The vascular system develops from fixed primitive angiogenic cells that are mesodermal in origin; this occurs in the middle of week 3 of fetal development. The process initiates with hemangioblasts, which divide into two categories: primitive hematopoietic progenitor cells (HPCs) and endothelial precursor cells (EPCs, otherwise known as angioblasts). Hematopoiesis begins in the yolk sac of the extraembryonic mesoderm, and flattened EPCs initiate vasculogenesis at the embryonic discs. EPCs form the vessels that eventually become arteries and veins through which blood will circulate.