Figure 1:

Evolution of gills slits and neural crest in hemichordates and chordates. Gill slits first appear in hemichordates, associated with the expression of Pax1/9 homologs. Snail, FoxD, and SoxE factors are expressed at the neural plate border in pre-vertebrate chordates, at locations where neural crest later evolves in vertebrates. In vertebrates, these factors function in neural crest cell delamination and migration. Low retinoic acid (RA) in pharyngeal endoderm specifies the anterior pharynx in pre-vertebrate chordates. Twist is associated with neural crest cell migration in gnathostomes, but does not function in this process in agnathans. Hemichordate redrawn from an image by C.B. Cameron, used with permission.