536 THE WONDER OF LIFE 



brates, or iu the %m& oj the blood, as in earthworms ; it 

 may be in the form of coloured spicules or calcareous 

 deposits, as in Akwjnarian corals; it may be in special 

 cells which oft*** *&&*- considerable activity the chroma- 

 tophores (so- F;# *7l 



Primary S*utkAm:e of Pigments. There have been 



relatively few- ^8Bp>it*nt inquiries into the physiological 



signifies? , ; -.4 v:$ir\-.^.-Ar> which is a very difficult problem; 



but it, ;,-*.*r h* amid that some pigmented substances 



rti't t-.f ?Ko. 'kiittii* of waste-products, like the green guanin 



: H toii**Vi' kidney, or the sulphur-yellow in the wings 



^s* buttrfbs, r the sepia of the cuttlefish ; that 



art- of ihe nature >f resent products, like the carmine 



run accumulates ifl the bod of the female cochineal 



c5T .W .nbH 9 Hl rri zitomtin'A yd 



<>f tnaj*\ Thus the gonads 



' ,...- ^ ; :.' 'iflt. 



Primary Si Coloration. 



The cross bars, the concentric lines, the zoned structure. 

 and the super jK>siuon of very thin lamellae produce inter- 

 ference colours, but what is their primary significance ? 

 The answer must be, that they are the ripple-marks ot 

 growth ; they are expressions of the fact that growth j* 

 rhythmic, not continuous. The familiar concentric li- 

 the stem of a tree express the difference between tit* 

 summer and the winter wood ; the lines on the sur 

 a shell are indices of periods of growth pum. 

 times of rest. 



As- ustration of the idea towards whi 



structural features are just 



