The development of blood vessels starts outside the embryo on day 15 or 16 in the mesoderm. Two days later, blood vessels start to form inside the embryo. The early vascular precursor cells called hemangioblasts give rise to angioblasts, which subsequently aggregate to create structures with cords and masses known as blood islands. These islands develop cavities that become the lumens of blood vessels. Angioblasts surrounding these cavities differentiate into the endothelial lining of blood vessels, while others form the layers of larger blood vessels. The blood islands continue to expand and merge, creating a complex network of blood vessels throughout the developing embryo. The concentration of vascular endothelial growth factor in the surrounding area decides if a blood vessel will develop into an artery or vein. By the end of the fourth week, the fetal heart starts pumping blood via this embryonic vascular network.