The three dimensional growth of the mouse isocortex was examined by plotting the variations in intermediate layer depth on orthogonal projections of the telencephalic surface at successive periods of development; a histological status was assigned to each depth. Thus portrayed, the development of the isocortex was seen as a propagated sequence of histological change, commencing at a rostral focus coextensive with the caudatopallial angle and thence spreading across the telencephalic wall. Growth was asymmetric about the focus of origin and terminated in a rostrocaudal direction as the spread of neuron production reached and extinguished a growth zone along the sagittal perimeter of the hemisphere. The possibility of mouse isocortical histogenesis representing a variation of a general mammalian pattern was noted, as was the evolutionary and methodological significance of the apparent coincidence of the origin of the gradient of isocortical neuron release with the region of cortex representing oropharyngeal structures. An alternative form of representation of the isocortical gradient, as the summation of a number of radial strips of tissue each with a similar history of neuron release and migration, was used to lay a foundation for a three dimensional model.