Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer), with the endoderm as the innermost layer. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm.[citation needed]The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of multiple systems.[citation needed]