82 Microscopic Histochemistry 



present locally at any given moment (both the hydrolyzed 

 DNA and the dyestuff are assumed to be soluble and mobile ) 

 should stain the chromosomes intensely and sharply in a 

 matter of a few minutes when a strong solution of the dye 

 prepared in the test tube from hydrolyzed DNA and Schiff's 

 reagent will give only a blurred and weak staining, even on 

 prolonged application. 



Additional proofs for the specificity of the Feulgen stain 

 and of the correctness of its localization are the staining of 

 isolated chromatin threads^^^ and the excellent agreement 

 between the Feulgen reaction and the results of localizing 

 ultraviolet spectrophotometry ( Caspersson ) }^^ 



In summary, it may be said that the Feulgen reaction is a 

 reliable method for the specific localization of DNA, pro- 

 vided that adequate unhydrolyzed and hydrolyzed but alde- 

 hyde-blocked controls are used in the case of doubt. The 

 nature of the positive Feulgen reaction in nervous elements 

 (in unembedded tissue only), reported by Liang^^"^ and 

 Chu,^^^ is not clear. 



The Feulgen reaction can be performed in tissue blocks;^^^ 

 however, the section technique is much more preferable. 



The optimal duration of hydrolysis depends on the type of 

 fixation. In the case of dichromate-free fixatives, the most 

 intense reactions are obtained between 8 and 12 minutes 

 (N HCl, 60°C.). Overhydrolysis, will gradually abohsh the 

 reaction. There is a difference between the resistance of 

 nuclei of various tissues, thymus nuclei being the most re- 

 sistant (up to 30 minutes ).^^^ After dichromate-containing 



105. Claude, A., and Potter, J. S.: J. Exper. Med., 77:345, 1943; Barber, 

 H. N., and Callan, H. G.: Nature, 153:109, 1944. 



106. Caspersson, T.: Nature, 153:499, 1944. 



107. Liang, H. M.: Anat. Rec., 99:511, 1947. 



108. Chu, C. H. U.: Science, 106:70, 1947. 



109. Lhotka, J. F., and Davenport, H. A.: Stain Technol., 22:139, 1947; 

 Lhotka, J. F., and Davenport, H. A.: ibid., 24:127, 1949. 



110. DeLamater, E. D., Mescon, H., and Barger, J. D.: J. Invest. Dermat., 

 14:133, 1950. 



