To demonstrate a causal relationship between neuronal damage and peripheral mesodermal anomalies, an operation was performed through oblique incisions in the dorsum of 247 chick embryos at stages 12-15 of development. In the region of the 24-30th presumptive somites, a three to four somite length of the right half of the neural tube was cut with fine glass sytlettes and removed with a 40μm glass pipette. Immediately after the operation and at intervals during continued incubation (37±, 75% R.H.), embryos were processed for histology. Fifty one embryos survived until day 14 when they were examined for malformations. Excision frequently resulted in a variety of reduction deformities: some embryos appeared normal; 31 displayed unilateral hemimelia or amelia. Examination of serially sectioned embryos fixed immediately and at varying intervals after the operation, demonstrated that this range of malformations was consistent with variation in the amount of neural tissue excised. Disruption of the mesoderm and regeneration of nervous tissue was minimal. It is concluded that the nervous system per se or some factor produced by the nervous system is essential for the development of the limb.