Cutaneous nerves of the embryonic chick wing do not develop in regions denuded of ectoderm. Development

Peripheral nerves travel to their targets along precise routes, and it is likely that different cues provide guidance at different stages of the journey. In a develop-ing chick limb, the cutaneous nerve fibres follow at first deep mixed nerve trunks, in company with motor axons; they branch from these trunks at predictable points and approach the skin; they then ramify profusely to form a plexus at a precisely defined depth beneath the ecto-derm, at exactly the same level as the blood vascular plexus. To analyse the role of signals from the target patch of skin in regulating cutaneous nerve develop-ment, we have ablated patches of dorsal wing ectoderm using short-wave ultraviolet irradiation at E4 (embry-onic day 4), approximately one day before nerves grow into the limb bud. The irradiated patches remai