Rat embryos (8-14 days) were fixed, included and cut. Others in all cases from the same litter, were crushed on smears, in toto or after dissection. A comparative study of the two types of samples is necessary to obtain good results. Primitive erythropoietic stem cells seem to arise by the extra-endothelial way in the yolk sac mesodermic layer. They disappear when blood islands are connected to the vascular net. Primitive red cells, Perls-positive, are always intravascular in hepatic buds, normoblasts always extravascular. When grown on a medium containing hematopoietic factors, primitive (megaloblastic) cells remain megaloblastic and mature, as do the (normoblastic) definitive red cells. On a poor culture medium (Parker 199) normoblasts do not become megaloblastic. The two series, yolk sac primitive and hepatic definitive red cells, seem to proceed from individual cellular clones.