The development of specific neuronal connections between motor neurons and muscles will be studied in the chick embryo. Intramuscular injections of horseradish peroxidase (HRP) will be used to map the motor neuron pools in the spinal cord supplying identified muscles of the leg and wing. Electrophysiological techniques will be used to assay the actual functional connectivity between the motor pools and the muscles. The innervation of identified muscles by specific motor pools will be studied in both normal and experimentally perturbed embryos at several developmental stages. Experimental perturbations are designed to produce abnormal developmental conditions. They will be accomplished in two ways: by grafting a supernumerary leg and by interchanging lumbar and brachial spinal cord segments. A comparison of the innervation of normal limbs information about the several developmental processes which contribute to the formation of the final adult pattern of specific neuronal connections.