128 SCIENCE PROGRESS. 



phores of the lower Vertebrata. The ancestors of the 

 Mammalia no doubt agreed with existing Amphibia and 

 Reptiles in possessing chromatophores, but these would 

 become useless as soon as the development of hair con- 

 cealed the skin from view. The same argument may be 

 employed in the case of birds, whose coating of feathers 

 would also render chromatophores of no further use. 

 Hence their degeneracy in birds and Mammals. That 

 they are not necessary for the nutrition of the epidermis 

 is well seen in the case of Primates. In Anthropoid Apes 

 the epidermis contains not only processes from cutaneous 

 pigment-cells, but frequently entire cells as well (Kolliker, 

 34). In the negro there are no entire pigment-cells in the 

 epidermis, only processes from sub-epidermal cells (Karg, 

 38 ; Kolliker, 35). Lastly, in the white races of man 

 pigment-cells are almost entirely absent, and are limited 

 to certain special parts of the body (nipple, scrotum, etc.). 



If I have thus rendered it probable that pigment-cells in 

 Mammals have not the value of nutritive cells, perhaps this 

 fact may reduce the bias which at present exists towards 

 the view of their imagined mesodermal origin. 



It is, of course, perfectly possible that the pigment-cells 

 of Mammals are not homologous with the chromatophores 

 of the lower Vertebrata, and have a nutritive function. 

 That would be a very interesting point to decide. 



Here my article must close. The literature upon pig- 

 ment-cells is enormous, and the study which, for purposes 

 of my own, I have made of it will not only, I trust, 

 lighten the labours of some of my fellow-naturalists who 

 are interested in the subject of animal coloration, but may 

 possibly be serviceable in showing the paths along which 

 new research can most profitably be conducted. 



I have been astonished to discover that, so far as I am 

 aware, there is not within the whole of existing literature a 

 single indubitable proof of the mesodermal origin of true 

 chromatic cells, and I have been unexpectedly led by my 

 survey of the facts to the opinion that chromatophores, as 

 defined at the beginning of this article, are universally of 

 ectodermal orioin. 



