In situ hybridization with a tal-1 antisense probe. (A) Mid-primitive streak stage conceptus with tal-1 mRNA expression localized to 2 extraembryonic mesoderm cells. Serial sections probed with antisense tal-1 gave similar results. (B) Late-primitive streak stage conceptus with tal-1 signal restricted to VYS mesoderm (closed arrow) and posterior embryonic mesoderm (open arrow). (C) Neural plate stage conceptus with tal-1 mRNA present in both VYS mesoderm (closed arrow) and posterior embryonic mesoderm (open arrow). (D) Early somite stage conceptus with low levels of tal-1 mRNA expression in yolk sac blood islands (closed arrow) and in regions of embryonic mesenchyme (open arrows). ac, amniotic cavity; ex, exocoelomic cavity; pc, proamniotic cavity; ep, embryo proper. Each scale bar represents 100 μm.