2IO On Melanin [Dec. 



less body, for example, the Stimulus which causes the secretion of 

 oxidase, or chromogen, or both, in the transformation of a color- 

 less pupa to a pigmented adult, (3) the cause of those white colora- 

 tions which are not albinic and which, when mated with colored 

 individuals, produce white offspring in the first generation, and 

 (4) the nature of tyrosinase, especially as to the cause of inactivation 

 of some varieties of tyrosinase by precipitation with alcohol while 

 tyrosinases of different origin are not affected by this treatment. 

 It is possible that lack of enzyme, or chromogen, changes in perme- 

 ability, and the presence of inhibiting factors may explain the first 

 three problems but as yet definite data are wanting, and it is along 

 these lines that future work will be continued. 



III. MELANOTIC PIGMENTS 



Methods of Isolation. In a survey of the literature relating to 

 the melanins I noticed a wide ränge in the methods by which the 

 pigments were isolated. In some preparations the pigment was 

 rubbed loose under water and, after washing with alcohol and ether, 

 was called pure. In other instances the Isolation was effected by 

 boiling with 5-6 per cent. sodium hydroxide, or boiling with fum- 

 ing hydrochloric acid. Indeed, instances were found where the 

 keratin structure was destroyed by the action of cold, concentrated, 

 nitric acid. In order to resist these reagents a substance should be 

 almost as stable as the noble metals and, although one usually 

 obtains a colored substance by any of these processes, it is highly 

 improbable that the substance has been isolated in the form in which 

 it occurs in nature. This is especially true of those "pigments" 

 which have been isolated by the action of strong acids. Almost all 

 of the proteins yield a black residue when hydrolized by strong 

 mineral acids and even pure carbohydrates, such as cane sugar, 

 yield a black residue when boiled w'ith strong hydrochloric acid. It 

 is very unfortunate that these black residues have been referred 

 to as melanins, for there is no evidence that they bear the slightest 

 relation to the melanin Compounds, and until such a relation is 

 shown they should not be confused with animal pigments. No such 

 confusion would result if they were referred to under the name of 

 humins. 



