Transgene expression at E7.5. In situ hybridization of a transversal section with CAT probe, shown as bright-field (A) or dark-field image (B). The transgenic embryo is located in the wild-type maternal decidua. CAT transcripts were detected in the mesodermal aggregates of the yolk sac from which blood islands are derived (arrow), and in the inner part of allantois (arrowhead). Scale bar represents 100 μm; al, allantois; am, amnion; d, decidua; n, neuroectoderm; ys, yolk sac.