1. 1. Deficiencies are produced in the spinal pteryla by X-irradiation of a portion of the trunk in 2 1 2 - day chick embryos. The abnormalities are characterized by the absence of several rows of feathers and by the formation of an apterium. 2. 2. The radiodestruction of a portion of the spinal cord has no effect on the spinal pteryla. 3. 3. On the contrary, radiolesions produced in the somites, and consequently in the dermatomes, result in the nondifferentiation of the feathers in part of the spinal pteryla. 4. 4. X-rays slightly delay the epidermal differentiation, but do not interfere with its final histological structure. They inhibit the transformation of the loose subectodermal mesenchyme into dense dermis. The resulting loose dermis is unable to form feathers. 5. 5. The mechanism by which X-rays stop feather morphogenesis is discussed. The epidermal radiolesions are certainly not responsible for the nonindividualization of a dense dermis. X-rays may act either on hypothetical factors of the densification of the subepidermal mesenchymal cells or by genetic alterations of these cells.