1. Limb primordia, both leg and wing, have been transplanted heterotopically in chick embryos on the 3rd day of incubation. Of 11 chicks hatched, 1 had a limb graft situated in the neck, 9 in the thoracic, and 1 in the lumbar region. These supernumerary limbs were supplied with nerves from the respective segments.

  2. Muscle tissues underwent degeneration and the joints were ankylotic. The covering skin and feathers were normal and sensitive to mechanical stimuli. Responses of the hosts to stimulation of the supernumerary limbs have been studied.

  3. The most common reflex response to light mechanical stimulation of the graft was a flexion reflex of the host’s ipsilateral leg. If the graft was situated in close vicinity of the wing, elevation of the host’s own wing could be elicited. These Timb-specific’ reflexes could be exclusively evoked from the grafts, but never from their neighbourhood. Four cases yielded more complex behaviour reactions always referred to one of the host’s own limbs, i.e. lame walking after stronger stimulation of the graft and cleaning of the host’s foot with the beak when a permanent, painful, stimulus was applied to the supernumerary limb.

  4. There was no correlation whatever between the type of the graft and the evoked ‘limb-specific’ responses which appeared only some days after hatching. Pain sensitivity and specific reflexogenic capacity were not strictly associated, since Timb-specific’ responses could be elicited from painless parts of the grafts and the reverse.

  5. According to anatomical investigation most of the grafts were supplied exclusively by intercostal nerves which never under normal circumstances convey limb signals and stimulation of which normally never evokes Timb-specific’ responses. Grafts producing only simple reflexes were generally supplied by a single segmental nerve, whereas complex behaviour reactions could be exclusively evoked from grafts with multisegmental inervation and some plexus formation among the supplying nerves.

  6. It is difficult to explain the observations made on supernumerary limbs of chicks on the basis of the Weiss-Sperry concept of ‘specific modulation’. As alternative hypothesis the capacity of ‘analysis’ and ‘recognition’ of specific afferent impulse patterns by the c.n.s. is suggested.

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