BIOCHEMICAL STUDIES 121 



(b) Most of the work on fixed cells has involved the use of 

 fixatives such as acetic alcohol. As indicated in Chapter 2, such 

 fixation may result in serious diffusion artefacts, and it is hardly 

 profitable to consider results on tissues which have been prepared 

 other than by freeze-drying. 



(c) In the interpretation of cytochemical results the data 

 which are available show, at best, only that at certain times in 

 the life of a cell there are high concentrations of proteins and 

 other compounds at certain sites in the cell. There is no infor- 

 mation available as to where these compounds come from, where 

 they are synthesized, and often there is little information as to 

 what their ultimate fate is within the cell. Caspersson has built 

 up a fairly elaborate system based on the assumption that all the 

 substances concerned move down their concentration gradients. 

 This, however, is a quite unsatisfactory hypothesis, for at least 

 two reasons. First, it is just as characteristic of living cells that 

 substances should move up, rather than down, concentration 

 gradients. Secondly, there can be no question of the substances 

 diffusing down the concentration gradients in the cell: if thermal 

 diffusion were concerned, all the substances in a cell would be 

 uniformly distributed over cells in a few seconds. Thus it is 

 clear that, since no such uniform distribution exists with proteins 

 and nucleic acids, these substances are not free to diffuse, and 

 consequently one cannot tell whether concentration gradients in 

 these substances have any relevance to their future distribution 

 in a cell. 



We are thus bound to conclude that, although much recent 

 work, especially that of Caspersson and his school and the stud- 

 ies of phage, is most stimulating and provides much valuable 

 evidence, the fact remains that the sites of synthesis of protein 

 within the cell are as yet unknown, and that the exact relation- 

 ship of nucleic acids to protein synthesis is also unknown. To 

 emphasize this point I wish to conclude this chapter by bringing 

 forward two quite independent alternative theories of the rela- 

 tionship of nucleic acid to protein synthesis, neither of which in- 

 volves direct participation of the nucleic acids in the formation 

 of the chemical bonds of peptide chains. 



That there is a marked degree of species specificity in the bio- 

 logical action of nucleic acids was clearly demonstrated by the 

 experiments of Avery, McLeod, and McCarty, in which it was 



