A model for programming the VBI. aVBI precursors in the dorsal lip at stage 10 rapidly escape FGF signaling by crawling over the blastocoel roof toward the animal pole where they encounter BMP signaling in the ventral animal pole ectoderm and commit to a hemangioblast fate (stage 12). They continue to migrate anteriorly and ventrally, and by neurula stages (14-16) they are found in the anterior ventral mesoderm just below the cement gland. pVBI precursors migrate much more slowly and experience prolonged exposure to FGF and BMP signaling through to tailbud stages. By stage 14, the anterior and posterior VBI compartments have met in the ventral midline; and although the aVBI experiences relatively low FGF and BMP signaling, the pVBI region is high in both signals. A block to FGF signaling allows the premature expression of erythroid genes in the posterior, but the high BMP levels do not permit premature expression of endothelial and myeloid genes posteriorly (for more details of the model, see “Discussion”). A indicates anterior; and P, posterior. Distribution of FGF/MAPK and BMP/pSmad1 as in Schohl and Fagotto.³¹