RECENT SCIENCE. 631 



as soon as a certain part of the skin receives no more blood, the 

 color-cells receive less oxygen, the black cells contract, and the 

 animal assumes a lighter color. But the effects of light are even 

 more interesting. Pouchet had shown that those fishes which 

 usually adapt their color to their dark or light surroundings cease 

 to do so when they have lost sight ; they remain dark even in 

 light surroundings.* The indirect effects of light through the 

 intermediary of the visual organs are thus certain. But Steinachf 

 has proved that light acts in a direct way as well — perhaps, we 

 may add, in the same way as it acts upon the chlorophyll grains 

 of the leaves. He glued strips of black paper to the skin of frogs 

 which were kept in the dark, and when these animals were 

 exposed to light, only the open parts of their skin returned to a 

 lighter color, while the covered parts remained dark. To avoid 

 all doubts, the experiments were repeated on skin separated from 

 the body, and photograms of letters and flowers, cut out of black 

 paper and glued to the skin, were reproduced upon it. Besides, 

 blind tree-frogs do not darken as the fishes do, and Biedermann 

 has proved that the chief agency of their changes of color is not 

 in the sensations derived from the eye, but in those derived from 

 the skin. Frogs, whether blind or not, become dark green, or 

 black, if they are kept in a dark vessel in a sparingly lighted 

 room. But when a larger branch with green leaves is introduced 

 into the vessel, they all recover their bright-green color, whether 

 blind or not. In some way unknown, the reflected green light 

 acts either upon the nerves of the skin, or, what seems more prob- 

 able, if Steinach's experiments are taken into account, directly 

 upon the pigment-cells. Moreover, the sensations derived from 

 the toes have also an influence upon the changes of color. When 

 the bottom of the vessel is covered with felt, or with a thin wire 

 net, the frogs also become black, recovering their green color 

 when a green branch is introduced in the vessel. 



We have here temporary changes of color produced by the 

 surroundings ; but various gradations may be traced between the 

 temporary and the permanent changes. Thus Lode provoked local 

 contractions of the pigment-cells in fishes by electrical irritations 

 applied locally. And Franz Werner's researches upon the color- 

 ing of snakes, recently embodied in a separate work, J show that 

 the temporary and irregular spots which appear in fishes and 

 frogs under the influence of artificial irritations are of the same 



* Direct observations have been made also by Alois Lode (Sitzungsberichte of the Vienna 

 Academy, 1890, vol. xcix, 3te Abtheilung). 



f TTeber Farbenwechsel bei niederen Wirbelthieren, bedingt durch directe Wirkung des 

 Lichtes auf die Pignientzellen, Centralblatt fur Physiologie, 1891, Bd. v, p. 326. 



\ Franz Werner, Ueber die Zeichnungen der Schlangen, Wien, 1890. 



