212 UADIATIO.N HlOLOGV 



the optical coiKlitioiis within the tissue arc tlicii more i'ax'orablc loi' tlu-.so. 

 measurcnuMits. This aclvaiitufj;c would appear to ho somewhat olTset by 

 the confusion which is introduced into the absorption picture hy the 

 probable presence, in unextracted material, of many absorbinji, com- 

 pounds of low molecular wci<>;lit, a situation like that which makes the 

 results of ultraviolet absorption stud}'' of li\iiiji; cells so inconclusive. 



When precipitation or a color reaction on the microscopic section 

 necessarily invoh'es use of a(|ueous reagents, then the frozen-dried sec- 

 tions must be rehydrated. This in effect appears to eliminate about every 

 advantage of the freeze-drying technicjue because extensive redistribution 

 can take place at once and can continue dining any subsecjuent steps of 

 the technifjue (Iloerr, 1943). Unless these technical difficulties can be 

 overcome, the sound view seems to be to admit that, except for micro- 

 incineration and X-ray absorption, precise intracellular localization is 

 practically limited to substances of high molecular weight, which diffuse 

 slowly and are easily converted into relatively insoluble masses, and to 

 the use of tests which demonstrate the smaller chemical groupings when 

 they are parts of these large molecules. Two substances of this charac- 

 ter, proteins and nucleic acids, bulk large in the composition of all cells. 

 Indeed, it is easy to see that, if these two constituents are removed, as 

 can be done with enzymes, the remainder is but an unrecognizable ghost 

 of a cell. As a matter of fact, the whole concept of the fixed cell is mainly 

 that of a nucleoprotein mass. Except for some of the special mito- 

 chondrial methods which preserve lipids, the vast majority of cellular 

 studies have been made on cells fixed in strongly acid fluids which, while 

 precipitating admirably proteins and nucleic acids, at the same time must 

 wash out smaller unattached cellular constituents, organic or inorganic, 

 to such an extent that the residue can hardly be great enough to be cyto- 

 logically detectable (see Pollister, 1952a). This predominance of nucleo- 

 protein in cellular composition has always been so obvious that it is possi- 

 ble to overlook its significance to cytochemistry. For example, in 

 Lison's Histochemie animale (1936) this point is not stressed, and there is a 

 distinct impression that the primary concern of histochemists should l)e 

 the localization of simple chemicals such as iron and amino acids. 



Whatever the special objectives of a study of intracellular localization, 

 a very obvious fact about cellular composition is that, in all cells, proteins 

 and nucleic acids occur in high enough concentration so that tests for 

 them fall within the visible, or measurable, range, and consequently tech- 

 nifiues for nucleoprotein demonstration are applicable to a great variety 

 of problems. 



3-2. NUCLEIC ACID STAINING AND TESTS 



Although the major features of morphology are distinct in living cells 

 and in fixed uncolored cells, the bulk of cytological researches have been 



