There are three layers of an embryo: the endoderm (innermost layer), mesoderm (middle layer), and ectoderm (outermost layer). The endoderm gives rise to many internal organs and the gut. The mesoderm gives rise muscles, bones, and connective tissues. The ectoderm gives rise to the nervous system and epidermis. Most musculoskeletal tumors are formed from the mesoderm, as this layer gives rise to musculoskeletal tissue.