A downhole apparatus includes an outer case connectable at upper and lower ends thereof in a casing string. A degradable plug is fixed in an interior of the outer case. A rupture disc is mounted in a port in the wall of the outer case. The port is positioned to communicate a degrading fluid to an axial flow passage defined in a wall of the outer case and the axial flow passage is configured to communicate the degrading fluid back into the interior of the outer case and into the degradable plug.