In situ hybridization with βH1-globin antisense (A through C) and sense probes (D through F ) of late-primitive streak (A and C), neural plate (B and D) and early somite stage (C and F ) conceptuses. (A) Late-primitive streak stage conceptus with no signal above background. (B) Neural plate stage conceptuses (E7.5), with βH1-globin transcripts localized to the mesodermal cell mass (closed arrow). (C) Early somite stage conceptuses with high levels of βH1-globin mRNA present in yolk sac blood islands (closed arrows), and blood cells within the allantois (arrowhead) and the dorsal aorta of the embryo proper (white arrow). (D through F ) Sections of equivalent staged embryos probed with sense βH1-globin gave no signal above background. ac, amniotic cavity; ex, exocoelomic cavity; m, mesoderm; pc, proamniotic cavity. Each scale bar represents 100 μm.