iS6 COLOUR IN NATURE chap. 



Origin of Pigments of Butterflies 



Let us now consider the meaning of this utilisa- 

 tion of waste products, and first of all their immediate 

 origin. Urech suggests that they arise from the 

 breaking down of the substance known as nuclein, 

 which has recently been rising in importance in the 

 eyes of physiologists. Nuclein is a complex substance 

 forming a considerable part of the nuclei of all living 

 cells ; it is derived from albumen, from which it chiefly 

 differs in containing a large amount of phosphorus. 

 According to Urech the nuclein of the leucocytes in 

 butterflies undergoes a process of breaking down, 

 yielding the nuclein bases, that is, such waste products 

 as uric acid, guanin, adenin, hypoxanthin, etc., and 

 also phosphoric acid and albumen. The nuclein 

 bases then become further modified into the pigments. 

 The different colours of the pigments Urech describes 

 as due to an increase in the molecular weight of the 

 substances, the yellow pigment being thus simpler 

 chemically than the red one. 



Mr. A. G. Mayer gives a somewhat different 

 account of the development of the pigment of the 

 scales in butterflies. We may consider first his 

 account of the development of the scales them- 

 selves, which is interesting in several respects, and 

 is illustrated by some very good figures. These 

 show clearly that the scales develop from prolonga- 

 tions of certain formative cells of the epidermis, 

 which project outwards beyond the level of the other 

 cells. The scales at first contain protoplasm, which 

 is continuous with the protoplasm of the formative 



