Regions of the mammalian telencephalon and their neurogenetic gradients. The schematic drawing on the left depicts the developing brain in side view. Different brain regions are depicted in grey and different telencephalic regions are depicted in colour as indicated. The rhinal fissure is shown as a white line. On the right side, a flat mount of the telencephalon is shown in the same orientation as the side view (after Bayer and Altman, 1991). The developmental gradients are indicated as arrows pointing to the latest developing regions.

Developmental origin of vertebrate telencephalic regions. (a) The neural plate viewed from the top. The prospective telencephalic regions are delineated in colour. (b) The developing brain at a later stage in side view. The dorsal and ventral telencephalon are depicted in lighter and darker blue, respectively, as shown in their regions of origin in (a).

Histogenesis of the developing mammalian isocortex. Cross‐sections through the developing isocortex at different developmental stages: (a) before neurogenesis; (b) just after onset of neurogenesis; (c) at midneurogenesis. The cortex is depicted with the pial (basal) side up. Dividing precursor cells are depicted in red; postmitotic neurons are in blue.

(a) Before neurogenesis, all cells appear similar. They span the entire thickness of the cerebral wall and proliferate. During the cell cycle the nuclei move between the ventricular and pial surface (interkinetic nuclear migration).

(b) After the onset of neurogenesis, the first postmitotic neurons settle underneath the pial surface. A subpopulation of these earliest neurons expresses the extracellular matrix molecule reelin (green dots). The precursor cells that remain attached to both surfaces are the radial glia cells.

(c) After further thickening of the cerebral wall, neurons migrate along radial glia fibres and settle in the middle of the former preplate. Consequently the preplate is split into the marginal zone (MZ) and the subplate zone (SP) situated above and below the newly forming cortical plate (CP). Almost all cortical layers in the adult develop from the cortical plate, whereas the most preplate neurons die at later stages. At midneurogenesis a secondary proliferative zone is formed, the subventricular zone (SVZ). VZ, ventricular zone.

Neurogenesis of the isocortex in different mammalian species. The grey bars depict the length of neurogenesis of the isocortex (i.e. the time between generation of the first and last neurons). The production of different layers is indicated in different colours. Neurons of the preplate are generated at the onset of neurogenesis in the cortex (grey region to the left of the blue bars).