Organic Substances 117 



anate^^ and later with fluorescein isocyanate.^^ In this way 

 intensely fluorescent carbamido-proteins can be produced, 

 which, if adsorbed on morphological structures, are easily 

 visible under the microscope. 



The method is rather complicated. Fluorescein isocyanate 

 itself is not available on the market but must be synthesized 

 by a laborious procedure; the conjugated antibody must be 

 carefully purified to eliminate its nonspecific components. 

 However, the method has already produced most valuable 

 results in the localization of several antigens^^ ( among others, 

 ACTH)2Mn the tissues. 



Urea 



Urea is precipitated by a solution of xanthydrol in acetic 

 acid as dixanthydryl urea, which forms beautiful rosettes of 

 small needle-shaped crystals. The crystals are insoluble in 

 water and in most organic solvents. 



This reaction is the only really specific test for urea.^^ Un- 

 fortunately, its usefulness in histochemistry is limited because 



( 1 ) the solvent severely damages histological structure and 



(2) the reaction is relatively slow, permitting considerable 

 dilution and displacement of urea before precipitation occurs. 

 For this reason the sensitivity of the reaction is low, and the 

 localization is only approximate. The failure of the test to 



25. Coons, A. H., Creech, H. J., and Jones, R. N.: Proc. Soc. Exper. 

 Biol. & Med., 47:200, 1941. 



26. Coons, A. H., Creech, H. J., Jones, R. N., and Berliner, E.: J. Im- 

 munol., 45:159, 1942. 



27. Coons, A. H., and Kaplan, M. H.: J. Exper. Med., 91:1, 1950; 

 Kaplan, M. H., and Coons, A. H.: J. Exper. Med., 91:15, 1950; Coons, 

 A. H., Snyder, J. C, Cheever, F. S., and Murray, E. S.: J. Exper. Med., 

 91:31, 1950; Hill, A. G. S., Deane, H. W., and Coons, A. H.: J. Exper. 

 Med., 92:35, 1950; Coons, A. H., Kaplan, M. H., and Deane, H. W.: J. Nat. 

 Cancer Inst., 10:1344, 1950. 



28. Marshall, J. M.: J. Exper. Med., 94:21, 1951. 



29. Policard, A.: Compt. rend. Soc. de biol., 78:32, 1915. 



