AbstractThe pattern of incororation of thymidine‐H3was studied in chick embryos in which overgrowth was experimentally induced in the optic tectum. Radioautograhs were prepared for experimental and control embryos ranging in age from three to five days' incubation at post‐label periods of one hours, and eight hours. Under conditions of overgrowth, S‐phase cells are situated not only in the periphery of the wall of the tectum, but are also found scattered among mitotic cells along the ventricular border. Labeled intraventricular cells are commonly seen in the defective brains. The onset of neuroblast differentiation, as indicated by a loss in ability of certain cells to incorporate thymidine‐H3, appears to be the same in the tectum of experimental and normal embryos and takes place toward the end of the fourth day of incubation. As development continues, the pattern of labeling becomes increasingly distorted in the severely affected potions of the overgrown tectum, and neuroepithelial cells become interspersed among differentiated neur