Thirteen Years of Homologous Function in Normal and Supernumerary Grafted Limbs

Abstract

For nearly twenty years it has been known that when a supernumerary limb in urodele amphibians is grafted adjacent to a normal limb, so that it acquires innervation from a portion of the normal limb plexus, the corresponding muscles of the two limbs always contract synchronously and with the same degree of intensity. This phenomenon has been called homologous function” by Weiss who, in order to explain the mechanism, proposed the so-called “Resonance Theory” of reflex activity. 1

In his earlier experiments, Weiss grafted fully differentiated limbs (fore and hind) of Salamandra maculosa larvae close to the normal. In making the wound for the insertion of the limb, one or more of the normal limb nerves were cut. These were found to regenerate into those muscles of the orthotopic limb which were rendered nerveless by the nerve section, as well as into the denervated muscles of the supernumerary limb. Weiss pointed out that there occurred an “at random” regeneration of the nerves, consequently homologous function could not be explained upon any structural specificity in regenerating nerve pathways.

Homologous function has been described also by the author 2 in limbs that have been grafted as embryonic rudiments. In these cases, the normal forelimb rudiment of Amblystoma embryos was excised and grafted 3 or 4 segments caudal to the normal position. The wound was left uncovered—thus original site (Fig. 1). Under such conditions one might obtain 3 limbs, viz., a regenerant and a double limb arising form the graft. When this occurred, synchronized movements of the homologous muscles ensured, regardless