Expression of the zic3 gene in chick embryos at HH stages 7 to 13. A: zic3 is expressed in the future forebrain in a limited and weak expression domain. It is broadly expressed in the future hindbrain. B: At stage 8+, weak zic3 expression continues in a limited region of the forebrain. It is expressed in the neural folds of the hindbrain and more weakly in the anterior trunk. C, D, D': In stage 9 embryos (7-9 somites), the first epithelial somites (somites I-III and sometimes somite IV) express zic3, while more anterior somites do not express zic3 (black arrows, C). zic3 is expressed at low levels in the presomitic mesoderm and at a slightly increased level in the area that will form the next somite (red arrowheads). D and D': posterior region of a stage 9 embryo. In D', the area that represents the neural plate and expresses zic2 (see Figure 2B) is outlined with red dots. The outer limit of the adjacent presomitic mesoderm, which expresses zic3, is marked by black dots. E: At stage 10, low levels of zic3 expression continue in presomitic mesoderm and in epithelial somites I-III. The neural folds do not express zic3. F: zic3 is first expressed along the entire brain in stage 11 embryos. Anterior somites do not express zic3 (black arrows). G, H: By stage 13, anterior somites (H, black arrowheads) and the optic vesicles first express low levels of zic3. The presomitic mesoderm expresses zic3, while there is very little or no expression of zic3 in epithelial somites (G). The brain and anterior neural tube express zic3 (H), while the posterior neural tube does not (G). Abbreviations as in legend for Figure 1; NP: neural plate; black arrows: somites not expressing zic3; black arrowheads flank somite showing weak zic3 expression; red arrowheads: increased zic3 expression in presomitic mesoderm.