“…To prove a common viral cause for skin and genital warts by this means, it would be necessary to inoculate successfully a cell-free filtrate of genital warts into the skin, and of skin warts into the genitals; the use of unfiltered extracts would not be acceptable, as living cells might be introduced and act as a tissue graft (Gross, 1961). Unfiltered penile wart extracts have raised skin warts on several occasions (Waelsch, 1918;Frey, 1924; Goldschmidt and Kligman, 1958), but filtered extracts only once (Serra, 1924). Vulval wart extracts have never been used; and the crucial experiment of successfully inoculating an extract of skin warts into the genitals has never been carried out.…”