Positional signalling and the development of the humerus in the chick limb bud.

The positional signal model for specification of the cartilaginous elements in limb development has been tested by examining the effect on the humerus of grafting a polarizing region to different positions along the anteroposterior axis of the limb bud at stage 16. The humerus between the host and grafted polarizing region was largely normal though there were variations in width, particularly the distal epiphysis. The humerus often showed mirror-image symmetry along the anteroposterior axis. When the grafted polarizing region was in a very anterior position, there were a few cases where a second humerus developed. Anterior to the graft an additional humerus often developed. This was associated with the splitting of the bud into two domains. It is suggested that these results are not consistent with a positional signal model and that an additional mechanism involving an isomorphic prepattern may be involved in the specification of the cartilaginous elements.