This investigation primarily consisted of labeling cells in forelimbs of the newt; irradiating the labeled limb; and then inducing regeneration with a non-irradiated skin graft. The labeled cells were obtained by injecting tritiated thymidine intraperitoneally into newts with regenerating right forelimbs. The cells in the regenerate which were undergoing DNA synthesis were labeled, and those which differentiated and ceased to divide remained labeled for long periods. The labeled regenerates

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... irradiated with 2000 R, and subsequently the irradiated limbs were re-amputated. Some limbs were supplied with a hind-limb skin autograft; others received no graft. The limbs without the grafts did not regenerate, whereas several of those with grafts did. Histological studies of the blastemas of the new regenerates indicated that labeled, irradiated cells were released from the internal tissues near the amputation surface and were incorporated into the blastema. These cells presumably formed a portion of the differentiated tissues of the new regenerate. Studies were performed to verify that the labeled cells in the blastemas were derived from irradiated tissues and were not derived from extraneous sources. This investigation substantiates reports that irradiation with 2000 R inhibits limb regeneration, and that at least some irradiated cells can contribute to a blastema formed on an irradiated limb.