244 CAROTINOIDS AND RELATED PIGMENTS 



would undoubtedly disintegrate animal tissues before the pigment 

 could crystallize out. It would be well worth the effort, however, 

 if a microchemical crystallization method could be devised which 

 would be applicable to animal tissues. 



The conception of a constant association of pigment with fat which 

 is suggested by the term lipochrome was no doubt in a measure re- 

 sponsible for the introduction of fat stains for demonstrating the 

 presence of such pigments in animal tissues. The writer has not 

 made a thorough study of the history of the use of this technic, since 

 the matter is not of great importance. It is, therefore, a little dif- 

 ficult to state whether the use of fat stains began with the idea of dem- 

 onstrating that a pigment in question was actually associated with fat 

 and therefore a true "lipochrome," or whether their use was suggested 

 solely by the term itself or by the statements encountered here and 

 there in the literature on plant lipochromes that one of the fat stains 

 (usually osmic acid) imparted its characteristic color to the pigment. 

 Whatever the origin of their use may have been it is obvious that the 

 present conception of the action of the fat stains, as shown by con- 

 sulting the modern handbooks on biochemistry and pathology, is that 

 they stain the pigments themselves. For example, Wells (1918) 

 states that the lipochromes "are characterized by staining by such 

 fat stains as Sudan III and Scarlet Red, and usually, but not con- 

 stantly, by osmic acid"; and Herxheimer (1913) makes practically 

 the same statement, without, however, making any reservations with 

 respect to osmic acid. 



As far as the true carotinoids are concerned this conception rests 

 upon the uncertain assumption that these pigments are actually 

 stained by such dyes as Sudan III, Scarlet Red and osmic acid. It 

 is possible that such is the case, but unfortunately the matter has 

 never been subjected to an experimental study; and until we have 

 further proof of the action of these and other fat dyes upon the 

 pure pigments it is not possible to state definitely that a positive stain 

 with a fat dye is a positive test for pigment of the carotinoid (lipo- 

 chrome) type. In fact, there is evidence which indicates that a posi- 

 tive stain with a fat dye is merely a test for the lipoid with which 

 the pigment is associated. 



Neumann (1902) states that when the fat cells of the bone marrow 

 and sex glands of frogs have become completely atrophied through 

 inanition (or during hibernation) the lipochrome which remains no 

 longer takes the osmic acid stain, but still gives the reaction with 



