Morphogenesis of the avian beak. (A) At embryonic stage (HH) 9.5 (), neural crest cells (dorsal view) migrate from the caudal forebrain (fb) and rostral midbrain (mb) and produce frontonasal structures (pink). Cells from the caudal midbrain and first two rhombomeres (r) of the hindbrain give rise to maxillary and mandibular derivatives (light blue). The hyoid arch forms from r4 (green) and the third arch forms from r6 (brown). Neural crest cells migrate through and around mesodermal mesenchyme (m; grey). (B) By HH25, the frontonasal (fn), maxillary (mx), mandibular (ma) and hyoid (hy) primordia (sagittal view) are surrounded by surface ectoderm (se; light purple), pharyngeal endoderm (pe; yellow) and forebrain neuroepithelium (fb; dark blue), and contain contributions from the neural crest, nasal placode (np; dark blue) and cranial ganglia (V, VII, IX; dark blue). Mesoderm (m) produces muscles, vascular endothelial cells, and some skeletal tissues. (C) By HH40, neural crest cells produce cartilaginous and bony elements in the facial and jaw skeletons (pink, light blue, green and brown) whereas mesoderm forms much of the caudal cranial vault (grey) and basicranium caudal to the pituitary. Based on data and drawings from a variety of sources (, ; ; ; ; ). (D,E) At HH25, the frontonasal (fn), maxillary (mx) and mandibular (ma) primordia of quail and duck appear equivalent in proportion and shape but not size (frontal view). Scale bars = 1 mm.