Timing and positioning of reopening of the occluded spinal neurocele in the chick embryo.

Occlusion and reopening of the spinal neurocele are important morphogenetic events occurring during rapid brain enlargement in amphibians, birds, and mammals. The purpose of the present study, a companion to a previous one on occlusion, is to examine reopening. We determined when reopening occurred and at what frequency, whether it occurred suddenly or gradually, the extents of the neurocele that were patent at particular stages of reopening, and the morphological pattern of reopening. Reopening begins at stages 14+ to 15 and is virtually complete at stages 16-17. Thus, reopening of the spinal neurocele occurs suddenly and occlusion is a relatively short-lived developmental event. Because rapid brain enlargement still occurs after reopening, occlusion probably functions principally in the initial phases of enlargement prior to closure of the posterior neuropore. Further studies on the mechanisms of occlusion and reopening are under way.