(A, B) ASVs and primitive coronary plexuses are present in the embryonic stages in mice (A) and quails (B). The blood vessels were visualized by immunohistochemistry. Pink arrowheads indicate ASVs originated from the cranial part of the outflow tracts (same to the following panels). (C) ASVs in 12.5 dpc mouse embryos branched from the root of the future carotid artery (a). Immunohistochemical staining of cryosections shows two orifices: at the root of the carotid artery (b) and at the boundary of the outflow tract and ventricle (c). In 13.5 dpc embryos, the secondary orifice developed at the aortic root instead of the carotid artery orifice, which was lost (d). (D) In quails at stage 30, the secondary orifice was formed at the aortic root, as in mice. Immunohistochemical and HE staining images of cryosections (a and b). Ao, aorta; LA, left atrium; RA, right atrium; Vent, ventricle. Scale bars: 500 µm (A, B), 100 µm (C, D).