The growth potentialities of induced skin tumors in mice; the effects of different methods of chemical carcinogenesis.

Iii their original description of the induced skin tumors of the rabbit, Yamagiwa and Itchikawa (14) detailed several morphological types of neo plasm with varying life histories. These tumors have been analyzed even more closely by Rous and Kidd (1@), who have shown that a series of graded lesions are produced by applications of either tar or pure hydrocarbons, ranging from warts that re gress to malignant metastasizing tumors. In the case of the mouse, it has been observed many times that a typical sequence of induced hyper plasia gives way to papilloma formation and that malignant transformation of some of these papil lomas occurs. Here, too, the phenomenon of re gression has been recorded. It has been noted that different carcinogenic hydrocarbons give rise to a variation in the type of neoplastic skin lesions in the mouse; in particular, the potent 9,10-dimethyl 1,@-benzanthracene has been observed by Brad bury et at. (7) to give rise to more papillomas than the other compounds used ; methylcholanthrene has been found to be an effective carcinogen when administered as a single large skin application (10), and the tumors so induced (6) have been found to be largely carcinomatous from the mo ment of their first macroscopic recognition. So far, no detailed survey of the tumors induced in the skin of the mouse, similar to that carried out for the rabbit, has been reported. Since it was demonstrated by Mottram (11) that a single application of a carcinogenic hydro carbon followed by repeated applications of croton