The histogenesis of the isocortical segment of the lateral telencephalic wall at the coronal level of the interventricular foramen was studied in mice between the ages of E10 and the adult. The proliferative activity of the periventricular germinal layers was correlated with changes in cell distributions in the intermediate layer. The appearances were consistent with a wave of differentiation moving across the ventricular layer from lateral to medial and a peak of neuron production occurring about E13. The sequence of changes was analysed using the concept of a radial unit composed of ventricular cells and their related progeny of neurons. The observed histological changes were interpreted as the result of radial units of similar productive history entering and completing the histogenetic sequence at successively later times along a lateromedial gradient. Some of the implications of this approach were examined and discussed in relation to the general evolutionary properties of such a system of histogenesis.