Fig. 1. Expression of T during limb development in chick. Whole-mount in situ hybridization with chick T probe. (A) T expression in lateral plate of the wing region at stage 15 (arrow), as well as weak expression in adjacent somites, and strong expression in notochord and tail bud. (B) At stage 18, T is expressed in a narrow strip of distal mesoderm underlying both wing and leg AER (arrows), as well as in somites, notochord and tail bud. (C) By stage 22, T expression is beginning to decline in wing, but is still very evident in leg bud (arrows). (D) Transverse section through wing region of a stage 15 embryo (as indicated in A) showing strong T expression in notochord (no), weak to moderate expression in wing lateral plate, somite (so) and nephric/Wolffian ducts (nd). (E,F) Transverse sections through wing (E) and leg (F) buds from a stage 18 embryo (as indicated in B), and (G) transverse section through leg bud of a stage 22 embryo (as indicated in C) showing T expression in the immediate subridge mesoderm beneath the AER (arrow) and continued weak expression in dermomyotome of somites (so) and in nephric ducts (nd). Note T expression is also seen in tail epithelial somites, at stage 22 (G). Signal development time was extended to visualize sites with weak expression (lateral plate, somites, limb bud). d, dorsal; v, ventral.