The first hemopoietic cells were found in the yolk sac of mammalian embryos. Based on this observation it was postulated that hemopoietic stem cells are generated in this location. In the last few years, however, increasing evidence indicates that there is an independent site of hemopoietic cell generation, in the embryo proper, designated the paraaortic splanchnopleura/aorta, gonad, mesonephros region. Precursors of hemopoietic cells are found in this region before circulation is established between the yolk sac and the embryo proper. In contrast to the hemopoietic cells found in the yolk sac, the ones found in the embryo proper are multipotent lympho-myeloid precursors that rapidly acquire the capacity to reconstitute the hemopoietic system of adult irradiated recipients. We propose that these are the founder cells of adult definitive hematopoiesis.