Figure 5  Model for the origin of the neocortex from the superposition of lateral and dorsal cortex. (A) Schematic view of the putative organization of the dorso-lateral part of the telencephalon in an early mammaliaform (pre-mammlian synapsid). In these animals the lateral cortex (LC, violet) may have expanded over the dorsal cortex (DC, green), and at some point some radial glial progenitor (dark cells) may have started to produce both LC and DC cells. Note that since the internal anatomy of these animals is unknown, this scheme was based on modern macrosmatic reptiles. Radial glial cells of other brain regions are omitted for clarity. The proposed homology with neocortical layers is indicated with roman nueral. (B) Tangential expansion of the progenitors of the proto-neocortical column gave rise to the establishement of the neocortex. Note that the more lateral part of the neocortex maintains a direct olfactory input. Abbreviations: H.comm., hippocampal commissure; AC, Anterior commissure.