320 A. C. R. Dean and Sir Cyril Hinshelwood 



there is a retardation of growth followed by a recovery which 

 indicates some active adjustment of the cells. This response 

 fails if the medium is much more acid than pH 6 or if its 

 acidity changes too rapidly, as occurs when the buffer breaks 

 towards the end of growth in a concentrated glucose medium 

 (Eddy and Hinshelwood, 1951). 



5. Cell Division 



Division of the cell may be advanced or delayed in relation 

 to other synthetic processes, and since those themselves vary 

 in rate earlier or later cell division may influence the ratio of 

 the constituents. 



Determination of the nucleic acid content of cells reveals 

 one major tendency, namely for the content of deoxyribo- 

 nucleic acid (DNA) to remain nearly constant in spite of 

 changes of conditions which considerably influence cell size 

 and growth rate (Boivin, Vendrely and Vendrely, 1948; 

 Mirsky and Ris, 1949; Caldwell and Hinshelwood, 1950). 

 This tendency may be strained too far and break down in 

 extreme conditions, but is clearly the reflexion of one major 

 regulating mechanism. There is on the other hand a much 

 greater variability in the ribonucleic acid (RNA) content, and 

 a general tendency is observable for the RNA to increase in 

 amount in conditions giving enhanced growth rate (Caspers- 

 son, 1947; Malmgren and Heden, 1947; Caldwell, Mackor and 

 Hinshelwood, 1950). 



The information stored in the DNA is recognized to be a 

 major factor determining heredity. Since, however, the cell is 

 probably a hierarchial organization, cytoplasmic structures 

 above the level of DNA codes may also have considerable 

 influence on the properties of cells formed by binary fission, 

 and play a part in determining the persistence over several 

 generations of biochemical characters impressed by adaptive 

 processes. 



There is a close linkage between the synthesis of nucleic 

 acids and that of proteins, and a good deal of inconclusive 



