Fig. 2. Morphology of wild-type (+/+) and Tbx4-homozygous mutants (-/-). Black arrowhead indicates the allantois or umbilicus. (A) An 8-somite wild-type embryo. The allantois has fused with the chorion and ectoplacental cone (epc). (B) An 8-somite Tbx4-mutant embryo shows a stunted, unfused allantois. (C) 9.5 dpc embryos partially dissected out of the yolk sac (ys). The allantois of the wild-type embryo has formed a vascular umbilicus connecting it to the placenta (p), whereas the mutant embryo is loose in the yolk sac and the allantois has formed only a small, amorphous stump. (D) 10.5 dpc wild-type and mutant embryos dissected out of their membranes. The mutant is hemorrhagic and has only the stump of an allantois, but it is otherwise normal and shows a distinct hindlimb bud similar to that of the wild type (red arrowheads). (E) Close-up of a wild-type umbilical connection at 10.5 dpc, with large umbilical blood vessels connected to the chorionic plate (ch) of the placenta. (F) Close-up of the unfused allantoic stump of a Tbx4 mutant at the same stage, which is not connected to the placenta and shows no coherent blood vessels. (G) Section through a normal 8.25 dpc allantois. The allantois has a funnel shape, the chorionic end of which contains loose, cavitated mesenchyme and a layer of cells tightly opposed to the chorion; the base has a more compact, uniform mesenchyme. (H) Section through the base of an unfused mutant allantois, showing dense cell packing and distinctive vesicles. (I) Detail of H, showing two of the characteristic mutant double-layered vesicles and cell debris from dying cells. (J) Detail of L, showing the mutant allantois at 9.5 dpc. The irregular mesenchyme contains dense condensations that have no apparent connections between them in serial sections. (K) Section through a normal 9.5 dpc allantois and embryo. Blue arrowhead indicates a blood vessel in the allantois. (L) Section through a mutant 9.5 dpc allantois and embryo. The allantois is unfused and the chorion is out of the section plane at the top. Embryonic tissues appear normal. (M) Transverse section through a wild-type 10.5 dpc embryo, with normal lung buds (lb), hindlimb bud (hl) and umbilicus. (N) Transverse section of a 10.5 dpc mutant in a similar plane as the embryo in M, as indicated by the lung buds. All structures appear normal. (O) A more ventral section of the same embryo showing a normal heart (a, atrium; v, ventricle), a small hindlimb bud (hl) and the remnants of an allantois.