English: Allocation of the germ layer derivatives to the embryonic head structures. (a) Regionalization of germ layer progenitors in the epiblast elicited by the graded signalling activity across the prospective anterior–posterior plane of the embryo. (b–d) Allocation of epiblast-derived cells during gastrulation to (b) the ectoderm tissues that contribute to the brain, neural crest and the surface ectoderm, (c) the mesoderm tissues in the cranial mesenchyme and the heart, and (d) endoderm tissues of the embryonic foregut. The fate maps of the progenitor tissues of the embryonic head reveals that the domains and boundaries of the progenitors in the three germ layers are generally aligned with each other, although a clear demarcation of head versus non-head progenitors is not yet evident at the late gastrulation stage. ade, anterior definitive (gut) endoderm; ame, anterior mesendoderm; amn, amnion ectoderm; ave, anterior visceral endoderm; crm, cranial mesoderm; en, endoderm; fb, forebrain; fg, foregut; hb, hindbrain; ht, heart; md, midbrain; mes, mesoderm; ncc, neural crest cells; n-ect, neuroectoderm; se, surface ectoderm.