Tumor agent transformations in Amphibia.

INTRODUCTION There is a reciprocal relationship between tu mor agents and cellular differentiation. A particu lar agent may be introduced into only one type of differentiated or partially differentiated tissue. At the same time, an agent from a donor neoplasm will, when introduced into cells of a new host, not only induce rapid growth but also cause the host cells to become identical in appearance and in secretions to the donor cells. Some years ago Murphy (@0) emphasized that these agents are concerned with cellular differentiation as well as with the initiation of abnormal growth. Murphy also pointed out the similarity of tumor agents to transformation agents of pneumococci. By 1914 the Rous group had discovered three different tu mors from which the agent could be removed (@4— @7). When these tumor agents were injected into muscles, it was noted that certain cells (possibly connective tissue cells) changed to resemble those of the tumor furnishing the agent. For example, one of the tumors, a chondro-osteosarcoma, caused cells lying within the muscle to produce the pe culiar cartilage and bone of the type found in the original tumor. A tumor agent may be passed through many generations and in each induce the same peculiar cell type. This indicates that some thing separable from cells can be added to other cells where it will be reproduced and at the same time direct the differentiation of the cells contain ing it. Duran-Reynals demonstrated (10, 11) that tu mor agents or viruses, as he called them, or micro somes, as they are called by Claude (7), from the Rous Sarcoma I of chickens could not only trans