Figure 5. Bichir embryos diverge from the common anteroposterior differentiation scheme by accelerated development of the entire hyoid segment.

(A, B) A cartoon of cranial neural crest migration (green), the first mesoderm (red), and pharyngeal pouches (yellow) in a typical vertebrate (A) and a bichir (B). Top are left lateral views, below are left horizontal sections. (A) In vertebrates, the sequential anteroposterior formation of cranial segments is well conserved, including pharyngeal pouches and cranial neural crest streams. (B) In bichirs, the entire hyoid segment is accelerated with earlier formation of the endodermal, mesodermal, and neural crest tissues, what constitutes a developmental basis for the appearance their external gills. Surface ectoderm in horizontal sections is shown in blue and primitive gut in ochre; B, branchial NC stream; H, hyoid NC stream; Ma, mandibular NC stream; pp I.-pp VI., pharyngeal pouches.