1. For a complete survey of the developments up to 1986, see
2. M. Green, J. H. Schwarz, and E. Witten, Superstring Theories, vols. I and II, Cambridge University Press, Cambridge, 1986;
3. M. Kaku, Introduction to Superstrings, Springer-Verlag, New York, 1988.
4. Y. Nambu, in Symmetries and Quark Model (edited by R. Chand ), Gordon and Breach, New York, 1970, p. 269;
5. T. Goto, Prog. Theor. Phys.
46, 1560 (1971).