[Examination of Weiss's modulation hypothesis in regenerating hind limb nerves ofXenopus laevis tadpoles].

1. In tadpoles ofXenopus laevis (stage 56), either the sciatic or the crural nerve was transsected in the right hind limb ("Ortsbein"). The central nerve stump was deviated and connected with the peripheral sciatic nerve stump of theautoplastically transplanted left hind limb. 2. Up to final observations between stage 62 and stage 66 the grafts developed "herkunftsgemäß" into normal, differentiated hind limbs. In all cases the deviated nerves grew into the grafts by regeneration and established the characteristic branching pattern of the sciatic nerve. 3. In grafted hind limbsone type of positional defect arises alongside normally placed hind limbs: permanent extension in the hip and knee joints. This positional defect may have been caused by the blocking syndrome, due to the operation. There is no explanation for functional defects in the hip and knee joints of normally placed hind limbs. The result of innervation of the nerves grown into the graft is judged on the basis of the motor function originating in the crurotarsal joint of the grafts, because of positional and functional defects in proximal joints. 4. Motor function in the crurotarsal joint is more complete after innervation by the sciatic nerve than after innervation by the crural nerve. The sciatic nerve can moreover produce coordinated swimming movements in the crurotarsal joints of all grafts. After innervation by the crural nerve onlyun-coordinated movements are to be seen in the crurotarsal joints. These results demonstrate that the corresponding motor neurons, originally innervating hip flexors, are not capable of myotypic response. The modulation hypothesis of Weiss can therefore be rejected in this case.