THE FUTURE OUTLOOK 135 



actions are quantitative is adequately explored, and that more 

 attention is paid to losses of light by scattering, etc. 



The biological problems which can be studied by cytochemical 

 methods are already numerous, and beyond doubt the number 

 will increase rapidly. A very profitable field to survey will be 

 that of finding the relationship between purified enzymes and 

 intracellular enzymes. To a considerable degree the biologist 

 must look upon purified enzymes as artefacts, and, as indicated 

 in the chapter on phosphatase, to proceed from knowledge of 

 purified enzymes to knowledge of their biological function may 

 be a long and difficult journey. It is certainly a journey which 

 can be shortened by cytochemical studies. Valuable examina- 

 tions will also be made of the extent to which the biochemical 

 theories of metabolic pathways are valid. At present almost 

 all the evidence on the nature of metabolic pathways has been 

 built up by in vitro studies of enzyme systems and by a limited 

 number of genetical studies. Some degree of confirmation of 

 these theories is being obtained by the use of isotopes, but it is 

 doubtful if isolation of isotope-tagged compounds from living 

 tissues will finally solve many problems. The chemist who can- 

 not think of a variety of alternative relationships between the 

 members of a given set of tagged intermediates is unusually lack- 

 ing in ingenuity. Cytochemical studies, by revealing the distri- 

 bution of enzymes and substrates in the different phases of cell 

 activity, can contribute greatly to this field. 



The biochemistry of tissues is at present a very rough-and- 

 ready affair. The histologist knows that all tissues are com- 

 pounded of a wide variety of cells, all of which must be indis- 

 criminately ground up together in the formation of "acetone-ex- 

 tracted powders," homogenates, breis, and microsome suspensions, 

 etc. The cytochemist knows not only that these various cells 

 are chemically different but also that even cells of the same type 

 may, in different parts of an organ, be widely different. For ex- 

 ample, the hepatic cells close to the central vein in a hepatic 

 unit may differ widely from the more peripheral cells in their 

 content of nucleic acid, phosphatase, fat, and glycogen, and so 

 they may be in markedly different physico-chemical states. The 

 combination of the present undiscriminating approach of the bio- 

 chemist and the more perceptive but undeveloped approach of the 

 cytologist should be richly rewarding. 



