Accessory limb production by nerve-induced cell proliferation.

The deviation of large limb nerves to a more proximal skin wound yielded a high proportion of accessory limb responses in different age groups of Ambystoma mexicanum (axolotls). In some instances the deviated nerve was positioned on skin previously grafted from an animal of different age and pigmentation from that of the host. Grafts were found not to be a necessary prerequisite for accessory limb induction, but the presence of wound epithelium was required. The rule of distal morphogenesis was expressed in reference to the level at which the nerve was cut, not in reference to the wound site where the accessory actually developed. The upper arm proved to be a more favorable site for accessory limb production than the flank or the leg under the conditions of the present experiments, in which little or no damage was done to the underlying muscles. The orientation of the accessory limb was extremely varied despite the uniformity of the surgical procedure.