[Regulation of nematocytec-production ofHydra attenuata Pall]

The number of nematocytes in the tentacles and in the ectoderm of the body-column ofHydra attenuata Pall. shows a great variability. The variability of the nematocytes in the tentacles is manifested, not only if different individuals of the same clone, but also if various tentacles of the same polyp are compared. 2. The percentage of the different types of nematocytes in the tentacles and in the body-ectoderm shows a greater stability than the absolute numbers. A comparison of the percentage of differentiated nematocytes of the body-ectoderm with those of the tentacles shows that in these two regions of the polyp the different types of nematocytes are differently distributed. 3. By a combined chemical and mechanical stimulation of the tentacles, 90% of the stenoteles are discharged unlike the isorhizas and desmonemes, which cannot be brought to explosion under the same conditions. 4. The reaction of the polyp following the selective discharge of stenoteles shows three different phases: a) As a first, short-term reaction to selective discharge, differentiated stenoteles already present in the body-ectoderm move into the tentacles. b) In a second phase, the synchronous mitotic divisions of the already determinated groups of stenotele-nematoblasts are interrupted to enable earlier differentiation. c) In a third, long-term reaction, the production of new stenoteles is increased. This increased production occurs on one hand by supplementary determination of interstitial cells to stenotele-nematoblasts, and on the other hand by supplementary mitotic divisions, i.e. by increasing the number of nematoblasts per group. 5. The intense discharge of stenoteles has two effects on the desmonemes: It causes an increased number of differentiated desmonemes already present in the body-ectoderm during the discharge of stenoteles to migrate into the tentacles as well as an increase in the production of nematocytes of this type. 6. Neither the production nor the migration of the isorhizas are influenced by the discharge of stenoteles. 7. The times of differentiation of the three types of nematocytes are: Isorhizas 1-3 days, desmonemes 3-4 days, stenoteles about 6 days. 8. The variability of the nematocytes in the tentacles and in the ectoderm of the bodycolumn, the moment of determination of the interstitial-cells differentiating into nematoblasts as well as the co-ordinated behaviour of stenoteles and desmonemes are discussed.