In the postimplantation mammalian embryo,primitive erythroid (EryPs) or red blood cells are the first hematopoietic cell type to be specified.They emerge in great numbers within the "blood islands" of the yolk sac (YS) and constitute the predominant circulating blood cell until a second wave of definitive,enucleated erythrocytes (EryDs)are produced by the fetal liver.Failure in primitive erythropoiesis is uniformly associated with embryonic lethality,indicating that EryPs are crucial for the transition from rapidly growing embryo to fetus.