692 



EVOLUTION OF ANIMALS 



Part V 



colorless 



ow 

 ii^Mr- blue 

 black 



SECTIONS OF SKIN TREE FROG 



Fig. 34.11. Sections of skin cells of a tree frog (Hyla), showing the relations 

 of the pigments and blue-reflecting crystals when the skin is bright green, dark 

 green, and yellow. Yellow pigment is contained in the cells next to the outer 

 skin cells or epidermis. In the next layer inward the cells contain blue-reflecting 

 crystals (guanin). The cells below these contain black pigment. A, bright green. 

 The yellow pigment is expanded. The cells with the blue-reflecting crystals are 

 covered by black pigment on one side. B, dark green. The yellow pigment is only 

 slightly expanded. The black pigment covers much of the cells that contain 

 crystals. C, yellow. Black pigment is greatly contracted. Yellow pigment is ex- 

 panded and blue-reflecting crystals are irregularly arranged. In brown, not shown 

 here, the crystals are almost covered and black and yellow are expanded, with 

 black dominant. (After Noble: The Biology of Amphibia. New York, McGraw- 

 Hill Book Co., 1931.) 



yellow produce the green. The skin is yellow when light is reflected only from 

 yellow pigment; the black pigment is then contracted and fails to give the 

 guanin crystals the dark background. Brown color occurs when black pig- 

 ment covers the guanin crystals; black and yellow are reflected and mixed. 

 The association of endocrine secretions and color changes is discussed in 

 Chapter 15. 



Skeleton. The skull is roughly triangular with bones firmly joined except 

 the loose attachments of the lower jaw. The cranium, a narrow bony box that 

 holds the brain, is similar in shape to the fish's cranium and a contrast to the 



