That Roentgen rays produce distinct changes in various tissues of the animal body is well known, but concerning the exact mechanism by which these changes are caused we know practically nothing. Marked changes may be produced in the blood and blood-forming organs by these rays under proper conditions and, as first shown by Heineke, the lymphocytes are especially sensitive to their action, disintegration taking place very rapidly. It is on this account that the rays are used in the treatment of leukemia. In animals it has been found that the lymphoid tissues themselves are markedly affected. Recently the effect of roentgenization on the thymus has been studied experimentally by Rudberg,1 and as the resulting cellular changes have been followed closely (in rabbits) a brief account of them is given.
Under the influence of the rays, the lymphocytes as well as the cells of the reticulum of the thymus rapidly