  In this paper the detailed CCWZ procedure for introducing fermions on the
world sheet of a string propagating in flat space-time is presented. The theory
of nonlinear realizations is used to derive the transformation as well as the
interactions of fermionic matter fields under arbitrary spinorial
representations of the unbroken subgroup. This demonstrates that even for
non-supersymmetric spinors, the interactions are still severely restricted by
the nonlinearly realized symmetry. We also explain how supersymmetric models
provide an example for this construction with Goldstinos as matter fields, and
how one can use the $\kappa$-symmetry of the Green Schwarz action in
particular, to verify this nonlinear transformation for a specific matter field
representation. We finally restrict the target space dimension without
reference to supersymmetry, but rather by imposing one-loop integrability on a
fermionic string that nonlinearly realizes Poincare symmetry. This singles out
the critical dimension $D=10$ for heterotic, GS and RNS supersymmetric strings.
