A group of mesenchyme cells at the posterior margin of the developing wing-the polarizing region-can have a dramatic effect on the pattern of structures which develop across the antero-posterior axis of the limb. If one grafts a polarizing region to the anterior margin of a wing bud so that this bud now has one polarizing region at the anterior margin and one at the posterior margin, the wing that develops has duplicated structures across the antero-posterior axis in mirror-image symmetry. That is, the normal pattern of digits 2 3 4 (reading from anterior to posterior) becomes 4 3 2 2 3 4. The ability of the polarizing region to specify additional digits from adjacent tissue can be progressively attenuated by gamma-ray radiation. If this attenuation is caused by progressively fewer cells remaining viable to signal, there should be a quantitative relationship between the number of polarizing cells used and the digit specified next to the graft. Here, two tests are reported which confirm this idea. The results suggest that the apical ridge cooperates with a small number of polarizing region cells in a monolayer to specify structures across the anterio-posterior axis of the wing.