Here we present ideas connecting the behaviour of the cranial neural crest during development with the venerable, perhaps incorrect, view that gill-supporting cartilages of an ancient agnathan evolved into the skeleton of an early gnathostome's jaw. We discuss the pattern of migration of the cranial neural crest ectomesenchyme in zebrafish, along with the subsequent arrangement of postmigratory crest and head mesoderm in the nascent pharyngeal segments (branchiomeres), in diverse gnathostomes and in lampreys. These characteristics provide for a plausible von Baerian explanation for the problematic inside-outside change in topology of the gills and their supports between these 2 major groups of vertebrates. We consider it likely that the jaw supports did indeed arise from branchiomeric cartilages.