i88 THE BIOLOGY OF THE FROG CHAP. 



Seasonal Changes. Most of the seasonal changes in the 

 skin are correlated with the sexual differences that occur 

 during the breeding season, and have been treated under 

 that head. There are some other seasonal changes, how- 

 ever, which occur apparently without regard to the develop- 

 ment of the sexual products. In the winter and early 

 spring frogs are darker in color than in summer, owing 

 probably in large part to differences of temperature. Ac- 

 cording to Donaldson the power of the skin to absorb water 

 is greater in summer than in winter. 



Color Changes. The power of the skin to change its 

 color in relation to surrounding conditions depends upon 

 changes which occur in the pigment cells, or chromato- 

 phores. Of these there may be distinguished the following 

 varieties : black pigment cells (melanophores), interference 

 cells (leucophores), golden pigment cells (xanthophores, 

 xantholeucophores), and in some species of frogs red pig- 

 ment cells. 



The black chromatophores are stellate cells with irregu- 

 larly branching processes. There is a single nucleus near 

 the center of the cell. The dark pigment is in the form 

 of numerous small brown or black granules of a substance 

 called melanin, which is a very resistant compound remain- 

 ing unaffected by most reagents. The black chromato- 

 phores are most abundant on the dorsal side of the body, 

 especially in the black spots where they are massed together 

 very thickly. On the ventral side they are almost entirely 

 absent over a considerable area. They are found mostly in 

 the superficial layer of the corium just below the epidermis. 

 Scattered chromatophores occur in the epidermis and the 

 deeper layers of the corium. They tend to aggregate in 

 regions which are most abundantly supplied with blood 

 vessels. The pigment of the chromatophores undergoes 



