Fig. 3. Development of the mouse embryo between E3 and E4. The upper panels are a schematic of the blastocyst development. A layer of cells (in red) covering the ES cells (orange) becomes the epithelium of the primitive endoderm. Later, some of its cells will populate the inside layer of the trophectoderm to form the parietal endoderm whereas that covering the ES cells will form the visceral endoderm. In the lower panels are embryos taken from mice with LacZ inserted into the hensin locus. The embryo on the right was removed at E3.5 and cultured for 24 hours in vitro. Hensin expression begins in the ES cells (E3.5) and eventually is concentrated in the primitive endoderm.