Parietal branches emerge from the dorsal aortae at their dorsal and lateral aspects; these are the paired intersegmental arteries. On both sides, the vertebral artery develops from the longitudinal anastomosis between the first (cervical) intersegmental arteries. Once the cervical intersegmental arteries degenerate, the vertebral artery is then fed by the 7th intersegmental artery only. In turn, the latter becomes the stem of the subclavian artery. The more caudal intersegmental arteries give rise to the intercostal and lumbar arteries. The caudal end of the aorta becomes the unpaired middle sacral artery.