Investigations on the effects of chromatic spinal section on the so-called 'morphological' colour changes of the minnow (Phoxinus phoxinus (L.)) with observations on the rate of pigment migration in the newly-formed melanophores

Abstract

Intact and chromatically spinal sectioned minnows of known background histories; were exposed to illuminated white and black backgrounds for 2 months. A definite skin area was photographed and the melanophores lost and formed in it were recorded. Loss and formation of melanophores occurred simultaneously in intact minnows whether placed on a white or on a black background although their relative numbers were very different in the two situations and depended largely on previous background histories. After chromatic spinal section, the loss of melanophores was reduced and formation increased in minnows on a white background, whereas the reverse was observed in minnows on a black background, these results followed irrespective of difference in background histories. two conclusions are drawn from the results. Firstly, that the chromatic nerve fibres play a significant part in 'morphological' colour changes of the minnow. Secondly, that a dineuroic control is involved. It is suggested that the pigment-aggregated transmitter substance promotes loss of melanophores and limits their formation and that the pigment-dispersing transmitter promotes their formation and limits their loss. In the second set of experiments minnows were exposed on a black background for varying periods to allow melanophore formation, the ages and time of appearance of melanophores were photographically determined. The background responses of the newly formed melanophores were assessed microscopically in fixed preparations of animals killed in at different time-intervals. The results were interpreted according to the generally accepted belief that slow and rapid colour changes indicate hormonal and nervous mediation respectively. The age of the melanophores was: found to be a significant factor in determining their background responses, the older melanophores being more responsive. It is suggested that the sensitivity to hormonal control develops first in the life of a pigment cell, to be followed later by sensitivity to nervous control, nervous control, however, does act appear simultaneously for w/B and B/w adaptations. It develops first for B/w colour change. This differential development is considered to provide evidence for double innervation of melanophores.