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The cytology, cytochemistry and ultrastructure of Interstitial cells differentiating into spermatozoa is described. Interstitial cells of Hydra fusca are induced to transform into spermatozoa rather than somatic cells by reducing the culture temperature from 20OC to 8°C, Within 30 days at 8°C, several testes containing spermatozoa appear as epidermal swellings. Spermatogenesis begins with the enlargement of the interstitial cell (diameter, 6,2p; volume, 128p $ ) , which lacks endoplasmic reticulum and has few small organelles. Subsequently, the transforming interstitial cell passes through three successive phases: mitosis, meiosis, spermlogenesis. The spermatogonium (6,2-12.5p; 123-I02^p3) in the mitotic phase is character ized by a more intense basophilia due to an abundance of ribosomes, absence of endoplasmic reticulum and the accumulation of DNA along the nuclear boundary and about the enlarged nucleolus. In the formation of the primary spermatocyte (6.1p; 122) i$) , the cytoplasmic basophilia diminishes, the nucleolus disappears and the nuclear DNA becomes more coarsely granular. After an extended prophase I, meiosis I is completed and the secondary spermatocyte (t.8p; 60y$) is formed. The second meiotic division ensues immediately. During this meiotic phase the chromosome number is reduced from 30 to 1 5. Finally. ii in spermiogenesis of the spermatid (3.9p; 3 the nuclear volume is reduced and four large mitochondria are aligned at the point where the flagellum is elaborated. Electron microscopic studies show up to four sperniatids connected by cytoplasmic bridges and each spermatozoan X 1.5j*) surrounded by an electron dense layer. Histochemical studies reveal that at the end of 30 days at 8°C the distribution of food materials in the gastrodermls of sexual hydra is altered as compared to that of non-sexual animals. Furthermore, some glycogen taken up by the gastrodermls may be passed through the mesoglea to the epidermal testes. The events of spermatogenesis and the effect of reduced temperature on the differentiating cells is also discussed.

CYTOLOGY AND ULTRASTRUCTURE OF DIFFERENTIATING INTERSTITIAL CELLS IN SPERMATOGENESIS IN HYDRA FUSCA