CYTOCHEMICAL TECHNIQUES FOR NUCLEIC ACIDS 83 



been extremely simple in design; a few have been elaborate, and because of 

 expense, and engineering skill required for their construction and mainte- 

 nance, are available to only a few laboratories. 



Some workers, particularly Pollister and Ris,^^^ have maintained that 

 information of considerable biological importance could be obtained with 

 simple apparatus, readily available to the average laboratory. This view 

 appears to have been fully justified from the large number of papers which 

 have appeared in the last few years, based on measurements from such 

 instruments. It is unfortunately also true that these instruments have been 

 misused by some. This has resulted in the appearance of a certain amount 

 of conflicting data in the literature, and some natural skepticism as to the 

 vahdity of cell photometry as a quantitative method. ^-^'^^^"'^^ 



Space does not permit discussion of the many technical details encoun- 

 tered in the photometry of nucleic acids. Many of the problems associated 

 mth these methods have been adequately treated in the review articles 

 mentioned above. A few points, however, deserve particular emphasis. In 

 many cases adequate calibration methods are much more important than 

 elaborate electronic circuits, and the need for careful testing of instruments 

 cannot be overstressed. In measurements by visible light of Feulgen-stained 

 nuclei, absorption curve analysis is readily carried out^^*''^^ so that amounts 

 of dye bound can be determined in nuclei of markedly irregular shape or 

 chromatin distribution, provided dye intensity is sufficient. ^*° A certain 

 amount of common sense is essential for the photometric study of any 

 tissue. Some nuclei are too pale to give dependable values, or light scatter 

 may be too large. Some nuclei may be too darkly stained and must be 

 measured at wavelengths off the peak absorption. Some are too irregular 

 in outline for accurate volume computations, or the sections may be cut so 

 thin that it is difficult to tell whether a nucleus is whole or cut by the 



" P. A. Cole and F. S. Brackett, Rev. Set. Instr. 11, 419 (1940). 



66 G. I. Lavin, Rev. Sci. Insir. 14, 375 (1943). 



6^ I. Gersh and R. P. Baker, J. Cellular Comp. Phijsiol. 21, 213 (1943). 



68 B. Thorell, Acta Med. Scand. 129, Suppl. 200 (1947). 



69 F. M. Uber, Am. J. Botany 26, 797 (1939). 



'6 A. W. Pollister and M. J. Moses, J. Gen. Physiol. 32, 567 (1949). 



" R. C. Mellors, Science 112, 381 (1950). 



" L. Lison, Acta Anat. 10, 333 (1950). 



" M. J. Moses, Exptl. Cell Research Suppl. 2, 75 (1952). 



'* H. Wyckoff, Lab. Invest. 1, 115 (1952). 



" B. Commoner, Science 110, 31 (1949). 



'6 D. Click, A. Engstrom, and B. G. Malmstrom, Science 114, 253 (1951). 



" H. Naora, Science, 114, 279 (1951). 



" L. Ornstein, Lab. Invest. 1, 250 (1952). 



'9 K. Patau, Chromosotyia 5, 363 (1952). 



8" K. Patau and H. Swift, Chromosoma 6, 149 (1953). 



