Epidermal tumors were induced in Swiss female mice by a topical application of 9,10-dimethyl-1,2-benzanthracene solution followed by repeated applications of croton oil solutions. Fourteen benign and malignant tumors were sampled 25 weeks after the treatment had begun, fixed in osmium tetroxide or potassium permanganate, and embedded in Epon. Sections stained with lead hydroxide were examined. In three tumors defects of the epidermal basement membrane were seen. These defects accompanied local invasion of the tumors. The possible mechanisms of the development of this unusual anatomical situation are discussed.

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