Human embryos at (i) 19 days, (ii) 22 days, and (iii) 25 days, in (a) median section, and (b) transverse section at plane indicated by dashed lines in (a). In (a), note reversal of the heart; in (a) and (b), formation of the digestive tube (gut) as it pinches off from the yolk sac. In M the neural tube closes and separates from the surface ectoderm. Neural tube becomes brain and spinal cord; neural crest turns into peripheral nervous system.