During the embryonic development of the cerebral cortex, young neurons migrate out into characteristic laminar positions and form specific axonal connections with other neurons. The birthdate of a neuron, and its tangential location in the ventricular zone, can serve as markers that predict its normal laminar fate and pattern of connectivity. In order to test whether cells of the developing cerebral cortex are committed to their normal fates, several types of transplantation experiment have challenged young cortical neurons to alter their identities or connections in novel environments. Other recent experiments have employed retroviral vectors to trace neuronal lineages in the cortex. Together, these studies suggest that phenotypic commitment involves a series of decisions. Certain developmental restrictions--for example, commitment to a laminar identity--may occur at or around the time of the cell's final mitotic division, whereas the refinement of area-specific axonal projections occurs as the neuron differentiates within the cortex.