Fig. 1. Comparison of Hoxb4 and Hoxb9 expression in the chick neural tube. Dorsal views of different stages of embryos hybridized with Hoxb4 (A-E) or Hoxb9 (F-J). Embryos are at stage 4 (A), stage 8 (B,F), stage 9 (C,G), stage 10- (H), stage 10 (D), stage 11 (I), stage 14 (E), or stage 17 (J). Expression of Hoxb4 remains at a fixed AP boundary in the neural tube once activated, whereas that of Hoxb9 regresses posteriorly in the later stages. White arrows indicate the initial boundary of expression. H^IN, Hensen's node. Horizontal black bars mark the boundary of expression in the CNS relative to the adjacent somite (s) number or rhombomere (r).