The location of neuron nuclei of different labelling intensities in autoradiographs of the anterior forebrain of two 22 day old mice which had been injected with [3H]thymidine at 11 and 12 days post-conception respectively was charted on photocollages of sections enlarges 175 times. The pattern of distribution of the heavily labelled nuclei, i.e. those nuclei belonging to cells most likely to have been born shortly after the time of [3H]thymidine injection, indicated that the inner two thirds of the neocortex is laid down along a ventro-dorsal gradient, i.e. the lateral neocortex starts to form before the dorsal; and that cells born at a particular time lie in cortical layer VI at the dorsal edge of the gradient is traced ventrally. Progressively more weakly labelled cells formed intermediate steps in this migration. A model or cortical growth fitting these findings is presented. Some inferences are also made about the possible role of the ganglionic eminences in providing cortical cells, at least during the initial stages of cortical histogenesis.