Fig. 2. Distribution of X-gal-positive cells in an embryo derived from a mating with a F₁ male lacking the floxed CAT gene. (A) Whole-mount embryo in the deciduum. All embryonic cells expressed ß-gal except for the layers of primitive endoderm origin. (B,C) Sections stained with Hematoxylin and Eosin. The staining pattern was similar to that of embryos after injection at the one-cell stage. Almost all cells were stained except for the parietal endoderm (arrowhead) and visceral yolk sac endoderm (arrow). Scale bars, 300 µm (A), 150 µm (B), 30 µm (C).