INTRODUCTION One of the major problems of neuro-embryology concerns the specific affinities of different types of nerve fibers to their respective end-organs. The patterns of selective connections which are the foundations of normal functioning are laid down in early embryonic stages. The effectiveness of the mechanisms which bring about the anchorage of motor fibers in effector organs and of sensory fibers in receptor organs is particularly clearly demonstrated in experiments in which the dorsal or ventral half, respectively, of the spinal cord of Amphibian embryos was removed. The fibers emerging from the isolated ventral half formed typical patterns of motor nerves alone, whereas nerves composed exclusively of sensory fibers formed peripheral patterns of sensory nerves (Harrison,'06; Hamburger,'29; Taylor,'44; Weiss,'47). The mechanisms by which these specific connections are established have not been elucidated as yet. Their analysis is facilitated by the fact that developing nerve centers are extremely sensitive indicators for the conditions which nerve fibers emerging from them encounter at the periphery. Any change in the milieu in which the outgrowing fibers spread have immediate repercussions on the corresponding nerve