  We show that, given an arbitrary shift, the lapse $N$ can be chosen so that
the extrinsic curvature $K$ of the space slices with metric $\overline g$ in
arbitrary coordinates of a solution of Einstein's equations satisfies a
quasi-linear wave equation. We give a geometric first order symmetric
hyperbolic system verified in vacuum by $\overline g$, $K$ and $N$. We show
that one can also obtain a quasi-linear wave equation for $K$ by requiring $N$
to satisfy at each time an elliptic equation which fixes the value of the mean
extrinsic curvature of the space slices.
