1. The contracted melanophores of both Anura (frogs and toads) and Urodela (salamanders) react by maximal expansion to pituitary extract; the active substance in the latter does not appear to be confined invariably to the posterior lobe of the mammalian gland. A positive melanophore response was obtained from the gland of a human abortus in the fourth month.
2. The removal of the whole pituitary gland may be accomplished in Urodele larvaæ (Amblystoma tigrinum) at any age (nine months to four years) without impairing their viability. After complete removal of the pituitary in Urodele larvæ, as in adult and larval Anura, the melanophores remain contracted and a state of permanent pallor ensues. The normal colour resulting from melanophore expansion can be re-established by injection of pituitary extract; but such animals regain pallor, although exposed to conditions in which melanophore expansion invariably occurs in normal individuals.
3. It is legitimate to conclude that pituitary secretion is the main factor in regulating the chromatic function throughout the Amphibia as a class. Fluctuating pituitary secretion in correlation with those conditions that evoke colour response in the frog (cf. the third paper of this series) provides a satisfactory basis for the interpretation of all the accredited bionomic data concerning colour response in adult Amphibia. Possibly adrenal secretion or some auxiliary mechanism plays a subsidiary part; but there are no satisfactory grounds for believing that nervous agencies directly influence amphibian melanophores; and there is reason to believe that even if amphibian melanophores are directly innervated, nervous control is not significant to the normal rhythm of colour change.
4. The study of amphibian colour response provides evidence not only of the presence of physiologically active substances in the pituitary, but functional activity of the gland in the intact animal. It does not appear, however, that the interpretation of colour response here put forward for Amphibia can be extended to Reptiles and Fishes.