THE CHROMATOPHORES OF ANIMALS. 127 



they are absent from that layer. He dismisses it as pro- 

 bably a " phenomenon of heredity ". 



The same investigator finds branched pigment-cells in 

 the epidermis of an embryo chick of fifteen days. Here 

 also he regards them as intrusive connective tissue cells, 

 although a difference from those of Viper embryos is found 

 in the fact that the pigment-cells already form pigment 

 while still within the connective tissue. Remarkably 

 enough, the pigment-cells both of the epidermis and of 

 the cutis are stated to disappear completely by the 

 twenty-third day. 



In Mammals branched pigment-cells are frequent both 

 in the epidermis and the cutis, but their origin is a subject 

 of much dispute. An excellent summary of the facts may 

 be found in Kolliker (35), who, although inclining to the 

 view that pigment-cells are for the most part of mesodermal 

 origin, does not go to such extremes as Aeby (36), Halpern 

 (37), and others, who deny the pigment-forming power of 

 the ectoderm altogether. 



The writings of certain investigators, e.g., Karg (38), 

 Halpern (57), betray signs of a certain physiological pre- 

 conception as to the function of pigment-cells, which it may 

 be well to deal with here. Karg is especially emphatic 

 upon the great role which intrusive pigment-cells must play 

 in the nourishment of the epidermis, while Halpern assures 

 us that the cells of the rete Halpighii take up pigment 

 through the breaking down of pigment-laden wandering 

 cells, and through absorption of parts of their protoplasm. 

 It is clearly imagined here that the pigments in these cases 

 are supplies of food material ; but it may be asked whether 

 the chromatophores of Fishes, Amphibians, and Reptiles 

 are also stores of reserve material. They are well known 

 to be nothing of the kind, and unless we deny the 

 homology between the pigment-cells of Mammals and the 

 chromatophores of the lower Vertebrata, we can afford 

 to be sceptical about views as to the nutritive value of 

 pigment-cells in Mammals. It seems to me to be much 

 more probable that the pigment-cells of Mammals are 

 simply degenerate representatives of the chromato- 



