It has been suggested that the ectoderm on the dorsal and ventral faces of the limb bud plays a part in controlling the pattern of cartilage differentiation. To test this, the dorsal wing bud ectoderm in the chick embryo was destroyed by irradiation with ultraviolet light at stage 17-19, at the very beginning of limb bud development, but the apical ectodermal ridge was spared. The irradiated ectoderm disappeared within 24 hr (by stage 23-24) and did not regenerate thereafter; thus the dorsal surface of the limb bud was kept denuded throughout most of the period of skeletal pattern formation. By 6 or 7 days after the irradiation (stage 35), when the rudiments of all the adult skeletal elements are normally present in recognizable form, the irradiated wings could be placed into two categories, those that were approximately normal in shape and those that had curled dorsally. All of these limbs were reduced in size, to varying degrees, when compared to their controls and lacked dorsal soft tissues. The limbs that were normal in shape, however, even though sometimes denuded over practically the whole extent of their dorsal surface, almost always had a complete and normally proportioned cartilage pattern, suggesting that ectoderm (other than the apical ectodermal ridge) does not exert any direct control over the development of the limb cartilage pattern. However, many of those limbs that had curled as a result of the irradiation did have major pattern deformities, suggesting that the topology of cartilage differentiation does depend on the shape of the limb bud.