1. W. Heisenberg:Nachr. d. Göttinger Akad. d. Wiss., 111 (1953).
2. H. P. Dürr:Zeits. f. Naturf,16 A, 327 (1961).
3. For a review of older work, see,W. Heisenberg:Introduction to the Unified Field Theory of Elementary Particles (London, 1966);H. P. Dürr:Acta Phys. Austriaca, Suppl.,3, 3 (1966).
4. R. E. Marshak andE. C. G. Sudarshan: inProceedings of Padua-Venice Conference on Mesons and Recently Discovered Particles, 1957 (Padova, 1958);Phys. Rev.,109, 1860 (1958);
5. R. P. Feynman andM. Gellman:Phys. Rev.,109, 193 (1958).