1. Two strains of mice, one with a high, the other with a comparatively low incidence of spontaneous tumors of the mammary gland, when painted between the shoulders with coal tar extract developed tar tumors with about practically identical frequency.
Possibly this result was to have been expected. If tumors of certain organs or tissues are specific in heredity, a conception for which there is some evidence, then assuredly a high degree of incidence for one tissue, as for example the mammary gland, does not necessarily mean a high incidence for another tissue, such as the skin, when subjected to tarring. To test the influence of heredity on the response of the skin to tar painting it would be best to employ strains of animals exhibiting differences in the incidence of spontaneous cancer of the skin. Unfortunately no such material was available. We can state on the basis of our experiments only that no hereditary differences in the response of the skin to tar painting were demonstrable in two strains of mice manifesting markedly different percentage incidences for spontaneous mammary growths. It is possible, of course, that the natural differences in the two strains may have been wiped out by the tar treatment. Such a view is suggested by the result of other work in this laboratory which has shown that tar painting increases markedly the incidence of tumors of the lung and destroys the resistance to transplantable tumors (unpublished work).
2. Mice from which spontaneous mammary tumors had been removed were treated with tar. The percentage incidence of the resulting tar tumors was similar to that met with in the controls, except possibly in the case of such animals as showed a recurrence of the spontaneous growth. In them the development of tar tumors seemed to be delayed and possibly prevented. The numbers involved are too small to be conclusive.
Murray has stated that "the induction of a fresh primary growth after a first has been definitely established, meets with a very intense resistance whether the first tumor be of the same parent tissue or another." His condusions are based partly upon certain retarring experiments but more especially upon a series of mice from which spontaneous mammary gland tumors had been removed, which were then tarred. Apparently these latter did not have recurrences of the spontaneous growth. Only 1 of them developed a papilloma and none carcinoma. The tar was applied twice weekly in Murray's experiment instead of 3 times as in ours and for this as for many other reasons a direct comparison is not possible. It is furthermore not clear what the expectation for his group would have been. Our conclusions, however, seem only partly to confirm his for we find that if any protection exists against a tar cancer after the ablation of a spontaneous mammary growth, it is only when a recurrence has taken place. From certain additional tarring experiments Murray concluded that if a second tarring is begun before cancer has developed from the first, susceptibility is increased. Truffi also has found a diminished refractoriness after tarring. Among our mice in which the mammary gland recurrence appeared during or after the tar painting, no increase in susceptibility but rather the reverse was found. Future experimentation must determine to which among several variables this divergence of results is to be attributed.