Transgene expression in sections of stained embryos. LacZ expression is specifically localized to the round hematopoietic cells in the yolk sacs of d7.5 to d8.0 (A-B), d8.0 to d8.5 (C), and d10.5 (D-F) embryos and is not observed in either the endodermal or endothelial layers. Arrowheads direct attention to the lacZ-positive hematopoietic cells between the endothelial and endodermal layers of the yolk sac. emb indicates embryo; ys, yolk sac; en, endothelial layer; and ve, visceral endoderm. Original magnification, × 100. Two samples are shown in panels A-B; 3 samples are shown in panels D-F.