328 ZOOLOGY. 



Bieilermann more recently has claimed that the color- 

 cells change their shape owing to the direct action of 

 light and of temperature. It appears that the slightest 

 change of temperature affects the mutual disposition of 

 the pigment-cells, aud consequently the color, of the frog. 

 It is enough to keep the animal in the haud to provoke a 

 contraction of its black cells. The amount of blood-supply 

 also has a definite effect. 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. That tlie light acts as a direct stimulus has 

 been proved by Steinacli, who glued strips of black paper 

 to the ski)i of frogs which were kept in the dark; when 

 they were exposed to the light only the open parts of their 

 skin returned to a lighter coloi', while the covered p)arts 

 remained dark. To avoid all doubts the experiments were 

 repeated on skin separated from the body, and jjliotograms 

 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 la.ro;e branch with sreen leaves 

 is introduced into the vessel they all recover their bright- 

 green color, whether blind or not. In some way uidiuown 

 the reflected green light acts either upon tlio nerves of the 

 skin, or — what seems more probable if Steiuach's experi- 

 ments are taken into account — directly ujjon 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 into the 

 vessel." 



