Development of motor activity follows a fairly standard sequence. The infant learns to lift its head, to turn over on its back, and to develop the muscular coordination for refined, visually directed hand movements and for sitting, crawling, standing, and walking, generally in that order. Motor development proceeds more rapidly than actual physical growth by the beginning of the second year. Bowel and bladder control is sometimes possible after 18 months. However, many normal, healthy infants show delayed response in one or several developmental activities, or may apparently skip a stage altogether.