322 OSCAR RIDDLE. 



oxidizing enzyme (tyrosinase) in the normal development of the 

 dark pigment of the integuments of living, growing animals (fly- 

 larvae Lucilia Ccesar}. At the same time he was able to prove 

 that, in the forms with which he worked, free oxygen is also an 

 indispensable factor in the development of the color. This work, 

 important and suggestive as it was then, is now made still more 

 valuable by new knowledge of the chromogen that is, the other 

 factor involved in the pigment formation. Without knowing 

 just what this chromogen might be, Dewitz was able to conclude 

 (p. 45), " We cannot doubt that we have here in the blood of the 

 larvae an enzyme under the influence of which a chromogen is 

 oxidized and forms a brown or black pigment." 



A year later Gessard ('03^) was able to show that in the 

 melanotic tumors of white horses not only tyrosinase but free 

 tyrosin is present. He concludes (p. 1088) : " Tyrosin is then 

 the chromogen, the oxidation of which by tyrosinase determines 

 the formation of the black pigment which is common to many 

 physiological and pathological products of the animal economy ; 

 and it can be said that the color of the negro is due to the same 

 reaction that produces the ink of the squid, or the black color of 

 some mushrooms." Gessard states, too, that when tyrosin is 

 oxidized with tyrosinase it gives a series of colors "rose, 

 rouge-grenat et brune." In a later work ('O3</) he made a 

 closer study of the color reactions of tyrosin in which he showed 

 that the presence of acids, alkalis and salts have marked effects 

 on the colors produced. 



The recent work (May, 1908) of Bertrand, is, however, of the 

 highest interest. He has been able to determine (i) the type of 

 substance of w/iic/t there are many representatives which can 

 by the use of tyrosinase be oxidised to melanin compounds ; (2) he 

 has shown that each one of these compounds passes through a series 

 of colors before arriving at the final stage of oxidation ; (3) that 

 this series varies somewhat as to the exact tint of the initial and 

 final colors, but that (4) the early stages of oxidation uniformly 

 give lighter colors than the later ones, the series usually running 

 from yellow to orange, through darker tints to brown or black. 



Bertrand's studies make it clear that any benzene nucleus with an 

 attached hydroxyl can be acted upon by tyrosinase and converted 



