3 W. L. Russell, L. B. Russell, and E. F. Oakberg, in Radiation Biology and Medicine, ed. W. D. Claus (Reading, Mass.: Addison-Wesley Publishing Co., Inc., 1958), chap. viii. 4 W. L. Russell, L. B. Russell, and E. M. Kelly, Science (in press). 5 E. F. Oakberg, Proc. Tenth Intern. Congr. Genetics, 2, 207, 1958. 6 W. L. Russell, L. B. Russell, J. S. Gower, and S. C. Maddux, these PROCEEDINGS, 44,...