Fig. 4. The posterior limit of expression of AmphiNodal (A-L) and AmphiOtx (M-R) in the endoderm is not affected by altered levels of RA signaling until the late neurula/early larval stage. Anterior to left. Whole mounts viewed from the left side and frontal sections in B,F,J viewed from dorsal side. The `x' in A,E,I shows the level of the section in B,F,J, respectively. Arrowheads indicate the posterior limit of expression in the endoderm. Scale bars=50 µm. (A) At midneurula (16 hours), AmphiNodal is expressed throughout the length of the endoderm. The posterior limit of expression in the endoderm at this stage is not affected by either RA (E) or by BMS009 (I). The frontal sections show that expression of AmphiNodal is restricted to the left side of the pharynx in controls (B), RA-treated (F), and BMS009-treated (J) embryos. By the late (24-hour) neurula, expression of AmphiNodal is restricted to the tail bud and pharyngeal endoderm in DMSO-treated controls (C), and is shifted slightly anteriorly by RA (G) and posteriorly by BMS009 (K). In early (30-hour) larvae, pharyngeal expression of AmphiNodal is reduced in the pharynx of controls (D), eliminated in RA-treated larvae (H) and expanded posteriorly in animals treated with BMS009 (L). (M) At the late (20-hour) neurula, AmphiOtx is expressed throughout the length of the pharynx in DMSO-treated controls. At this stage, the posterior limit of AmphiOtx expression is not affected by either RA (O) or BMS009 (Q). In 30-hour larvae, endodermal expression of AmphiOtx is restricted to the pharyngeal endoderm in DMSO-treated controls (N) and its posterior limit is shifted anteriorly by RA (P) and posteriorly by BMS009 (R).