Fig. 1. Head mesoderm develops into muscle according to its local environment. (A) Scheme of operations at HH8-9^-; dorsal view of embryos, anterior to the top. Quail-derived head mesoderm from rhombomere 2 levels was grafted orthotopically as control (B), anteriorly next to the anterior midbrain (C) or posteriorly next to otic levels/rhombomere 5-6 (D). In E,G, head mesoderm from midbrain levels was grafted next to rhombomere 2; in F,H, head mesoderm from otic levels/rhombomere 5-6 was grafted next to rhombomere 2. (B-H) Lateral view of the heads of operated embryos at HH19; anterior to the top, dorsal to the left, ventral (branchial arches) to the right. Quail tissues were detected with the QCPN antibody (brown), the lateral rectus EOM with Lbx1 (B-F; blue; note: additional expression in the neural tube), and muscle anlagen in general with Myf5 (G,H, blue). Note that the molecular markers showed normal expression patterns. Scale bar: 200 µm. do, dorsal oblique EOM; dr, dorsal rectus EOM; hy, hyoid arch; lr, lateral rectus EOM; m, midbrain; ma, mandibular arch; nt, neural tube; ov, otic vesicle; r, rhombomere; som, somite; vr, ventral rectus EOM.