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In a previous contribution (Hogben and Slome, 1931) evidence was brought forward to show that the white background response does not depend on the same mechanism of coordination as the black background response, which is produced by reflex liberation of a hormone (“B” substance) of the pars intermedia in the pituitary gland; and experiments pointing to the existence of another internal secretion (“W” substance), connected directly or indirectly with the activity of the pars tuberalis, were described. The existence of separate receptor components of the retina controlling the two systems was left for subsequent enquiry. Of two possible hypotheses concerning the nature of the receptive mechanism, the most likely one is illustrated diagrammatically in fig. 1. In normal situations, when an animal is illuminated on a black background, light can only fall on the floor of the retina. If it is aquatic, the maximum divergence of any two rays which strike the eye is twice the critical angle for air and water, so that in the absence of reflexion of rays from sur­rounding objects below the surface of separation all rays will presumably be brought to a sharp focus in shallow water. There are thus three distinct possibilities which arise from the way in which the animal is illuminated if, as in Xenopus , the eyes are situated on the top of the head: ( a ) in darkness no part of the retina is stimulated, the same being true of the eyeless animal; ( b ) when the animal is exposed to a black background only a sharply localized region of the retina is stimulated; ( c ) when the animal is exposed to a white background the whole of the retina is illuminated owing to the scattering of rays in all directions from the surroundings. For convenience of description the usual black back­ground situation will be described hereafter as one in which only the “floor” of the retina is stimulated, and the white background situation as one in which the floor and the “ periphery ” of the retina are both stimulated together. If then, the receptor elements of the floor and periphery initiate different systems of reflex arcs the phenomena of the background response in Amphibia and Reptiles may be interpreted as follows. In Reptiles we may suppose that stimulation of floor reflexly excites the melanophores to expand, while stimulation of peripheral photoreceptors excites them to contract, being presumably prepotent in the final common path. In Amphibia two alternatives may be con­sidered: ( a ) that floor elements reflexly excite liberation of “B” and that peripheral photoreceptors, being prepotent, reflexly inhibit libera­tion of “B"; ( b ) that floor elements reflexly excite liberation of “B”, and peripheral photoreceptors reflexly excite production of the antago­nistic substance “W” in quantity sufficient to over-ride the effect of “B”. The crucial test of the truth of the general hypothesis that the floor and peripheral elements of the retina initiate different processes of coordina­tion was suggested by Keeble and Gamble (1904-6) in their experiments on Crustacea. If it is true, a normal animal illuminated from below in a black tank with a white top should react in exactly the same way as a normal animal when illuminated in a black tank from above. On the other hand, a normal animal illuminated from below in a black tank with a black top should react like an eyeless animal in the same situation, because the floor elements would not be subject to stimulation. The experiment may be varied as indicated below. In order to obtain signi­ficant results two classes of precautions must be carefully observed. One is that the physical dimensions of the tank must not exceed certain limits, since the maximal divergence of two rays is rigidly fixed when an animal is illuminated from above. The other is that there must be no air-water interface to permit reflexion of the incident rays downwards. Aside from the fact that no bubbles must be allowed to collect, this condition presents a practical difficulty if the animal has to come to the surface to breathe.

The pigmentary effector system VIII—The dual receptive mechanism of the amphibian background response