4 INTRODUCTION 



Holter, who blazed new trails into the wilderness of cellular 

 chemistry. It has, however, been somewhat unfortunate that 

 many of the lessons which can be learned from the work of 

 Hardy and Fischer and others have been overlooked in more 

 recent studies. 



In the development of rigorous methods of investigation in 

 cytochemistry, it is necessary to look at each problem from 

 three points of view: as a chemist, as a physicist, and as a bi- 

 ologist. From the point of view of the physicist, the main 

 problems arise from diffusion and adsorption artefacts, from 

 estimating errors which may arise from the state of aggregation 

 of the substance which is being studied, from the degree of 

 molecular orientation of the substance, and from the scattering 

 of light within a specimen. No qualitative study can be re- 

 garded as satisfactory which does not involve the elimination 

 of diffusion and adsorption artefacts. Correspondingly, no quan- 

 titative study can be regarded as satisfactory unless the de- 

 gree of aggregation of molecules in the specimen, the orientation 

 of the molecules in the specimen, and the scattering of light 

 within the specimen are properly taken into consideration. 



From the chemist's point of view the main problems are to 

 use methods which are of sufficient specificity and which shall 

 be quantitative and accurate to a known extent. A further prac- 

 tical problem which frequently arises is to find methods which 

 will be to a sufficient degree inert, i.e., which do not damage 

 the specimen. The problem of chemical specificity is a particu- 

 larly difficult one. The reason for this is that chemical reac- 

 tions are not carried out by molecules as a whole. They are 

 usually carried out by a very small number of atoms in a mole- 

 cule. They are often affected to a marked degree by a rather 

 larger number of neighbouring atoms; but the greater part of 

 the molecule may well have no effect on whether a particular 

 reaction occurs or not. Thus the specificity of a chemical re- 

 action is limited to supplying information as to whether a par- 

 ticular chemical group is present in a particular part of a cell. 

 Information as to the nature of some of the other neighbouring 

 groups in the same molecule may sometimes be obtained by 

 studying the rate at which a chemical reaction proceeds. But 

 it is impossible to identify the whole of any molecules, other 

 than the simplest, by carrying out chemical reactions. Thus, in 



