Fig. 1. Transplantation design for this study. Medial or lateral halves of newly-formed somites or pieces of segmental plate mesoderm from quail embryos were grafted into pockets cut into chick head mesoderm. Also shown are the sites of origin within head paraxial mesoderm of extraocular and jaw muscles discussed in this paper. These foci have been identified by orthotopic transplantations (Noden, 1986; Couly et al., 1992) and retroviral injections (Noden, 2002), and were confirmed in this study. The lateral rectus and pyramidalis primordia arise medial and ventral to the first branchial arch progenitors.