The injection of Scharlach R into the skin about rabbit papillomas resulting from virus inoculation causes them to invade the underlying tissue and form large, fleshy masses beneath the surface. Histologically these appear malignant, and they frequently invade the blood vessels. Covering young papillomas with a layer of collodion causes them to burrow downwards with result in discoid masses which enlarge progressively, both by expansive growth beneath the epidermis and by invasion. Such masses, like the nodules resulting from implantation, have the papillae turned toward their interior, the apparent reverse of the condition of affairs when the growth is situated on the skin surface. The reasons for this are analyzed. The peculiarities of the host influence skin papillomas not a little, as is plain from the forms they assume; but the epithelial changes induced by the virus take a single direction, and no significant variations from type have been encountered.
Local or generalized retrogression of the experimentally induced papilloma is not uncommon. The histological alterations that take place are identical with those attending retrogression of the epidermoid tumors, and the reactive changes taking place in the surrounding tissue are also like those about such tumors. The slowing and cessation of growth that occur secondarily in the case of virus-induced skin papillomas are associated with the formation under them of a dense layer of connective tissue, and to this their behavior is attributable. Similar findings have been often recorded for tumors, notably for the epidermoid cancers produced in rabbits by tarring.