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THE AMERICAN NATURALIST 



[Vol. XLV 



and others, but when one assumes (as all investigators 

 agree in doing) that pigment formation is somehow 

 related to the blood as source of nutrition or supply, the 

 segregation of the pigmented cutis cells at this level 

 becomes intelligible on other grounds, i. e., necessary 

 closeness to the capillaries of papillae. Only those layers 

 of the epidermis next the border line (/. e., next the capil- 

 laries) have the pigment granules of the prevailing size 

 and color for normal pigment cells. Thus my evidence 

 points to a dependence of both cutis and epidermal cells 

 upon the same source (the blood of the capillaries) for a 

 sine qua non of pigment formation, and an independence 

 of each with respect to the other as a necessary source 

 of supply or even as an aid to pigmentation. That the 

 blood constituent is not haemoglobin the arguments 

 of Karg and the observations of Meirowsky seem con- 

 clusively to prove. That it is not an iron-containing 

 element (e. g., haemosiderin) I have demonstrated by 

 the method of testing with potassium ferricyanide as 

 used by Brown (1910) for the liver. Chemical analy- 

 sis by Abel and Davis (1896) also has shown that the 

 melanic pigment of the hair and skin of the negro 

 is free of iron. The evidence at hand, as furnished by 

 Chittenden and Albro, von Fiirth, Spiegler, Gessard, 

 Riddle, Meirowsky, and others seems to render it very 

 probable that in vital melanogenesis we are dealing with 

 a proteid substance (tyrosin; trophoplast; chromo- 

 gen) acted upon by an enzyme or oxidase (tyrosinase) and 

 that one (probably the former) is supplied by the cell 

 (nucleus) and the other by the blood. 



The fact that the pigment granules, in epidermal cells 

 that are not packed with them, are segregated in the distal 

 portion, indicates that they are responsive to the influence 

 of light. However, the further discussion will not be com- 

 plicated by a consideration of this possible factor. The 

 following discussion will accept as well supported the 

 position that pigment is formed in the epidermal cells— 

 the analogous formation of pigment in ganglion cells 

 gives further support— by virtue of a cellular metab- 



