In the never-ceasing discussion on the origin of cancer, much attention has always been given to that continuity between normal and carcinomatous epithelium which is so frequently observed at the edges of tumors. Many pathologists hold that the transition here noted is evidence at once of the primary seat of the growth and of its manner of origin. Certainly this evidence would seem strong, and those who do not accept it are few. Ribbert has long held that a secondary union really explains such pictures, and Borrman,1 his pupil, has collected many specimens which histologically further this view. Experimental evidence for it, however, has been lacking.
In the course of a comparison of the conditions regulating the growth of transplanted tumor and a transplanted normal tissue2 I have examined many grafts in which mixed tumor and embryonic tissue have grown intimately together in the new host. A few