The effect of x-irradiation in vitro with 25 to 500 R on the development of isolated submandibular and sublingual glands of the mouse was studied on explants at the thirteenth day of gestation, incubated on a clot of chick plasma and embryo extract. Growth was quantitatively estimated by counting epithelial buds formed during 48 hours of culture in controls and irradiated glands. The average number of counts was plotted against the dose, in order to construct a dose-effect curve and a regression line. Growth was found to be progressively inhibited with doses of radiation increasing from 100 to 500 R. However, an anomaly was observed in the range between 25 and 100 R, where a lower dose seemed to have a stronger inhibiting effect than a higher one. This rather surprising and apparently significant effect was consistently obtained in two different sets of experiments. Secretion was studied in two conditions: (1) by radiation applied at explanation time, and histological study in the following 5 days of culture, and (2) by radiation applied after 3 days of culture, when the glands have either just begun to secrete, or after 5 days when they have already progressed to secrete, and examination after 1 or 2 days of further cultivation. In both cases, the synthesizing activity of cells, expressed by the appearance of secretion, seemed to be related neither to the age nor to the dose applied, but only to the degree of development of acini. On the other hand, no significant qualitative change in secretion could be detected by the methods applied.

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