaculation of the mature spermatozoa occurs in clusters at different time intervals. 6. Under optimum conditions (9-12° C; pH 7.3) the viability of the spermatozoa is about 4 1/2 hours. 7. Fertilization is favoured by the following factors: synchronisation of the male and female sexual periods, longer duration of the male sexual period, ejaculation of the spermatozoa in clusters, short spaces between the sexual partners in a population.