A review of the basic embryology concerning the fate of the aortic arches is described herein. Briefly, the first two pair appear and disappear early and are of no consequence. The third pair remain as the common carotid arteries. The fourth pair persist ~s different structures. The left arch becomes the arch of the aorta, and the right arch becomes the innominate artery. The fifth pair appear briefly then degenerate. The sixth develop into the pulmonary artery on the right side and the left becomes the ductus arteriosus until birth. When breathing begins muscular contraction closes the ductus arteriosus. The resulting structure is the ligamentum arteriosum.