Fig. 1. Spatial and temporal separation of the events of neurogenesis during the generation of the spinal cord (D,E,G,I,M-P are tissue sections). (A,B) Summary of the gene expression domains in the developing embryo (medio-lateral extent of expression is not indicated). (A) Stage 7+ embryo, CNP, caudal neural plate; PS, primitive streak; S, first formed somite; PSM, presomitic mesoderm; dashed lines, underlying tissues. (B) Stage 8 embryo. PNT, preneural tube; NT, neural tube. (C-E) The first neurons appear opposite recently formed somites (arrowhead) as revealed by NeuroM expression in a stage 8– embryo. (F-I) NeuroM expression extends caudally within the neural tube and appears adjacent to the most recently formed somites (arrowheads) in stage HH8+ (F,G) and stage HH9 (H,I) embryos. (J-L) Sax1 expression in the regressing CNP and PNT prefigures neurogenesis and is down regulated in the neural tube at about the level of the most recently formed somite (arrowheads). (M-P) NeuroM-expressing cells (blue, arrowheads) do not incorporate BrdU (FITC/green) in stage 12 neural tube. Double exposures (M,O), bright field (N), FITC only (P). Scale bars, (C,F,H,J,K,L) 200 µm; (D) 100 µm; (E,G,I) 50 µm; (M) 20 µm; (N,O,P) 10 µm.