The three somatotypes ? endomorphic, mesomorphic, and ectomorphic ? are basic classifications of animal body types according to the prominence of different basic tissues types, roughly: digestive, muscular, and nervous tissues. They form the core of a theory, developed in the 1940s by American psychologist William Sheldon, associating body types with human temperament types. This linkage is fairly simplistic and is seen as outdated in physiological science, but the account of somatotypes is still probably a valid, if limited way to sort basic body types.He concluded that the physique of men can be divided into the contribution of three fundamental elements: the somatotypes. He named his somatotypes after the three germ layers of embryonic development: the endoderm, that develops into the digestive tract, the mesoderm, that is to become muscle, heart and blood vessels, and the ectoderm that is to form the nervous system