The developmental potencies of the regeneration blastema of the axolotl limb.

1. The developmental potencies of limb regeneration blastemas of the axolotl (Ambystoma mexicanum) were tested by transplanting them to the flank or to the orbit under various experimental conditions. 2. Early upper-arm blastemas transplanted singly to the damaged flank musculature or to the orbit can form forearm skeletal elements in addition to hand cartilages (digital and carpal elements). 3. Fusion of the mesenchyme of several early upper-arm blastemas into one transplant leads to the formation of a single regenerate. In this case an upper-arm element may differentiate in addition to the more distal limb regions. 4. In both transplantation sites single hand plates of paddle-shaped upper-arm blastemas form exclusively hand structures. Combination of mesenchyme of several hand plates, however, can result in the differentiation of forearm elements in addition to hand structures. In one case even an upper-arm element was formed. 5. The single transplantation of carpal blastemas of various stages leads to the development of hand structures only. Combination of the mesenchyme of several carpal blastemas on the flank shows no improvement of regional differentiation. When, however, the same combination experiment is performed in the orbit, the carpal blastemas of the oldest stage used are able fo form forearm elements as well. 6. The first conclusion that can be drawn from these results is that the early upperarm blastema possesses the information for the development of all the structures lost by amputation. Morphogenetic stump influences are not necessary for the formation of these structures. 7. The second conclusion is that the limb pattern in the transplants is established autonomously, because mesenchyme of several blastemas combined in random orientation gives rise to a single, normally oriented regenerate. 8. The third conclusion is that mesenchyme which is already differentiating into hand structures at the time of transplantation (hand plate of paddle-shaped upper-arm blastema, and oldest carpal blastema) upon dedifferentiation can be induced to form more proximal structures as well. Therefore this mesenchyme still possesses the information for the development of structures which it would never have formedin situ. 9. In the transplants that do not regress completely digital elements are always formed. According as regression is less extensive, or the mass of transplanted mesenchyme is enlarged experimentally, carpal elements are realized next, then forearm elements, and finally also the upper-arm element. This suggests that in the normal blastema distal differentiation tendencies appear first - under the influence of the epidermis - while successively more proximal differentiation tendencies arise as the blastema increases in mass. The disto-proximal succession of differentiation tendencies would continue until definitive differentiation begins in the most proximal mesenchyme, whence it proceeds in a proximo-distal direction. 10. Since transplanted mesenchyme can form more structures than are lost by amputation if its mass is increased, it is likely that during normal regeneration the level of amputation determines what will be formed by regulating the mass of the blastema, while inhibitory stump influences may prevent the development of supernumerary structures from the blastema.