The hematopoietic system of vertebrates is derived from the mesodermal germ layer in early embryogenesis. Various animal models have been used for the study of hematopoiesis, from early stages in the visceral yolk sac or its analog, to the later stages where hematopoiesis is observed in intraembryonic areas surrounding the aorta, genital ridge and pro/mesonephros. Using the mouse as a model, we describe what is known about mammalian embryonic hematopoiesis and put it in the context of hematopoietic cell formation in avian, amphibian and fish embryos. Evolutionary comparisons and recent experimental evidence show that there are two embryonic sites of developing hematopoietic activity in the mouse before fetal liver hematopoiesis and suggest that, during ontogeny, two successive waves of hematopoietic activity may contribute to the blood system of the adult.