(A) A wing bud of a chick embryo at 4 days of incubation. The scanning electron micrograph shows a dorsal view, with somites (the segments of the trunk of the embryo) visible to the left. At the distal margin of the limb bud a thickened ridge can just be seen—the apical ectodermal ridge. (B) Expression patterns of key signaling proteins and gene regulatory factors in the chick limb bud. The patterns are depicted schematically in two imaginary planes of section through the limb bud, one (horizontal) to show the dorsoventral system and the other (vertical) to show the anteroposterior and proximodistal systems. Sonic hedgehog, BMP2, and Lmx1 are expressed in the mesodermal core of the limb bud; the other molecules in the diagram are expressed in its epithelial covering. Almost all the molecules shown have homologs that are involved in patterning the Drosophila wing disc. (A, courtesy of Paul Martin.)