If the somitic mesoderm of a 2-day chick embryo is destroyed by X-irradiation, the adjacent limb develops with a normal pattern of connective tissues, but is devoid of muscle. The innervation of muscleless wings produced in this way was examined in silver-stained whole mounts, fixed 3 to 8 days later. The main nerve trunks and their cutaneous branches developed normally; but the nerve branches which in a normal limb would lead to individual muscles were generally absent. In almost all those exceptional cases where muscle nerve branches were present, muscle was found to be present also, despite the X-irradiation. Where there was no muscle, the muscle nerve branches apparently did not even begin to form. As a control for side effects of the X-irradiation, wing buds were grafted from normal to irradiated embryos and vice-versa, and again analysed for their innervation. The results confirmed that the absence of muscle nerve branches was due to the absence of muscle cells in the limb. Thus (1) the routes taken through a limb by the main mixed nerve trunks and by their cutaneous branches are determined by the connective tissues, and not by any mechanisms requiring muscle cells; but (2) muscle cells are necessary to provoke the formation of the side branches leading to the sites of individual muscles.

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