Cellular compartmentation was studied in the germinal matrices (the primary neuroepithelium and the subventricular zone) of the rat cerebral cortex at successive stages of embryonic development. Three sets of materials were used: methacrylate-embedded brain sections from normal embryos, autoradiograms from embryos labeled with [3H]thymidine, and methacrylate-embedded sections from embryos exposed to 200 R X-ray. Examination of normal specimens showed that between Embryonic Day 12 (E12) and E15 the cortical germinal matrix consists only of a primary neuroepithelium. By Day E16, a subventricular zone has formed in the early developing ventrolateral aspect of the cortex. The subventricular zone grows in depth for several days, while at the same time the depth of the neuroepithelium decreases. Examination of short-survival thymidine radiograms revealed that the labeled cells do not form a continuous band in the neuroepithelium but aggregate in patches reminiscent of bunches of grapes strung one a line. It is postulated that this vertical periodicity is due to the alternation of cell aggregates with short and long cell cycle times. Finally, examination of the cortical neuroepithelium in rats exposed to 200 R X-ray showed that there is an alternation of radiosensitive (collapsing) and radioresistant (intact) patches that roughly correspond in size to the labeled and unlabeled patches seen in autoradiograms. Additional observations concern the onset of local cell proliferation in the white matter at late stages of fetal development and the transformation of the neuroepithelium into a matrix producing ependymal cells.