Organic Substances 71 



amino groups. Such substances with multiple acidic group- 

 ings are the chromotropic substances. Depending on the 

 strength of the acidic groups, the degree of metachromasia 

 they produce may vary considerably. Some types of meta- 

 chromasia can exist only in an aqueous medium and at a neu- 

 tral or slightly acid reaction ( desoxyribose nucleic acid); 

 others will resist acid down to pH 3 and mounting in glycerol 

 (some connective-tissue polysaccharides). The extreme de- 

 gree is resistant both to acid and to dehydration by acetone or 

 alcohol, although the latter agents almost invariably decrease 

 the color contrast. Lison^^' ^^ has shown that the alcohol-re- 

 sistant form of metachromasia ( "true" metachromasia ) is due 

 to the presence of sulfuric esters of large molecular size. Until 

 recently the only known substances occurring in animal tis- 

 sues and exhibiting true metachromasia actually were such 

 sulfuric esters (heparin, chondroitin-, and mucoitin-sulfuric 

 acids ) . However, in 1940 Bignardi^^ showed that, if glycogen 

 is treated with chromic acid until it becomes negative by the 

 iodine stain and negative or weakly positive by Schiff's reac- 

 tion (more than 3 hours' exposure to 4 per cent chromic 

 acid ) , it will exhibit true metachromasia. Francini^^ was able 

 to show the same change in the case of starch. Glycogen 

 made metachromatic by long chromation also strains in- 

 tensely with mucicarmine. The intestine of some species 

 (e.g., the hedgehog) contains both mucous and mucoid cells 

 ( the latter stain poorly with mucicarmine and are not meta- 

 chromatic ) . It is a highly interesting fact that, if sections of 

 such a tissue are subjected to prolonged chromation, the 

 mucinous cells will lose their typical staining properties 

 while the mucoid cells become metachromatic and muci- 

 carmine-positive.^^ The explanation of these findings is not 



62. Lison, L.: Compt. rend. Soc. de biol., 118:821, 1935. 



63. Bignardi, C: Atti Soc. ital. sc. nat, 79:85, 1940. 



64. Francini, E.: Nuovo gior. bot. ital., 47:531, 1940. 



65. Bignardi, C: Boll, di zool, 10:219, 1939; Bignardi, C: Atti Soc. nat. 

 e mat. Modena, 71:59, 1940. 



