Fig. 3. Smad1^-/- allantois morphogenesis is abnormal. Three allantois phenotypes are observed in Smad1-deficient embryos at 9.5 dpc. The broken line in A,B represents the plane of section in (A',B'). (A,A') Some embryos develop a balloon-like structure containing pooled blood. (B,B') In other mutant embryos, the allantoic rudiment is compact and forms a dense mesenchyme with blood apparent throughout the structure. Frontal sections of wild-type (C) and mutant (D) embryos. (C) The wild-type allantois has prominent blood vessels and has fused uniformly across the entire surface of the chorionic plate. (D) In a proportion of Smad1-deficient embryos, the allantois fuses peripherally and non-uniformly. ab, allantois blood vessel; al, allantois; am, amnion; bl, blood; cp, chorionic plate; da, dorsal aorta; hg, hindgut diverticulum; va, vitelline artery.