XXIV CHOICE OF METHODS 



By a Photoelectric Microphotometer it is possible to estimate quantitatively 

 reactions like that of Feulgen for Thymonucleic Acid which give distinctive 

 colors and numerous stains which are specific for tissue components and can 

 be standardized in their action. But the data obtained are relative, that is 

 it can be said that the reaction is say 60 per cent greater in one specimen than 

 in another. The absolute amount of the component demonstrated per gram 

 of tissue cannot yet be arrived at. 



Several Enzymes (phosphatase, dopa-oxidase, arginase) can now be micro- 

 scopically identified and their position within cells determined. By close com- 

 parison of enz5Tnatic properties with the cellular composition of tissues, the 

 localization of many others can be inferred. 



In the case of these and other microchemical methods the treatment of the 

 tissue after excision and before the special procedures are commenced is of con- 

 sequence. Even in the preparation of routine frozen sections, and much more 

 so when the specimens are fixed, dehydrated, cleared, imbedded and sectioned, 

 there are many opportunities for the loss of chemical substances and of change 

 in their position in the tissue and within cells. The best way to hold the com- 

 ponents in the positions they occupy in the living state is to instantaneously 

 freeze the tissue and dehydrate in vacuum while still frozen, thus avoiding all 

 fixatives, by the Altmann-Gersh technique. Moreover, the reagents used in 

 testing must contact all the tissue equally for unequal contact may well be 

 followed by stronger reactions in some areas than in others. 



Quite recently chemical analysis has been accurately focussed, not merely 

 on cells, but on parts of cells. Nuclei, Mitochondria and many other cellular 

 components including even Chromatin Threads can now be collected en masse 

 by Centrifugation of broken up cells and analysed. This is a departure of con- 

 sequence. 



Finally standard qualitative chemical methods are often appUcable on a 

 microscopic basis. The reader wishing to do so may well consult Chamot, E. 

 E., and Mason, C. W., Handbook of Chemical Microscopy. New York: John 

 Wiley & Sons, 1940, vol. 2, 439 pp. Another book that will be found of service, 

 especially for analysis on microscopic slides, is Benedetti-Pichler, A. A., In- 

 troduction to the Microtechnique of Inorganic Analysis. New York: John 

 Wiley & Sons, 1942, 302 pp. Sometimes one is held up by having to deal with 

 some unfamiliar chemical substance in which case aid may be secured from the 

 large and comprehensive "Dictionary of Organic Compounds" edited by Heil- 

 bron and published in 3 volumes, 1934, 1936 and 1938, by Oxford University 

 Press, New York. No attempt is made in this dictionary to include dyes but 

 thousands of other organic compounds are conveniently arranged in alphabetical 

 order. If the wanted material is some sort of medical preparation seek informa- 

 tion in the following reference books. (1) New and Nonofficial Remedies, 

 1946. Chicago: Am. Med. Assoc, 770 pp.; (2) The National Formulary. 



