Organic Substances 133 



II. The exogenous pigments are mainly carbon and various 

 metals or metal-protein compounds, possibly also sulfides. 

 Carbon can be recognized by its opaque blackness and by 

 its absolute resistance to all bleaching agents and solvents. 

 Metallic pigments have been discussed in the section on 

 "Metalhc elements" (p. 29). 



III. The endogenous pigments fall into several classes ac- 

 cording to their origin and chemical nature. Some of the 

 individual pigments are chemically well-defined entities; 

 others are a conglomeration of a number of related sub- 

 stances which probably represent successive stages in the 

 evolution of an ultimate colored substance. The most im- 

 portant endogenous pigments will be divided into three 

 groups : 



1. Hematogenous pigments include hemoglobin and its 

 degradation products, some of which contain ferric iron and 

 some of which are iron-free. 



2. Phenolic pigments are formed by the oxidation and 

 polymerization of catechol and hydroquinone (possibly also 

 tyrosine ) derivatives. 



3. Lipogenous pigments result from the oxidation and 

 polymerization of unsaturated fatty acids. 



Other chemically well-defined pigments, such as porphy- 

 rins and carotenoids, very seldom occur in the tissues of 

 higher species in concentrations high enough to be visible 

 under the microscope. Crustacean tissues, on the other hand, 

 may be very rich in carotenoids. 



1) Hematogenous pigments.— 



A) Iron pigments.— a) Hemoglobin is demonstrated by its 

 peroxidase action (p. 162). It should be remarked that un- 

 altered hemoglobin is best identified by the zinc-leuco 

 method, but its immediate degradation products, such as 

 those seen in renal tubules 1 or 2 days after a hemolytic reac- 

 tion, may not stain at all. They can be demonstrated by the 

 benzidine method. It can be shown in test-tube experiments 

 that the recolorization of zinc-leuco dyes by peroxide is a 

 true enzymatic reaction requiring hemoglobin itself, whereas 



