We studied, using avian embryos, mechanisms underlying the three-dimensional assembly of the dorsal aorta, the first-forming embryonic vessel in amniotes. This vessel originates from two distinct cell populations, the splanchnic and somitic mesoderms. We have unveiled a role for Notch signaling in the somitic contribution. Upon activation of Notch signaling, a subpopulation of cells in the posterior half of individual somites migrates ventrally toward the primary dorsal aorta of splanchnic origin. After reaching the primary aorta, these somitic cells differentiate into the definitive aortic endothelial cells. This Notch-induced ventral migration is mediated by EphrinB2 and by an attractant action of the primary aorta. Furthermore, long-term chasing of cells by transposon-mediated gene transfer reveals that the segmentally provided endothelial cells of somitic origin in the dorsal aorta ultimately populate the entire region of the vessel. We demonstrate the molecular and cellular mechanisms underlying the formation of embryonic blood vessels from mesenchymal cells.