The study of malignant tumors by the usual methods hitherto in vogue has not yielded many results of fundamental importance. A serious drawback to this study is encountered in the non-inoculability of human tumors in animals. The occurrence in certain animals of transplantable tumors furnishes the investigator with welcome opportunities to institute various lines of experimentation looking, for instance, to the production of immunizing and curative substances. Thus Jensen<sup>2</sup>of Copenhagen was able to work with an inoculable, spontaneous tumor in a white mouse. He has carried the experimental inoculation through nineteen generations, and other experiments have been made with a view to producing active as well as passive immunity—at the same time as the tumor cells have been subjected to various procedures calculated to throw some light on their properties. This tumor occurred originally subcutaneously, and was in structure a carcinoma, which character it subsequently maintained. Subcutaneous injections