198 CAROTINOIDS AND RELATED PIGMENTS 



These results have been confirmed by subsequent investigations by 

 others and support the thesis that all animal chromolipoids are derived 

 from the carotinoids of the food, and, either unchanged or slightly 

 modified, are the cause of the yellow to red chromolipoid colors of all 

 species of animals. 



Carotinoid pigmentation of animal tissues is not, however, universal, 

 even among animals whose diet is normally rich in these pigments, as 

 shown by Palmer (1916). This fact offers a very attractive prob- 

 lem for research. Several methods are suggested for attacking it. 



