[Further data on the roles of skin and internal tissues in limb regeneration of the newtPleurodeles waltlii Michah. (Amphibia Urodela)].

In order to specify the role of skin and internal tissues in limb regeneration, we have replaced stump skin by skin from one side or from two opposite sides of the limb or from the flank. 1. After transplantation of limb skin, from one side regenerates are partially abesnt, normal, or multiple. Experimental variations were 90‡ rotation of the proximo-distal axis of the skin, proximo-distal heterogeneity in the grafted skin and the introduction of a unique quality (e.g. dorsal quality) in the grafted skin at the level of amputation. After a critical analysis only the third variations helps towards an explanation of the results. 2. After transplantation of limb skin from two opposite sides of the limb regenerates are normal or multiple. 3. After transplantation of back skin regeneration are normal or absent. To explain the results obtained after limb skin transplantation we make the following two suggestions: a) The development of principal or supernumerary morphogenetic centers requires contact between the tissues of the two opposite sides. The tissues may be of the same character (skin or internal tissue) or may be different (skin and internal tissue). b) Antero-posterior and dorso-ventral gradients are present in both skin and internal tissues. Either one or the other type of tissue can direct the pattern of morphogenesis and give rise to asymmetry of the regenerate. When only one of them has a predominant morphogenetic influence, regenerates may be either normal or partially absent. When skin and internal tissues have equivalent morphogenetic influences, a contact between skin from one side and internal tissues from the opposite side would lead to the development of supernumerary formations.