998 ROSE Fig, 2— Regeneration when two limbs were grafted together at a wide angle and both hands were amputated. The right limb, which had received an X-ray dose of2000 R, failed to regenerate a hand and did not interfere with the regeneration of a hand on the unirradiated left limb. [From J. C. Oberpriller, J. Exp. Zool., 168: 4129 (1968}.] morphogenetic controls. How this is done is being studied. The first results have been obtained. The studies are based on the fact that X-rayed limbs can recover regenerative ability if normal limb tissues are grafted to them,!?.!3 Skin that has not been X-rayed or just the epidermis!‘ covering an X-rayed limb can cause the regeneration of a perfect limb, The question has been whether the graft of normal tissue provides all the cells for regeneration or whether the graft in some manner enables the X-rayed tissues to participate, One can study an early step in the failure of X-rayed tissues to regenerate by providing them and norrial limbs with tritiated thymidine. In both the normal and the X-rayed limbs, the epidermis picks up tritiated thymidine and epidermal cells continue to divide, as can be seen On autoradiographs, The X-rayed limbs can function for many years, but regenerative ability does not return, It can return, as noted above, if the limb is provided with a normal epidermis, Then, as part of the series of steps in regeneration, the internal X-rayed cells of muscle, connective tissues, and nerve sheath do make new DNA and proceed to regenerate, The last question is what the normal graft does to reinstate regenerative ability. A comparison of normal stumps, X-rayed stumps,