Complex sentence: Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm.Initially, the endoderm consists of flattened cells, which subsequently become columnar.
Split into simpler sentences: Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm. The endoderm consists at first of flattened cells, which subsequently become columnar.