. Animal hormones; a comparative survey. Hormones. 76 KINETIC HORMONES—I this diurnal rhythm for a time under constant conditions. They show a Umited background response, becoming darker on an illuminated dark background than they are on a white or yellow background. In constant light, the melanin disperses in response to moisture, under the control of what is probably the same darkening hormone, since its secretion is also stimulated through the nervous system (Giersberg, 1928). This was established by putting the insect into a humid box, with its head projecting through a diaphragm into the
. Animal hormones; a comparative survey. Hormones. 76 KINETIC HORMONES—I this diurnal rhythm for a time under constant conditions. They show a Umited background response, becoming darker on an illuminated dark background than they are on a white or yellow background. In constant light, the melanin disperses in response to moisture, under the control of what is probably the same darkening hormone, since its secretion is also stimulated through the nervous system (Giersberg, 1928). This was established by putting the insect into a humid box, with its head projecting through a diaphragm into the dry air outside (Fig. 3-11); this induces darkening of the whole animal from head to tail in about half an hour. If a ligature is put round the body to prevent the circulation of the blood and hormone to the tail end, the darkening only affects the part in front of the ligature. If the ventral nerve cord is cut at a level just outside the humid box, no darkening takes. Fig. 3-11. The stick-insect, Carausius, with the hinder part of the body in a moist chamber and the head and thorax projecting through a membrane. The pigment dispersion, caused by the moisture, is transmitted by a darkening hormone from the suboesophageal ganglion. This acts only upon the head and pro- thorax because of the ligature just behind them (from Giersberg 1928). place, because the stimulus from the damp skin is not conducted to the brain. But if the same animal, with the nerve cord cut but the body unligatured, is then reversed, with its head in the box and the tail left out, the whole body darkens, because the stimulus from the skin of the head can reach the brain, which therefore stimulates the secretion of the hormone. This then circulates freely to all parts of the body. Secretion of the darkening hormone has been located histologically in the suboesophageal ganglia and confirmed experimentally by injection of extracts into animals from which the brain had been removed. Headless animals, lacking
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