  We report results of molecular-dynamics simulations for a glassy polymer melt
consisting of short, linear bead-spring chains. It was shown in previous work
that this onset of the glassy slowing down is compatible with the predictions
of the mode coupling theory. The physical process of `caging' of a monomer by
its spatial neighbors leads to a distinct two step behavior in the particle
mean square displacements. In this work we analyze the effects of this caging
process on the Rouse description of the melt's dynamics. We show that the Rouse
theory is applicable for length and time scales above the typical scales for
the caging process. Futhermore, the monomer displacement is compared with
simulation results for a binary Lennard-Jones mixture to point out the
differences which are introduced by the connectivity of the particles.
