Transgene expression by the -950 promoter at initial stages of yolk sac blood island development. LacZ expression is not apparent in d7.0 to d7.5 embryos (A), but becomes detectable as a ring of expression (indicated by the arrows) at the blood islands by d7.5 to d8.0 (B-D). The expression profile becomes more diffuse by d8.0 to d8.5, leading to a punctate pattern throughout the yolk sac (E-F). In panel B, P indicates posterior, and A indicates anterior. Original magnification, × 75. Three embryos are shown in panels B-D; 2 embryos are shown in panels E-F.