Gesell observed that maturational development always unfolds in fixed sequences: an embryo's heart is always the first organ to develop, then the central nervous system (the brain and spinal cord), followed by the peripheral organs. After birth, babies first gain control over their lips and tongues, then their eye movements, followed by control over their neck, shoulders, arms, hands, buttocks, fingers, legs, and feet. There is a genetic cephalocaudal (head-to-foot) trend in both prenatal and postnatal development.
