This three-layered embryo—ectoderm, mesoderm, endoderm—is the basis of every organ in the human body. The ectoderm will give rise to everything that faces the outer surface of the body: skin, hair, nails, teeth, even the lens of the eye. The endoderm produces everything that faces the inner surface of the body, such as the intestines and the lungs. The mesoderm handles everything in the middle: muscle, bone, blood, heart.
