The activities of the melanophore system of Phrynosoma in comparison with that of Chamaeleo may be stated categorically in the following way.
I. The blanching of Phrynosoma blainvillii is due
(1) to the action of its concentrating nerve-fibres on its melanophores,
(2) to the action of a hydrohumour, probably adrenalin, on the same cells,
(3) to the direct response of these cells to darkness, and
(4) to high temperatures.
II. The blanching of Chamaeleo is due to the first, third, and fourth of these factors. Whether the second factor is effective in this lizard is not yet determined.
III. The darkening of Phrynosoma is due
(5) to the action of a pituitary neurohumour on its melanophores,
(6) to the direct response of these cells to strong light, and
(7) usually, to low temperatures.
There are no dispersing nerve-fibres known in Phrynosoma.
IV. The darkening of Chamaeleo is due to the sixth and probably to the seventh of these factors. This lizard presents strong indications of possessing dispersing nerve-fibres. Whether it darkens from a pituitary neurohumour or not is unknown
