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Two- to three-day old chick embryos were treated with thymidine-H/sup 3/ (10 to 50 mu c per embryo; specific activity 1.6 or 1.9 curies per millimole) through a window in the shell, and incubation was continued for 12 to 55 hours longer. All embryos incubated after treatment for 12 hours or longer displayed cytological changes characteristic of radiation injury. Injury could be avoided and satisfactory radioautographs still obtained by diluting the thymidine-H/sup 3/ one hundred to five hundred times with unlabeled thymidine, thus reducing the specific activity to 4 to 20 mu c/ mu M. (auth)
Sauer et al. (1961) studied this question.