1. The common dogfish, Mustelus canis, as is well known, exhibits two temporary extremes of tint, one dark, the other light. The dark phase is induced by a secretion from the pituitary gland which is carried in the blood, hence a substance soluble in water (a hydroneurohumor). The light phase is under the control of nerves and cannot be excited by blood from a light fish.
2. When an olive oil or ether extract is made from the fins of a light dogfish and this extract is injected into a dark fish, large light spots may appear in from 1 to 2 days and persist for several days. These light spots, which may be called secondary spots, are not to be confused with certain small and very temporary light spots, the primary spots, which occur soon after the injection and which are believed to be purely operative in origin.
3. The secondary light spots are not due to the death of the integumentary tissue, for, after their formation, they can be made to disappear by the action of obstetrical pituitrin and will subsequently reappear.
4. They are produced by some substance extracted from the light fins by ether or by olive oil. They are not produced by sea water, ether, or olive oil alone.
5. The extracted substance, which can resist dry heat up to at least 110°C., owes its limited range of action in the dogfish to its inability to dissolve in water. It is soluble in oil (a liponeurohumor).
6. This liponeurohumor is believed to emanate from the nerve terminals concerned with the concentration of melanophore pigment and to spread through the fatty components of the integumentary cells.