1. For a recent review, see: Clegg,A.B.: Proc. of the 4th Intern. Symposium on Electron and Photon Interactions at High Energies, Daresbury 1969.
2. Albrecht,W., et al: Nucl. Phys. B25, 1 (1971).
3. Nuthakki,V.P.R., et al: Daresbury preprint DNPL/P 50 (Sept. 1970).
4. Amaldi,E., et al: Nuovo Cimento 65A, 377 (1970).
5. Kajiura,N., et al.: Lettere Nuovo Cim. 3, 714 (1970).