10 INTRODUCTION 



three. It is thus quite misleading to suppose that all the 

 granules must have all three of these substances, as has been 

 deduced from the procedure of isolation of granules. 



It should also be emphasized here that the phenomenon of 

 lack of biological identity of cells, even of the same cell type, 

 illustrates a general weakness of deductions drawn from bio- 

 chemical studies of extracts. As a general rule, no tissue can be 

 regarded as biologically homogeneous, and deductions made on 

 the biological level after disintegration of tissues must normally 

 be treated with considerable reserve. Only in the case of rela- 

 tively homogeneous cell preparations, such as those of yeast, 

 bacteria, ova and sperm, can such studies be regarded as re- 

 ferring to a single chemically homogeneous cell population. 



In maceration experiments, it is also inevitable that activa- 

 tion and inactivation of enzymes will occur to a degree which 

 may often present formidable difficulties. How this problem 

 can be coped with is not at all clear. There is also the difficulty 

 described by Marjorie Stevenson, that one is tempted to sup- 

 pose that all enzymes found in a cell must have a function. Dr. 

 Stevenson was inclined to think this must be the case until she 

 found an enzyme in bacteria which could act upon chlorate. She 

 could not believe that this activity could be of any value to the 

 cell since chlorate never appears in its normal environment. 

 One hesitates to agree entirely with this argument: it may well 

 be that the enzyme which acts upon chlorate also acts upon 

 some other substrate which is normally found in the environ- 

 ment, or which from time to time occurs in the environment. 

 And another difficulty in the interpretation of enzyme activities 

 is to know whether any individual enzyme is functioning in con- 

 nection with a particular physiological activity which is under 

 observation. Indeed, it does not necessarily follow that all the 

 enzymes found in a cell are necessarily functioning at the same 

 time. It seems quite possible that some enzymes may be pres- 

 ent in cells at all times, but have a function to fulfill only at 

 exceptional periods in the life of the cell. 



When all these experimental hazards and difficulties of inter- 

 pretation are considered, it must be clear that it will be a long 

 time yet before the results of maceration procedures can be 

 either evaluated or interpreted. 



