ABSTRACT The columnar organization of forebrain cortical neuron production allows the neuroepithelium to be analysed in terms of a simple growth model. The pattern of neuron release within each column is repeated with great regularity across the surface of the developing cerebral hemisphere, resulting in the accumulation of cortical neurons in radial stacks above the proliferative ventricular epithelium. The time course of recruitment of adjacent tissue into neuron production accounts for the observed rostrocaudal and laterodorsal gradients of cortical neuron release. The present study plots the history of this front of neuron production across the surface of the developing cerebral vesicle in the ferret and interprets the resulting maps of cortical development stages in terms of a unified growth model that links the radial and tangential aspects of neuron production. These observations are discussed in relation to other studies of gradients in developing nervous tissue and their implications for the study of experimentally induced abnormalities of brain development.