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. 
