Dramatic changes occur in the radial glial fiber system of the murine forebrain in the course of neocortical histogenesis. Initially, prior to substratification of cortical and subcortical anlagen between E13 and E14, the system is uniformly radial in alignment. It appears to achieve maximum density and to be highly uniform in structure throughout its radial span, both in terms of apparent fiber density and the pattern of arrangement of fibers in fascicles. Subsequent to E14, concurrently with rapid growth of the cerebral wall and with the differentiation of the cortical substrata and intermediate zone, the subcortical segment of the system becomes laterally arced while the transcortical span of the system remains radially aligned. Although the spacing between fascicles changes little, there is an apparent general drop in fiber density associated with a progressive reduction in the number of fibers per fascicle. The changes in relative positioning of the fibers and the apparent decline in fiber density are most dramatic within the developing cortical plate and subplate and may be of specific significance for the migration and radial assembly of the neurons in the supragranular neocortical layers.