The following images are from a coronally-sectioned head of an E16 rat embryo exposed to tritiated thymidine on E15–24 hr survival. Note the heavily-labeled cells just outside the germinal zones, while the germinal zones themselves contain more lightly-labeled cells. Germinal zones in the brainstem are shrinking. The heavily-labeled migrating cells are postmitotic young neurons generated ON E15. By now, UNLABELED cells (generated before E15) take up much more space outside the germinal and migratory zones–even in the lateral parts of the cerebral cortex, where you can see the first appearance of the cortical plate.
