Sagittal sections of the dorsal neocortex in the embryonic rat illustrating the successive emergence of developing cortical layers. Toluidine blue stain, 3-mm methacrylate sections. Abbreviations: ch1, extracellular channel 1; ch2, extracellular channel 2; CP, cortical plate; iz, intermediate zone; ne, neuroepithelium; pl, primordial plexiform layer; SP, cortical subplate; sv, subventricular zone. Reprinted with permission from Bayer and Altman (1; p. 26).