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b'The differentiation of nerve cells and nematocytes in Hydra attenuata has been investigated\\nby labelling interstitial cell precursors with PHJthymidine and following by autoradiography\\nthe appearance of labelled, newly differentiated cells. Nematocyte differentiation occurs only\\nin the gastric region where labelled nematoblasts appear 12 h and labelled nematocytes 72\\u201496 h\\nafter addition of 2H thymidine. Labelled nerves appear in hypostome, gastric region, and basal\\ndisk about 18 h after addition of 3H thymidine. The lag in the appearance of labelled cells\\nincludes cell division of the precursor as well as differentiation since nerves and nematocytes\\nhave 2JI postmitotic nuclear DNA content.\\nA cell flow model is proposed for interstitial cells and their differentiated products. Stem\\ncells occur as single interstitial cells or in pairs. Per cell generation about 60 % of the daughter\\ncells of stem cell divisions remain stem cells and about 40 % differentiate nerves and nematocytes.\\nNerves differentiate directly from stem cells in about 1 day. Nematocyte differentiation\\nrequires 5-7 days including proliferation of a cluster of 4, 8, 16 or 32 interstitial cells and\\ndifferentiation of a nematocyst capsule in each cell. The numbers of interstitial cells and nematoblasts\\npredicted by the cell flow model from the rates of nerve differentiation (900 nerves/day/\\nhydra), nematocyte differentiation (1760 nematocyte nests/day/hydia) and stem cell proliferation\\n(stem cell cycle = 24 h), agree with the numbers of these cells observed in hydra. The\\nnumber of stem cells per hydra is 3000-6000 depending on assumptions about the time of\\ndetermination. The ratio of nematocyte to nerve differentiation averaged over the whole hydra\\nis 3:1. In the hypostome and basal disk interstitial cell differentiation occurs exclusively to\\nnerve cells while in the gastric region the ratio of nematocyte to nerve differentiation is about\\n7:1.'