Fig. 1. Time course of Bmp and Msx RNA expression in ventral ectoderm of the chick early limb bud. RNA in situ hybridization of alternate sections from the hindlimb region of (A-F) an early stage 17 (29 somite) embryo and (G-L) a late stage 17 (32 somite) embryo; (M-Q) whole-mount of late stage 18 embryos which were subsequently cryosectioned (forelimbs). Hybridization with probes as indicated and immunolocalization of EN1 protein (I). Brackets outline the domains of expression in the ectoderm (except in F, where the bracket indicates mesenchymal domain), arrowhead in J indicates position of the AER. (A-F) Dorsal (medial) towards the left and ventral (lateral) to the right. (G-L and M-Q) Dorsal at top, ventral at bottom. In N-Q, expression on the dorsal side is in the mesenchyme, not the ectoderm.