XIV ORIGIN OF PIGMENTS 297 



Of all suggestions of this kind, those of List for 

 Vertebrates are the clearest and most definite, and 

 may be summarised here. 



In the first place, List accepts without reservation 

 the view that pigment does not originate in epi- 

 dermal structures, but is carried to them by wander- 

 ing leucocytes. He believes that the .pigment 

 originates within the blood-vessels by the degenera- 

 tion of red blood corpuscles ; that it is taken up by 

 leucocytes ; and that these with their burden follow 

 the track of the blood-vessels outwards from the 

 corium to the sub-epithelial layer. The pigment 

 granules are to be looked upon as excretory pro- 

 ducts, which are in part taken up by the epithelial 

 cells and gradually eliminated as these degenerate. 

 In bony fish, and apparently in Amphibia, pigment 

 arises in the embryos from the degeneration of the 

 yolk, but the pigment which is produced later prob- 

 ably arises from blood pigment. 



Similarly the migration outwards of pigmented 

 cells in the leech is often regarded as a process of 

 excretion. 



Criticism of these Conclusions 



As to the whole question, it is probably too soon 

 to attempt to draw conclusions, but one or two 

 points may at least be touched on. In the first 

 place, an obvious difficulty in the way of regarding 

 all pigments as waste products, or as derivatives of 

 respiratory pigments, is that the great majority of the 

 researches hitherto carried on have almost entirely 

 omitted to consider the pigments soluble in alcohol. 



