AbstractIn this experiment 12‐day pregnant mice were treated with 5‐fluorodeoxyuridine, an inhibitor of DNA‐synthesis, to examine the influence of the drug on the central nervous system of the embryos. More specifically the sites of cell degeneration, the type of cells affected, and the elimination of cellular debris were investigated.Approximately two hours after treatment with FUdR, mitotic activity in the neocortex ceased and the first degenerating cells became visible in the outer zone of the neuroepithelial layer, e.g., the DNA‐synthetic zone. During the following 8 to 11 hours degenerating cells increased in number and were seen throughout the neuroepithelial layer, suggesting either that other than DNA‐synthesizing cells were affected or that degenerating cells moved from the outer zone towards the inner zone. Since all dying cells were labeled when tritiated thymidine was given shortly before FUdR treatment, it is concluded that DNA synthesizing cells have moved to the lumen despite blockage of DNA synthesis and degeneration of the nucleus. At 24 hours after treatment all degenerating cells were found in the outer region of the neuroepithelial zone, indicating that they had moved from the inner zone (near the lumen) to the outer region. Hence, the degenerating cells did not remain in the DNA‐synthetic zone, but apparently moved or were pushed from the outer to the inner zone, and vice versa.When a search was made for cells cleaning up the debris, relatively few macrophages were seen. In addition only a few normal neuroepithelial cells could be identified as containing phagocytosed material. Many degenerating cells apparently were not phagocytosed but underwent lysis, leaving temporarily large holes in the tissue. It is thus suggested that in the embryo the large amount of cellular debris is eliminated mainly by autolysis, partially by macrophages, and also to some extent by neighboring neuroepithelial cells.Despite the presence of many degenerating cells, a number of mitotic cells were seen 11 hours after treatment, indicating that the drug had ceased to act. During the following 24 hours the neuroepithelial zone is reconstituted, but few if any neuroblasts are formed. Hence, despite the extensive repair which follows the action of a chemical compound, a permanent shortage of neurons results.