The human cerebral cortex includes billions of neurons organized into six sheetlike layers. More than a century ago, Santiago Ramón y Cajal appreciated the astonishing diversity of cell types in the brain, but even today, it is unclear how the different neuronal cell types are assembled and distributed across the distinct anatomical areas of the cortex to support its diverse functions. We know that excitatory neurons of the cortex are born from a uniform population of radial glia. During development, radial glia line the fluid-filled ventricles and extend long fibers that connect with the outer pial surface. Newborn neurons migrate along radial glia fibers to the outer layers of the tissue, where they form the cerebral cortex.