192 



LIGHT AND PROTOPLASM 



[CH. VII 



however, chiefly from a consideration of vegetative apostro- 

 phe, has been led to the conclusion " that the movements of 

 chlorophyll have no relation whatever to benefit or injury 

 experienced by the grains, nor necessarily to the well-being of 

 the protoplasm." 



/3. The Rearrangement of Pigment in Animal Cells in Response 

 to Light. One of the striking cases of this effect of light is 

 seen in the pigment cells of the skin of the chameleon, as 

 described by KELLER ('95, pp. 144, 162). He has found that 

 the dark color of the (illuminated) skin is due to the rich 



ll 



cu. ep. 



FIG. 57. Vertical section through a black dermal papilla of Chamaeleo vulgaris. 

 ep, epidermis; CM, cutis; p, black pigment cells; p', processes of the cells con- 

 taining pigment; y r , yellow pigment cells. (After KELLER, '95.) 



branching at the base of the epidermis of black pigment cells 

 lying deep in the cutis (Fig. 57). In the dark, the pigment 

 granules stream out of the branches into the cell body, but the 

 branches themselves are undisturbed (Fig. 58). So long as 

 the black pigment has this central position, the skin appears 

 whitish. The light, on the contrary, causes the pigment, which 

 is probably carried passively in the plasma, to move centrifu- 

 gally. Whether the direct response to light of the pigment 

 cells of the frog, as described by STEINACH ('91), is of the 

 same nature, or due to contractions of the pigment cells, re- 

 mains to be determined. 



Again, in the retina of the compound eyes of Arthropoda, 



