Reprinted in *Erice 1982, Proceedings, Unified Field Theories In More Than 4 Dimensions*, 434-446, and in *Ekspong, G. (ed.): The Oskar Klein memorial lectures, vol. 1* 67-80, and in *Chalk River 1983, Proceedings, An Introduction To Kaluza-Klein Theories*, 10-23,
See also *Appelquist, T. (ed.) et al.: Modern Kaluza-Klein theories* 76-87, and *De Sabbata, V. (ed.) et al.: Unified Field Theories of more than four dimensions including exact solutions* 434-446,