FIGURE 2.

Modes of neurogenesis in the vertebrate cerebral cortex. (A) Before the onset and during early stages of neurogenesis, the neuroepithelium divides symmetrically to expand in a lateral fashion, increasing the neurogenic domain. (B) In vertebrates with a small cortical field, the radial glia divide asymmetrically to generate neurons in a direct manner. (C) Indirect neurogenesis generates intermediate progenitors that divide symmetrically resulting in increased neuronal output and expansion of the cerebral cortex. The appearance of a duplication of the radial glia layer in outer radial glia allows further radial and lateral expansion and gyrification of the cortex.