Somatotype is a theory proposed in the 1940s by the American psychologist William Herbert Sheldon to categorize the human physique according to the relative contribution of three fundamental elements which he termed somatotypes, classified by him as ectomorphic, mesomorphic, and endomorphic. He created these terms borrowing from the three germ layers of embryonic development: The endoderm,, the mesoderm, and the ectoderm. Later variations of these categories, developed by his original research assistant Barbara Heath, and later by Lindsay Carter and Rob Rempel, are used by academics today.