Fig. 7. A second category of Eomes^N/+; Nodal^LacZ/+ double heterozygous mutants fails to specify the anterior primitive streak and exhibit partial axis duplications. A proportion of compound heterozygous embryos correctly specify the AP axis, but show reduced expression of DE markers (A,B) Hex and (C-D') Cer1 at E6.5 and E7.5, respectively. Anterior axial midline tissues are lost, as seen by (E,F) truncated brachyury expression anterior to the PS (asterisk) and (G-H') absence of Shh (arrows indicate remaining expression in the node). (H,H') Head folds are present at E8.0 in mutant embryos (asterisks), but (I,J) anterior neural identity and Otx2 expression is lost by E8.5. (K-N) At E9.5 double heterozygous mutants exhibit severe anterior truncations, and various degrees of heart defects, ranging from (L) looping defects (arrowheads) to (N) severe malformations with cardia bifida, while posterior development remains grossly unaffected. (O,P) A proportion of Eomes^N/+; Nodal^LacZ/+ compound heterozygous mutants show duplications of the node and notochord, as revealed by Shh expression marking both node and notochord, or (Q-R') staining for the Nodal^LacZ expression in the node. Embryos with node duplications lack Nodal expression in the left lateral plate mesoderm (arrows).