A free string quantum model is constructed directly in the light-cone
variables in the range of dimensions 1 < D < 25. The longitudinal degr
ees of freedom are described by an abstract Verma module with the cent
ral charge restricted by the requirement of closure of the non-linear
realization of the Poincare algebra. The spin content of the model is
analyzed. In particular for D = 4 the explicit formulae for the charac
ter generating functions of the open and closed strings are given and
the spin spectrum of the first twelve excited levels is calculated. It
is shown that for the space-time dimension in the range 1 < D < 25 th
is light-cone quantum model is equivalent to the critical massive stri
ng and to the non-critical Nambu-Goto string. (C) 1998 Elsevier Scienc
e B.V.