GFP expression of construct 3 in yolk sac and vitelline vessels (E11.5). Brightfield (i) and fluorescence (ii) images are shown. (A-D) Yolk sac. (E) Vitelline vessels. Note that qualitatively similar results are obtained with construct 2 (C), but no expression is seen with construct 1, and no nonspecific fluorescence is detected with nontransgenic embryos. Original magnification, × 40 for all panels.