Germ Layers

During early embryonic development, the blastula reorganizes into three primary germ layers that give rise to all tissues and organs. The outer ectoderm forms skin, hair, nails, and the nervous system. The middle mesoderm develops into muscles, bones, connective tissue, kidneys, gonads, and the cardiovascular system. The inner endoderm lines the digestive and respiratory tracts and produces glands like the liver, pancreas, and thyroid. These layers fold and migrate through gastrulation, setting the body plan that persists through adulthood.