PIGMENTATION OF THE VERTEBRATE SKIN 591 



from the epidermis, greatly conditions the color pattern of the species. The 

 color of these accessory structures is dependent upon three main factors: 



( 1 ) presence or absence of pigment, 



(2) presence of air, and 



(3) iridescence. 



Pigment and air are dominant factors, for example, in the color exhibited 

 by hair and feathers. The presence of air diminishes and distorts the effects of 

 the pigment which may be present. The property of iridescence is to be dis- 

 tinguished from the color effects due to the presence of certain pigments; the 

 latter absorb light rays and reflect them, whereas iridescence is dependent upon 

 the diffraction of light waves from irregular surfaces. Iridescence is important 

 in the color effects produced by the plumage of a bird or the skin surface of 

 many fish, reptiles, and amphibia. 



2. Color Patterns 



In the vertebrates whose manner of life dictates a close association of the 

 body with the environmental substrate, the underparts have less color than 

 the parts exposed to the light rays coming from above. Also, within the general, 

 colored areas, there are certain spots, lines, bars, and dark and light regions 

 which follow a definite pattern more or less peculiar to the variety, subspecies, 

 or species. These color patterns tend to be fixed and are determined by the 

 heredity of the animal. Consequently, they are related to the genie complex 

 in some way. However, in many species the tone of the color patterns may 

 be changed from time to time by changing environmental conditions as men- 

 tioned on page 594. 



3. Manner of Color-pattern Production 

 a. Role of Chromatophores in Producing Skin-color Effects 



Work in experimental embryology has demonstrated fairly conclusively that 

 the pigments necessary for color formation are elaborated principally by cer- 

 tain cells known as chromatophores. Chromatophores are pigment-bearing 

 and pigment-elaborating cells. Various cells may produce pigment, but chro- 

 matophores are cells specialized in the function of pigment elaboration. 



The distribution and activities of chromatophores vary in the different ver- 

 tebrate groups. For example, in fishes, amphibia, and many reptiles, three or 

 probably four kinds of chromatophores are present in the dermis, namely, 

 melanophores, lipophores, guanophores, and (possibly) allophores (Nobel, 

 '31, p. 141). By their presence and arrangement, the chromatophores pro- 

 duce specific color patterns. Moreover, the expansion and contraction of the 

 pigmented cytoplasm of some or all the chromatophores effects changes in 

 color, for the contracted or expanded state determines the types of light rays 

 which will be absorbed or reflected. The rapid color changes in certain tree 



