Ontogenetic Correspondences in the Supine and Prone Postures of the Human Infant *

In the early human fetus the arms are much longer than the legs. At birth, arms and legs are almost equal in length. In the young adolescent the legs are longer than the arms. This progressive reversal in proportions reflects evolutionary changes wrought over many millions of years. The gluteal musdes of the newborn infant are notably under-developed, a condition comparable to that of the chimpanzee. The average (American) infant stands without support at the age of 56 weeks. During the first year of life the legs, feet, arms, and hands undergo not only somatic transformations but even more pronounced functional changes, manifested in action patterns and postural attitudes. A comparison of the development of supine and prone postures should throw some light on the mechanisms of behavior ontogenesis. What determines the forms of postural behavior? Topographic relationships of joints and musculature delimit the scope and possible planes of movement. But the actual movements are generated by the central nervous system. To ascertain the developmental morphology of postural behavior we must study the form characteristics of the movements themselves, as they occur at advancing stages of maturity. Previous studies have defined stage by stage the developmental progressions of supine and prone behavior in the normal human infant (Gesell and Thompson,5 Ames,' Gesell and Ames4). From thirteen to forty-two infants were individually observed at each lunar month interval throughout the first year of life. Their postural and locomotor responses in naturalistic and induced situations were recorded. Abundant cinema records were also made on 16 mm. film. By the method of cinemanalysis (Fig. I), these films were sub-