LacZ staining of whole mount E7.5 to 8.5 days postcoitum (dpc) SCL^lacZ/lacZembryos. (A) E7.5 embryo showing speckled staining (arrow) in the yolk sac. (B) Sagittal section of E7.5 embryo indicating that lacZ staining is confined to the extraembryonic mesoderm (m); endoderm (e); amnion (am). (C) Lateral and (D) anterior views of early somite (E8) embryos demonstrating specific lacZ staining of vascular structures inSCL^lacZ/lacZ embryos (left). No staining is seen inSCL^w/w fetuses (right); cvp, cerebral vascular plexus; avp, allantoic vascular plexus; en, endocardium; da, dorsal aorta. (E) Ventral view of early somite (E8.0)SCL^lacZ/lacZ embryo, demonstrating migration and alignment of angioblasts to form the dorsal aorta (da), allantoic vascular plexus (avp), and the vitelline vein (vv), which receives blood from the yolk sac. (F) E8.5 embryo showing the somitic arteries (sa) sprouting from the dorsal aorta.