A problem of considerable importance in practical pathology, as well as in cancer research, is the identification and classification of malignant tissues. The characterization of tissues as malignant or cancerous derives from a biologic conception of these states; yet in most laboratories of pathology the distinction is based solely on morphologic considerations. Thus, the assessment of properties and potentialities becomes an interpretation of fixed appearance, rather than a judgment based on actual behavior.
Morphology indicates character, but the proof of malignancy lies in behavior. Many tissues and tissue states bear a remarkable resemblance to cancer, but their fate in the organism and their significance to it place them in entirely different categories. Early embryonic tissues frequently resemble cancer so closely that the distinction cannot be made on the basis of a histologic examination of a section, and the definition depends on the fact that in the primary host and in