Stages 9 and 10: Anterior–posterior axis reads left to right in both images. Scale bar = 1 mm. Stage 9 (42 h). Seven somites. Stage 10 (2 days). 10 somites. Abbreviations: aip, anterior intestinal portal; amm, anterior margin of invaginated lateral mesoderm; anp, anterior neural pore; ao, area opaca; ap, area pellucida; bi, blood islands; fb, forebrain; fgl, foregut lumen; hb, hindbrain; hm, head mesoderm; hn, Hensen’s node; lbf, lateral body fold mesoderm; mb, midbrain; mfh, margin of free head; mm, mesomere; nf, neural fold; nh, rhombomeres of hindbrain; no, notochord; nt, neural tube; onp, open neural plate; op, optic vesicle; ps, primitive streak; si, sinus terminalis; sm, somite mesoderm; usm, unsegmented somite mesoderm; v, ventricle of heart; vas, area vasculosa; vit, area vitellina; vv, vitelline vein; zj, zone of junction.