1. C. Lovelace:Phys. Lett.,28 B, 265 (1968);J. A. Shapiro:Phys. Rev.,179, 1345 (1969).
2. D. Amati:Lectures Given at the Latin American School of Physics, La Plata, Argentina (July 1970), Ref. TH 1231 CERN;G. Veneziano:Lectures Given at the Ettore Maforana School (July 1970).
3. S. L. Adler:Phys. Rev.,139, B 1638 (1965).
4. S. Weinberg:Phys. Rev. Lett.,17, 616 (1966).
5. H. Harari:Lectures Given at the Brookhaven Summer School (August 1969).