Dr. P. Ross, discussing the question from the clinical point of view, concentrated on the neoplastic effects and what he called the degenerative effects. There were two types of growth—the simple papilloma and the malignant epithelioma. So far as the simple papilloma or papular pitch wart was concerned, the latent period could be as short as six months from commencing contact with the materials pitch or tar. The appearance of this papilloma was not unlike that of the small wart. At times it was a keratotic patch and at other times a small pedunculated papilloma. The end results, if left alone, might be resolution and healing, but it might become malignant and sometimes it became hyperkeratotic and formed a cutaneous horn.