Analysis of the reactions produced in tissue by irradiation or the administration of chemotherapeutic agents reveals that such factors do more than merely interfere with cellular division. Some modalities inhibit the completion of mitosis in cells with the capacity of multiplication, some interfere with premitotic nuclear metabolism, and others inhibit an earlier cytoplasmic phase of synthesis. A comparable interference with cytoplasmic synthesis and an inhibition of differentiation are exercised by some of the drugs on the somatic cells which lack the ability to divide, while irradiation in therapeutic doses and other chemical agents do not interfere with differentiation. Although tumors in general are presumed to arise from reserve cells, spontaneous differentiation occurs not uncommonly. The possibility of enhancement of the tendency to differentiate is a relatively neglected but important effect of treating neoplasms by irradiation and selective chemotherapy.

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This project has received support from the Jane Coffin Childs Fund, The American Cancer Society, and the Atomic Energy Commission. Microphotographs by Dale Gillette.

Read at the International Congress of Clinical Pathology, Washington, D.C., September, 1954, and at the 121st Meeting of the American Association for the Advancement of Science, Berkeley, California, December, 1954.

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