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.