Comparison of zic1-4 gene expression in chick embryos at stage HH 18/19. A-D: whole embryos; E-H: close-up side-views of the eye and hindbrain regions; I-L: dorsal views of midbrain, hindbrain, and anterior trunk; M-P: dorsal views of the mid-trunk; Q-T: dorsal views of the tail. A, E, I , M, Q: zic1 is expressed in the dorsal brain and in the trunk neural tube, except in the tail tip (Q). zic1 is strongly expressed in all somites, including caudal somites (Q). B, F, J, N, R: zic2 is expressed in the dorsal brain and trunk neural tube. zic2 is strongly expressed in the neural tube of the tail tip (B), but more weakly in caudal somites (R). zic2 is expressed in the developing eye and periotic mesoderm (B, F) and in the limb buds (B, N, R). C, G, K, O, S: zic3 is expressed in the dorsal brain and spinal cord and in the somites. zic3 is expressed in the developing eye (C, G) and in the mesoderm of the tail (C, S). D, H, L, P, T: zic4 is expressed in the forebrain (D) and midbrain (D, L). No significant expression is detected in the hindbrain or in the trunk. zic1-4 are expressed in the optic stalk (E, F, G, H). Tel: telencephalon; Di: diencephalon; Mes: mesencephalon; Met: metencephalon; Myelen: myelencephalon; ov: optic vesicle; white bracket indicates extent of periotic mesoderm; each pair of arrowheads outlines a somite. The stage of the embryos is HH 18 with the exception of A, E, N, R, S, T: HH 19 embryos.