. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 710 Comjparative Animal Physiology of both concentrating and dispersing nerve fibers, color changes seem pre- dominantly determined by hormones. In its activity in the eel, the B-sub- stance is believed to be assisted by a W-substance.^^^ That direct innervation does play some role in color changes in this fish is seen in the limited back- ground response after hypophysectomy. The B-substance is found to be slighdy less important in normal color changes in the catfish Ameiurus. Hypophysectomized catfishes contin
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 710 Comjparative Animal Physiology of both concentrating and dispersing nerve fibers, color changes seem pre- dominantly determined by hormones. In its activity in the eel, the B-sub- stance is believed to be assisted by a W-substance.^^^ That direct innervation does play some role in color changes in this fish is seen in the limited back- ground response after hypophysectomy. The B-substance is found to be slighdy less important in normal color changes in the catfish Ameiurus. Hypophysectomized catfishes continue to show color changes in response to black and white backgrounds but show only an intermediate degree of darkening on '^ Injection of posterior- lobe extract will, however, completely blacken these fish. Here we must assume that a blood-borne B-substance supplements the action of dispers- ing nerve fibers in the normal responses to black backgrounds; there is as yet no evidence for the operation of a W-substance in this species. The killifish, Fundulus, on which a vast amount of research has been done, is a species in which the dominant mechanism of melanophore control is nervous. Color changes are very rapid, only a minute or two being re- quired for nearly maximal color change. These changes continue to occur. W(B) Fig. 273. Diagram of the controlling mechanism of melanophores in the eel, Anguilla. L, incident light; W(B), white or black background; DR, dorsal retina; VR, ventral retina; CNO, central nervous system; P, pituitary; A, adrenergic fibers; C, cholinergic fibers; B, blood and Ivmph; DM, dispersed melanin; CM, concentrated melanin. From Parker.^=^ in hypophysectomized specimens.^^^ Furthermore, injection of extracts of posterior lobe into pale fish on a white background does not produce sig- nificant darkening. Since such extracts will induce darkening in denervated areas of the skin we must conclude that the chromatophores are normally influenced to some
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