AbstractLeft caudal neural crest was ablated by diathermy in 2‐day‐old chick embryos, to injure the region of sensory sciatic innervation before the appearance of the hind‐limb bud. Embryos were fixed immediately, 24 hr and 14 days later, for examination of the effects of the ablation upon neural crest, neural tube, somitic mesoderm, and subsequent limb development. All embryos examined immediately and at 24 hr after ablation showed damage to the left neural crest, but the injury extended deeper into neural fold or tube, across the midline, and into somitic mesoderm. Of 17 chicks which survived to 16 days, 7 had gross deformity of the left leg and 10 had morphologically normal legs. Deformed chicks had a significant reduction in the mass of dorsal root ganglia on the operated side but no reduction in the area of the spinal cord. Operated chicks without limb deformities showed reduction of spinal cord but not of dorsal root ganglia. This suggests that limb morphogenesis may be quantitatively related to neural crest but not to neural tube. This possibility is considered in the light of previous publications. However, the experimental limitation of the diathermy method leaves the final conclusion open to interesting speculation and to future examination by improved experimental d