COLOUR IN NATURE 
CHAP. 
156 
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 
