Cortical Plate Formation. A schematic representation of human neocortical development is shown. A. The VZ prior to the onset of neuronal migration contains symmetrically dividing progenitor cells. B. The first neurons to leave the VZ form a sparse preplate layer at the edge of the VZ (GW<7). C. Axons of the preplate neurons along with the axons projecting from subcortical areas, form the intermediate zone. D. The preplate is split into an outer MZ and a deeper subplate zone by the developing cortical plate (beginning GW7–8). E. In the mature cortex, only the cortical layers and the underlying white matter pathways are evident. VZ=ventricular zone, MZ=marginal zone, PP=preplate, SP=subplate, SZ=subventricular zone.