You have a larger proportion of bone, muscle, and connective tissue than thin or heavy persons. Near the end of your first trimester of development cells begin to differentiate into three layers. The middle layer (mesoderm) develops into the bones, bone marrow, cartilage, and muscles, as well as the heart, veins, and arteries. Constitutional psychologists categorize individuals with a greater proportion of muscle and circulatory system like you, as mesomorphs, after the middle layer of the body mesoderm).