22 Microscopic Histochemistry 



It follows that readings of transmission obtained through 

 a variegated area have a very hmited quantitative signifi- 

 cance, except in the case of reasonably uniform colored par- 

 ticles of good transparency. Such relatively favorable condi- 

 tions obtain, for instance, in thin sections stained by the 

 Bauer-Feulgen method for glycogen,^ as could be shown by 

 the fairly satisfactory agreement between colorimetric read- 

 ings and the results of chemical analysis. The difiiculties 

 and pitfalls of quantitation by absorption colorimetry were 

 adeptly summarized by Click, Engstrom, and Malmstrom.* 



The same principle holds true for semiquantitative judg- 

 ments arrived at from gross inspection. A well-known ex- 

 ample of this is the change in color of the skin of some am- 

 phibians and fish. The same fish may appear almost black or 

 practically white, depending on the state of expansion or 

 contraction of the chromatophores, although the amount of 

 pigment per unit area remains unchanged. 



These simple facts are often ignored in histochemistry. In 

 a semiquantitative way, a reaction is often called intense or 

 the tissue is stated to contain large amounts of a substance 

 if the section shows a widespread reaction. Quantitatively, 

 transmission of areas grossly variegated in black and white 

 is measured by photometers, and conclusions as to the con- 

 centration of substances are drawn from the data.^ This 

 practice is incorrect scientifically. 



For a semiquantitative (accuracy about ±50 per cent) 

 evaluation of microscopic color reaction, model experiments 

 can be used to great advantage. It is likely that under suit- 

 able conditions even true photometric quantitation could be 

 achieved. This has been attempted by Marza and Chiosa^ 



3. Deane, H. W., Nesbett, F. B., and Hastings, A. B.: Proc. Soc. Exper. 

 Biol. & Med., 63:401, 1946. 



4. Click, D., Engstrom, A., and Malmstrom, B. G.: Science, 114:253, 

 1951. 



5. Cleland, K. W.: Proc. Linnean Soc, N.S. Wales, 75:35, 1950. 



6. Marza, V. D., and Chiosa, L.: Bull, d'histol. appliq. a la physiol., 

 12:58, 1935. 



