  We examine the solution of the constraints in spherically symmetric general
relativity when spacetime has a flat spatial hypersurface. We demonstrate
explicitly that given one flat slice, a foliation by flat slices can be
consistently evolved. We show that when the sources are finite these slices do
not admit singularities and we provide an explicit bound on the maximum value
assumed by the extrinsic curvature. If the dominant energy condition is
satisfied, the projection of the extrinsic curvature orthogonal to the radial
direction possesses a definite sign. We provide both necessary and sufficient
conditions for the formation of apparent horizons in this gauge which are
qualitatively identical to those established earlier for extrinsic time
foliations of spacetime, Phys. Rev. D56 7658, 7666 (1997) which suggests that
these conditions possess a gauge invariant validity.
