Fig. 1. The mode of neural-progenitor-cell division, and its effect on the direction of cortical expansion and growth (lateral expansion versus radial growth). (A) Lateral expansion (double-headed arrow), in which one AP generates two AP daughter cells, occurs as a result of the symmetric, proliferative division of APs (blue). (B,C) Radial growth (double-headed arrow) occurs as a result of either (B) asymmetric, neurogenic divisions of APs (blue), in which one AP generates a further AP as well as one neuron (red), or (C) asymmetric, differentiative divisions of APs (blue), in which one AP generates one AP and one BP (orange), which in turn generates two neurons (red) and is thereby consumed (dimmed orange).