1. The chromatic effector speed of the two species has been measured by perfusing fully dark animals with saline until they were fully pale and then introducing B hormone into the perfusate to observe the reverse change. It was found that:
(a) Complete expansion of the melanophores of Xenopus takes 100 min. Complete contraction of the melanophores of Xenopus takes 70 min.
(b) Complete expansion of the melanophores of the eel takes 30 min. Complete contraction of the melanophores of the eel takes 20 min.
Adrenalin exercises a potent contracting effect on the melanophores of the eel even in the presence of excess B. It has no direct effect on the melanophores of Xenopus.
2. Increased concentration of B hormone in the perfusate above the threshold dose does not increase the speed of melanophore expansion in either animal. This implies something approaching an all or nothing reaction on the part of the melanophore itself. The usual interpretation of the "all or nothing" behaviour of effectors is that it is in large measure due to the well-known "all or nothing" action of their efferent nerves.
3. Using eels of approximately the same weight estimations of:
(a) concentration of B hormone in the circulation of a dark fish,
(b) concentration of B hormone in the pituitary of a dark fish,
(c) total blood volume,
indicate that the distribution of B between pituitary and circulation in a dark animal is of the order pituitary 1, circulation 10.
4. Caustic soda treated extracts of posterior lobe pituitary which evoke the same rise of melanophore index as untreated extracts do not maintain the expanded condition of the melanophores for the same period. The factors involved are discussed.