The apparent 'corneal specificity' of sensory neurons.

SINCE the first fundamental papers of Paul Weiss (1924), a considerable body of evidence has been accumulated in favour of the assumption that different tissues or organs transplanted heterotopically exert some specific modulatory influence on neurons, the processes of which happened to grow into the grafts; the grafts in turn provide these neurons with capacities present in the neurons normally supplying these tissues or organs. These observations led to the formulation of a 'resonance' hypothesis, but later Weiss himself elaborated a somewhat more cautious concept of 'specific modulation' (see Weiss, 1955), which was corroborated and extended in masterly manner in different fields of neurogenesis by Sperry (1951) and his collaborators. Strong support was given to this concept by an experiment of Weiss (1942) in which, by tactile stimulation of the cornea of a supernumerary eye implanted into the region of the nostrils or the ear capsule, an ordinary corneal reflex response could be elicited in the ipsilateral eye of the host. The cornea of the implanted eye being innervated by the trigeminus like that of the normal eye, this experiment was not completely convincing. But when Kollros (1943) implanted supernumerary eyes between the ear capsule and the base of the gill, a region innervated by the vagal nerve, stimulation of the cornea yielded also corneal reflexes of the host's ipsilateral eye. Thus the assumption of a 'corneal specificity' appeared to be well justified. Working for other purposes with limbs of urodele larvae transplanted to the base of the gill, it happened that the distal part of an implanted limb was bitten off by another animal. Mechanical stimulation of the regeneration blastema evoked a corneal reflex of the ipsilateral eye, an effect never met with when an intact implanted limb was stimulated. This observation being inconsistent with the assumption of a 'corneal specificity' the matter was more closely investigated, and the results will be presented here.