1. Compare some recent books and reviews and the literature cited therein:
2. R. P. Feynman, Photon-Hadron Interaction, Addison Wesley, New York, 1972.
3. F. E. Close, Partons and Quarks, Daresbury: Lecture Notes Series, No. 12, DNPL IR31 (1973).
4. P. V. Landshoff, J. L. Polkinghorne, Physics Report 5c, 1 (1972).
5. Proc. Intern. Symp. on Electron and Photon Interaction at High Energies, Bonn, 1973.