A set of cellular and epithelial movements that transform the blastula stage of embryonic development into a more complex arrangement of three germ layers. The outer germ layer, which will be exposed to the external environment, is the ectoderm. It gives rise to the epidermis and nervous system of the developing organism. The inner layer of cells, the endoderm, forms the inner lining of the digestive tract and its derivatives. An intermediate layer, the mesoderm, develops into a diverse array of structures, including the skeleton, muscles, kidneys, and circulatory system. The ectoderm and endoderm are usually epithelial, consisting of closely packed sheets of cells connected by tight junctions. The mesoderm is sometimes epithelial, but at other times it is mesenchymal, forming a loose arrangement of cells surrounded by extracellular material. The three germ layers are formed in nearly all animal embryos, while the movements that generate them vary.