ABSTRACT
The first inkling that cancer formation is not the result of a sudden, single cellular change, but rather a long complex process occurred when it was discovered that the application of coal tar to the ears of rabbits did not cause skin cancer unless it was applied repeatedly for many months. Since then evidence from a variety of sources has substantiated the fact that carcinogenesis occurs in stages which take a long time to complete. This chapter reviews the evidence of the for cancer in general and for intestinal cancer in particular. There is strong evidence from epidemiological data that cancers in humans also develops slowly and in multiple steps. The most striking example of this comes from data on lung cancer, which is described in a review by R. Doll. The phenomenon of two-stage carcinogenesis has been observed in a variety of tissues. The mouse skin is without question the best system for the study of the process.
