COX-1-inhibited embryos are unable to form distinct dorsal aorta and posterior cardinal vein. Cross-section of anterior (A, C) and posterior trunk (E, G) in untreated embryos displays distinct dorsal aorta and posterior cardinal vein. Indomethacin-treated embryos do not form two vascular tubes but display one distended vessel both in anterior (B, D) and posterior sections (F, H). ant, anterior; post, posterior. All embryos were sectioned after staining with flk1.