Fig. 6. Roles of bon and sur in the development of endoderm and prechordal plate. (A-D) Dorsal view of 90% epiboly embryos. (E-L) Dorsal view of 80-90% epiboly embryos. (M-P) Dorsal view of 90% epiboly embryos. (Q-T) Lateral view of three-somite-stage embryos. (A-D) Expression of goosecoid in prechordal plate in wild-type (A), bonMO (B), MZsur (C) but not in MZsur;bonMO (D). Notice the recovery of gsc expression in MZsur at 90% (C) compared with 50% epiboly (Fig. 7C). (E-H) Expression of axial in dorsal midline and endoderm progenitors. Endodermal cells are reduced dramatically in bon (F) and MZsur;bon mutant (H) embryos but are less reduced in the MZsur mutant embryo (G). In the MZsur;bon mutant embryo (H), anterior expression corresponding to prechordal plate is missing and the remaining midline expression is compressed along the anterior-posterior axis and broadened laterally. (I-L) Expression of the endodermal marker sox17 is partially reduced in MZsur (K), strongly reduced in bon (J) and almost absent in the MZsur;bon mutant embryo (L). Expression of the endodermal marker casanova (M-P) is strongly reduced in bonMO (N), and almost absent in MZsur;bonMO (P). Notice that the expression of sox17 and casanova in dorsal forerunner cells (vegetal-dorsal expression domain) is not affected. Notochord expression of ntl is reduced and discontinuous in MZsur (S) and reduced and compressed in MZsur;bon (T).