238 Discussion 



the course of growth. A possible explanation of the above-mentioned 

 discrepancy is that the orthophosphate we isolate from the tumour is 

 an artifact and has a lower specific activity than that utilized in the 

 building up of the DNA molecule. If the genuine orthophosphate has 

 about twice the specific activity of the isolated one, the above-mentioned 

 discrepancy clearly disappears. We must furthermore consider the 

 possibility that the precursor of DNAP is not orthophosphate P but 

 the phosphorus of a compound formed in the cell boundary under 

 participation of highly active extracellular P. In experiments carried 

 out on composite homogenates made up from isolated nuclei and isolated 

 cytoplasm fractions from rabbit liver tissue Davidson observed recently 

 that the acid-soluble fraction of the cell sap contains a more effective 

 precursor of DNA than inorganic phosphate. 



Holmes: In one particular series of regenerating livers Dr. Richards 

 found that a number of cells had formed an amount of DNA which 

 brought the content up to an octoploid level. If many cells synthesize 

 this large quantity before they divide, the earlier calculations of the 

 amount of new DNA required for each mitosis must be altered. 



It has been found that the octoploid content of DNA is the usual 

 content in the Ehrlich ascites tumour cell immediately before mitosis 

 (Kelly, L. S., and Jones, H. B. (1956). Fed. Proc, 15, 108). Furthermore, 

 the same has been found to be true of the Krebs ascites tumour cells 

 (Richards, B. M., Walker, P.' M. B., and Deeley, E. M. (1956). Ann. 

 N.Y. Acad. Sci., 63, 831). 



