Fig. 4. Pattern of gene expression following head-into-head or trunk-into-head mesoderm transplantation. Column headings indicate the donor tissue; row headings are the gene products assayed. Sites of ectopic expression (black arrows) are evident in all heterotopic categories. myf5/myod and paraxis are expressed in aggregates, and also in cells dispersed around the site of implantation. lbx1 is evident in ectopic aggregates but not in dispersed cells. Neither paraxis nor lbx1 mRNA was detected in the first branchial arch (BA1). DR, dorsal rectus; VO, ventral oblique.