Scheme for the ontogeny of in vitro, hESC-derived hemangioblasts (A), and its congruence with the developmental timeline of normal human YS development (B). PC, progenitor cell; MESO, mesoderm progenitor; BL-CFC, hemangioblast progenitor; PRIM, primitive hematopoiesis; DEF, definitive (YS-like) hematopoiesis; PS, primitive streak; 2° YS, secondary yolk sac.