Transmission of Neurohumors in Animals by Other Means Than Blood and Lymph

Abstract

It has been assumed that the contraction or expansion of dermal melanophores in fishes whereby these animals take on a light or a dark tint is dependent upon substances carried in the animal's blood and lymph (Parker 1 ). Such an assumption appears to be justified by the fact that if a small amount of dissolved adrenalin is injected into the muscles of a dark chub, Fundulus heteroclitus, this fish within a quarter of an hour or less will become very light-colored. That the normal changes in tint shown by such a fish are brought about by other means than by dissolved substances carried in blood and lymph seems very probable from the following observations.

If a short transverse incision is made in the tail of a light-colored Fundulus, a dark stripe extending from the cut to the posterior edge of the tail will quickly appear. This stripe will gradually become lighter and lighter till in a few days it is no longer distinguishable from the rest of the tail. The transverse cut severs a number of radiating nerve fibers that pass out into the fish's tail and the dark stripe itself represents an area of denervation thus produced. That this stripe should be dark is due to the fact that the severance of the nerve fibers excites the discharge of an expanding neurohumor whereby the melanophores are made to enlarge and thus to darken the skin.

On placing a light-colored fish, whose stripe has become equally light-colored, on an illuminated dark background, the body of the fish will become fully dark in a little less than 2 hours. The stripe, however, will remain light-colored for some time, changing slowly to dark only in the course of a day to a day and a half.