  To establish a standard for the distinction of reptation from other modes of
polymer diffusion, we analytically and numerically study the displacement of
the central bead of a chain diffusing through an ordered obstacle array for
times $t < O(N^2)$. Our theory and simulations agree quantitatively and show
that the second moment approaches the $t^{1/4}$ often viewed as signature of
reptation only after a very long transient and only for long chains (N > 100).
Our analytically solvable model furthermore predicts a very short transient for
the fourth moment. This is verified by computer experiment.
