Fig. 3. Eomes mutant epiblast cells fail to contribute to the endoderm layer. Fate-mapping analysis of the epiblast via Sox2.Cre and the ROSA26^R reporter allele show that the outer endodermal layer (arrows) overlying the embryo of (A-A'') E7.75 control embryos is entirely derived from ROSA26-positive epiblast cells. (B-B'') Eomes^N/CA; Sox2.Cre; ROSA26^R/+ mutant epiblast cells fail to contribute to the endoderm layer (arrows). Occasionally, single lacZ-positive cells are found in the endodermal layer, most probably owing to perdurance of Cre expressed in the early ICM from the Sox2.Cre transgene.