Expression pattern of transcription factors Hey1 and Irf1 in the developing endoderm. a: Transverse section through anterior endoderm at NF32 showing expression of hey1 restricted to the outer periphery of the endoderm. b: Expression of hey1 is more diffuse within the endoderm of the NF36 embryo, transverse section. c: NF44 whole gut showing expression of hey1 throughout the stomach, liver and pancreas. d: Transverse section through anterior endoderm at NF32 showing expression of irf1 throughout the entire endoderm. e: Transverse section at NF36 shows restricted expression of irf1 along the dorsolateral endoderm. f: NF44 whole gut showing expression of irf1 throughout the stomach, liver, pancreas and in the intestine. dle, dorsolateral endoderm; ve, ventral endoderm; e, endoderm; l, liver; st, stomach; p, pancreas.