(1) It is shown that the number of cartilage elements in an experimentally produced reduplicated limb depends on the width of competent tissue between pieces of flank tissue. (2) Seventy well formed reduplications were examined on the assumption that the difference in the number of elements between them results from small differences in graft position. (3) All the reduplications are symmetrical along their entire length. (4) All possess the most posterior structures at both edges with other elements arranged in between in the correct serial order. (5) The existence of vestigial and of branched elements in the midline suggest that the cases examined come from a potentially continuous series. (6) All these characteristics can readily be explained by assuming the existence of a continuous U-shaped anteroposterior gradient, and of threshold responses by the cells which together control the pattern of cell differentiation. (7) It is suggested that the gradient "deepens" as more distal levels of the limb are laid down. (8) Most but not all of the cases can be arranged in a single series in which proximal and distal levels expand coordinately. (9) Possible mechanisms for the establishment and maintenance of the gradient are discussed.