With the waning of the tonic neck reflex beginning with the 8th to 12th week, and disappearing, in most instances, by the 16th week, the infant begins to become bilateral and makes symmetrical movements and engages his hands in the midline usually over the chest while in a supine position. The developmental significance of such behavior is considered--for example, its participation in the emerging sense of self and its role in the consolidation of emerging ego skills. Consideration is given to the possible implications of faulty midline behavior for development, and to whether failure to engage in an optimal amount of midline behavior, in interaction with other factors, can be used to alert observers to possible future developmental disturbances.