Disclosed herein are cell cultures comprising PDX1-positive endoderm cells and methods of producing the same. Also disclosed herein are cell populations comprising substantially purified PDX1-positive endoderm cells as well as methods for enriching, isolating and purifying PDX1-positive endoderm cells from other cell types. Methods of identifying differentiation factors capable of promoting the differentiation of endoderm cells, such as PDX1-positive foregut endoderm cells and PDX1-negative definitive endoderm cells, are also disclosed.