1‘ = 23 - depth is near the base of the hair follicle which comprises the deepest reservoir of ‘epithelial cells of the germinal layer, it was suggestive that this might be a critical The suggestion gained region in the observed carcinogenesis. significance from the observations that most of the tumors are similar to hair follicles, and that in the non-ulcerogenic dose range the number of tumors per rat was in nearly constant ratio (1/2000-1/4000) with the number of atrophied hair follicles. Thus the carcinogenesis in this experiment was remarkably correlated with the dose to and specific damage of a particular skin structure. When exposures were made with strive and sieve patterns of roughly 1 mm scale, geometrical effects were observed: most notably the cancer induction in the sieve geometry was suppressed at doses of 1700 rad but not at doses of 2300 rad.° The reduction, however, was again consistent with the reduction in damage as characterized by atrophied hair follicles. To summarize this important experiment, a high incidence of cancer was observed after intense local doses of radiation, and the carcinogenesis was proportional to the damageor disordering of a critical architectural unit of the tissuc, the hair follicles, .,