The foregoing experiments have shown that the causative virus is usually "masked" in the large, disorderly, fissured and inflamed papillomas of cottontails when antiviral antibody is present in quantity in their blood, though virus can be recovered as a rule from the smaller, discrete, well ordered papillomas of these rabbits, almost irrespective of the amount of antibody in the blood of the individuals bearing them. Other findings are described which indicate that the masking of the virus in the large fissured growths is due to serum antibody present in them as result of exudation or hemorrhage, which neutralizes the virus when the growths are extracted or preserved in vitro. The local conditions that favor extravasation of serum (and the accumulation of antibody) prevail as a rule in the large, confluent growths arising after virus has been sown broadcast on scarified skin, but to lesser extent or not at all in the discrete papillomas that occur naturally or as result of tattoo inoculation.
The state of affairs is notably different in the papillomas of domestic rabbits. The virus is regularly masked in these, and usually masked completely, even when there is little antibody in the blood and the local conditions do not favor its extravasation into the growths. The findings indicate that something other than antibody is primarily responsible for the masking in this species.