The relation between tritium content of individual cells and grain count obtained in autoradiographs of squashed cells was investigated. The tissues used were root meristems of Tradescantia paludosa and intestinal epithelium of the mouse. The relation between grain count and tritium content is affected by self-absorption which depends on the thickness of the labeled cell. Therefore, squashed preparations were sectioned to determine the uniformity of thickness of nuclei. In a preparation of mouse cells, thicknesses were 1.18 ± 0.35 µ, and in a preparation of Tradescantia cells, 2.97 ± 0.35 µ. The effects of similar and larger variations in thickness upon grain count were studied in material squashed with different pressures; no marked correlation was found. The lack of correlation is explained by the geometric relation between labeled nuclei and the emulsion. By counting grains and directly measuring tritium content in a glass proportional counting tube in the same preparation, the yield of grains per disintegration was measured in Tradescantia cells and found to be 1 grain for 10.9 disintegrations with AR 10 autoradiographic film and 1 grain for 19.3 disintegrations for NTB nuclear track liquid emulsion. Latent image fading may pose a problem with long exposures; the conditions of its occurrence are as yet not well known.

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