THE CO‐ORDINATION OF VERTEBRATE MELANOPHORE RESPONSES

Abstract

Summary1. Two classes of melanophore responses to light can be distinguished: (a) A primary, direct or non‐visual response involving the expansion of the melanophores in light and their contraction in darkness. This non‐visual response may be a co‐ordinated response or the melanophore may be an independent effector. (b) A secondary or visual response when the melanophores contract if the visual field is coincident with a light‐scattering field, and expand if the animal is exposed to superior illumination in surroundings which do not reflect or scatter light coming from above.2. Co‐ordination of melanophore responses in elasmobranchs and Amphibia is by means of pituitary hormones circulating in the blood.3. A direct nervous control of melanophores has been superimposed on the more archaic humoral mechanism to a varying extent in different genera of teleosts. Sufficient is now known of this group to formulate tentatively a general scheme applicable to all forms, according to which the two components–humoral and nervous–are developed to a greater or less extent in various genera.4. Co‐ordination of melanophore responses in some reptiles is solely humoral. In others only nervous agencies have been discovered. Insufficient data are available to construct a scheme showing the phyletic relationship of the two systems between the members of the group.5. The kind of evidence that is acceptable for the existence of the various co‐ordinating systems is discussed.6. The merits of the bihumoral, double innervation and neuro‐humoral hypotheses are discussed.7. An evolutionary scheme is appended which may tentatively be taken to indicate that direct nervous control is phylogenetically a later development than humoral control.