Organic Substances 87 



some of them do not actually extract nucleotides but rather 

 alter them sufficiently to abolish their normal staining char- 

 acteristics. Most of the others, however, do break them down 

 to small, soluble fragments which will diflFuse out from the 

 section. 



The efficiency of all extraction procedures greatly depends 

 on the type of fixation. Alcohol-, formalin-, and dichromate- 

 fixed tissues are attacked with increasing difficulty in the or- 

 der mentioned. Resistance to extraction also varies, with the 

 type of tissue. The procedures will be divided in two groups : 

 nonenzymatic and enzymatic. 



a) Nonenzymatic procedures.— Hot water will abolish the 

 staining of DNA by methyl green, while the Feulgen reac- 

 tion remains unchanged. ^^^ Exposure to 5 per cent citric acid 

 at room temperature for 12 hours has the same eflFect.^^^ Ac- 

 cording to Mirsky and Pollister,^^^ nucleoproteins can be ex- 

 tracted from tissues with M NaCl; the use of this method has 

 not been attempted histochemically. 



Strong acids, extract RNA first, and, on longer exposure, 

 DNA is also removed. For instance, the Feulgen type of hy- 

 drolysis (N HCl; 60° C.) removes RNA in about 3-10 min- 

 utes ;^^^ DNA is not completely removed up to 50 minutes. ^*^^ 

 Five to 10 per cent perchloric acid removes RNA in 4-18 

 hours at 5° C. and in about 2 hours at 25° C.^^^ DNA is not 

 completely extracted at the latter temperature in 22 hours; 

 at 70° C. both DNA and RNA are extracted in 20 minutes.^^^ 

 Five per cent trichloroacetic acid also extracts both nucleo- 



133. Brachet, J.: Arch, de biol., 53:207, 1942. 



134. Mirsky, A. E., and Pollister, A. W.: Proc. Nat. Acad. Sc, 28:344, 

 1942. 



135. Deane, H. V^.: Am. J. Anat., 78:227, 1946; Vendrely, R., and 

 Lipardy, J.: Compt. rend. Acad, sc, 223:342, 1946; Vendrely-Randavel, C: 

 Compt. rend. Soc. de biol., 143:294, 1949. 



136. Ogur, M., and Rosen, G.: Fed. Proc, 8:234, 1949. 



137. Erickson, R. O., Sax, K. B., and Ogur, M.: Science, 110:472, 1949; 

 Seshachar, B. R., and Flick, E. W.: Science, 110:659, 1949; Sulkin, N. M., 

 and Kuntz, A.: Proc. Soc Exper. Biol. & Med., 73:413, 1950; Koenig, H.: 

 J. Nat. Cancer Inst., 10:1346, 1950. 



