Fig. 3. Transverse optical sections of NT defects. (A) Control embryo at stage 16. (B,C) Xdd1-injected embryos at stage 16. (D) Control embryo at stage 17. (E,F) Xdd1-injected embryos at stage 17. Arrows indicate neural folds in all panels. Note that at each stage, neural folds in Xdd1-injected embryos are wider than controls at the same stage. Midline tissues are less organized in Xdd1-injected embryos at stage 16, though organization is better at stage 17. In many embryos, a prominent bulge was observed in the midline of the open neural plate (Fig. 3F). Although thinning of the neural plate has been suggested to contribute to tube closure (Poznanski et al., 1997), because it was not consistently observed (Fig. 3E), this midline bulge of the neuroepithelium cannot be the primary cause of the open NT phenotype in Xdd1-injected embryos.