The cellular and environmental regulation of hematopoiesis has been generally conserved throughout vertebrate evolution, although subtle species differences exist. The factors that regulate hematopoietic stem cell homeostasis may closely resemble the inducers of embryonic patterning, rather than the factors that stimulate hematopoietic cell proliferation and differentiation. Comparative study of embryonic hematopoiesis in lower vertebrates can generate testable hypotheses that similar mechanisms occur during hematopoiesis in higher species.