ABSTRACT
Minnows were subjected to spinal section at different levels between vertebrae 4 and 15, and the degree of dispersion of all the main melanophore regions was recorded in terms of the melanophore index.
Records were made of the times required to reach equilibrium (a) when the fish were placed after the operation on a black or white illuminated background, and (b) when the fish at equilibrium on a black or white background were subjected to background reversal.
These records show that the times necessary for the melanophores to reach equilibrium on a given background are of the same order at all the vertebral levels investigated.
There is considerable variation in the degree of dispersion of similar melanophore regions of different individuals under the same conditions of operation and background. The nature of these individual differences is not known.
Apart from the possibility of incomplete section of chromatic nerve fibres near the 15th vertebra, there appears to be no correlation between the chromatic behaviour and the level of the operation ; i.e. there is no indication that any activity of the spinal paling centre is being affected by spinal section at the different levels.
The results of experiments involving the elimination of the spinal paling centre were tested statistically. Within the limits of this treatment, based upon the melanophore index, it was concluded that the spinal paling centre plays no part in these colour changes.
Spinal section was carried out anterior to the 1st spinal nerve in order to interrupt the path of von Gelei’s dispersing fibres. Statistical treatment of the results indicates that these fibres are playing no part in these colour changes.
Experiments involving combined spinal and autonomic section confirm the conclusions given in paragraphs 6 and 7.
Since no activity of the nervous system arising from the spinal paling centre or resulting from the background through von Gelei’s dispersing fibres appears to be involved, the colour changes of these spinal minnows in response to illuminated backgrounds must be controlled by hormones alone.
In these spinal fish the various melanophore regions do not all react equally in terms of the melanophore index. Thus, those of the lateral stripe and associated dark pattern tend to have relatively higher M.I. values under all conditions.
Also, the factors involved in the first paling of the freshly operated animal on a white background are not the same as those involved in the later black-to-white colour change.
