ARTICLES 609 



We return now to the fact that a slowing up of the rate 



of growth of the tumour leads to an increase in the ratio of — ; 



n 



although the tumour has increased in size it cannot at once 



be said that both N and n have increased. If, for instance, 



the cells suffered an undue enlargement in size, this for a 



time would simulate growth ; such a possibility can be ruled out 



in the case in question, for no cell enlargement was observed. 



Hence although the rate of growth was diminished, N and n 



both increased (since subsequent growth was continuous), 



but N increased proportionally more than n. 



We are therefore led to the conclusion that a diminution in 

 the rate of cell growth subsequent to irradiation means that 

 some change has been brought about in the processes that 

 govern the growth of the young cell to the mature form. 

 Further, although the dividing cell may be more vulnerable to 

 radiation than the growing cell, this does not show itself in 

 an undue lengthening of the time required for division ; it 

 might even show a curtailment of this period. The time for 

 division is so short in comparison with the time for growth 

 that observations of the kind recorded would not establish 

 with certainty that any such change had occurred. 



The statement that is warranted, however, is that if the 

 rate of growth of the tumour is delayed, it is due to the pro- 

 cesses of growth (as apart from division) having been hindered. 



This fact is not without significance from a bio-chemical 

 standpoint ; with cell destruction and its disintegration 

 products we are not only concerned. Information is much 

 needed as to the changes which are set up in the metabolism 

 of cells after irradiation ; the considerations just set forth 

 show us that with quantities of radiation which produce 

 effects less than lethal, we are essentially concerned with some 

 alteration in the processes which govern the growth (which 

 will include the normal intake and output) of the cell. 



We now return to the consideration put forward on page 607 

 that in the irradiation of a tumour it is desirable to arrange 

 that the majority of the cells forming the tumour shall, for 

 some period during the exposure, be in the act of division. 

 This may be illustrated from the data already given. Sup- 

 pose that a tumour which doubles its volume in four days is 

 exposed to the gamma rays from 100 milligrams of radium 



