  The goal of this paper is the analytical validation of a model of Cermelli
and Gurtin for an evolution law for systems of screw dislocations under the
assumption of antiplane shear. The motion of the dislocations is restricted to
a discrete set of glide directions, which are properties of the material. The
evolution law is given by a "maximal dissipation criterion", leading to a
system of differential inclusions. Short time existence, uniqueness,
cross-slip, and fine cross-slip of solutions are proved.
