Fig. 4. CDK1 activity declines rapidly with polar body extrusion during meiosis II. Small groups of eggs were assayed, at the times indicated, for CDK1 activity by the ability of CDK1 to phosphorylate histone H1 in an in vitro assay. Eggs sampled at 1 hour had extruded their second polar body and eggs sampled at 8 hours had pronuclei. The shaded bars indicate that the polar body (pb2) and pronucleus (pn) formation must have occurred during this time. CDK1 activity declined rapidly in activating eggs, such that the largest decrease occurred in eggs that had extruded their pb2. Values are means ± standard deviation (n=5, separate experiments) in H1 kinase activity normalised with respect to non-activated eggs.