Expression of the zic1 gene in whole mount chick embryos at HH stages 7-11 determined by in situ hybridization. A: In stage 7 embryos, zic1 is expressed at very low levels in the most anterior portion of the head fold (white arrow). B: zic1 is expressed in the neural folds of the future forebrain (white arrow) and hindbrain (white arrowheads) at stage 8. It is not expressed in the future spinal cord or in somites (black arrows). C: By stage 9, zic1 is expressed in the anterior region of the future forebrain and in a domain of the future hindbrain (white arrowheads). D: Very weak zic1 expression in anterior somites is first observed at stage 10 (black arrowheads). E (dorsal view), F (side view): zic1 expression in anterior somites becomes more robust by stage 11 (black arrowheads), as does zic1 expression in the anterior portion of the trunk neural tube (E). In the brain, zic1 continues to be expressed in the anterior prosencephalon (best seen in F, white arrow) and in the limited hindbrain domain (best seen in E, white arrowheads). zic1 is not expressed in the optic vesicles. G: At stage 11, anterior somites express zic1, while it is not expressed in posterior somites. zic1 expression in the trunk neural tube diminishes towards posterior regions. H: zic1 is not expressed in the neural tube, neural folds, or somites of the posterior trunk in stage 11 (and older) embryos. Overall, expression levels of zic1 are very low in these early stage embryos. FB: future forebrain; MB: future midbrain; HB: future hindbrain; HN: Hensen's Node; PS: primitive streak; NF: neural folds; Pro: prosencephalon; Mes: mesencephalon; Rhom: rhombencephalon; white arrowheads: hindbrain expression domain of zic1; white arrow: forebrain expression domain of zic1; black arrows: somites not expressing zic1; black arrowheads flank somites expressing zic1; the anterior of the embryos in all figures is to the left.