THERE is an old saying that “what goes up must come down.” This expression could be paraphrased to read “what goes up might come down,” and applied to a graphic representation of the fascinating process of biologic differentiation as exemplified in embryonic development. Do some tissues “slip back down the embryonic stairway”? If so, does this fact play a significant role in neoplasia, either benign or malignant? Is there any way to induce such tissues to redifferentiate for a second, or many times? Is it possible even to hasten the normal embryonic process of differentiation? What role, if any, is played by so-called tissue organizers? Is the influence of such organizers limited to the embryonic era or could undifferentiated neoplastic cells perhaps be redifferentiated under such influence? Could, in fact, normal adult cells even be over-differentiated by such exposure? This communication does not answer any one of the foregoing questions. The results reported do, however, suggest that if modern techniqu...