220 COLOUR IN NATURE chap. 



outgrowths, and that in fishes in general the epidermis 

 is little specialised ; in accordance with this we find 

 that the epidermis is usually almost entirely devoid 

 of pigment, or when, as in the flounder {Pleuronectes 

 flesus), some pigment is present, it has no effect on 

 the visible coloration. In birds, on the other hand, 

 where the epidermis gives rise to greatly specialised 

 outgrowths, the feathers, the pigment of the body 

 is found in these epidermal structures. There is, 

 however, a general consensus of opinion that in all 

 cases the pigment originates either in the dermis or 

 in deeper connective tissue cells and migrates out- 

 wards to the epidermis, in the cases where it is 

 found there. Many would regard this as a proof 

 that pigments are in essence waste products, and 

 that this migration outwards is to be interpreted as 

 a process of excretion, the connective tissue cells or 

 amcebocytes carrying the waste pigment from the 

 essential organs of the body outward to the inert 

 skin. For an elaborate hypothesis of this kind the 

 reader may be referred to a paper by Mr. H. E. 

 Durham ; it is doubtful, however, whether such 

 generalisations are not as yet premature. 



To return from this digression to the immediate 

 subject of the skins of fishes, we find that, accord- 

 ing to Cunningham and M'Munn, the elements im- 

 portant in coloration occur in two layers in the skin, 

 and the two layers differ considerably in the upper 

 and lower surfaces of the fish. In most cases the 

 outer layer occurs in relation to the scales, while the 

 inner layer lies close to the surface of the muscles, 

 but in the mackerel the loose scales contain no 

 colouring elements. The outer layer consists of 



