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Blood vessel formation within the embryo begins from about 22 days gestation. Endoderm deep to the visceral mesoderm induces the latter to form angioblasts. Angioblasts cluster throughout the yolk sac to form angiocysts, regions of differentiating cells:
At about day 18 of development, lateral folding divides the visceral mesoderm and the angiocysts in the same regions into two groups of cells:
The development of the branchial arch arteries provides continuity between the primordial heart and the dorsal aortae. Budding and proliferation of endothelial cells also ensures that the dorsal aortae make contact with the vitelline and umbilical arteries and veins to complete the circulation of the early embryo.