Three distinct cell populations contribute to blood formation during development in Xenopus laevis. Two of these populations, the so-called anterior and posterior ventral blood islands (VBI) contribute to primitive blood whilst the third population contributes to definitive blood and the major vessels. Fate mapping data have shown that the two primitive blood populations are derived from opposite sides of the blastula, and require the movements of gastrulation to converge at their final position in the embryo.