606 SCIENCE PROGRESS 



by the cell. The evidence for these theories has hardly stood 

 the test of criticism or further experiments, and it may be noted 

 in passing that even though any one of these substances might 

 be shown to suffer decomposition under irradiation it would 

 tell us singularly little of the processes which, once established, 

 lead to the destruction of the cell. 



If little is known of the processes entailed in such gross 

 changes as finally lead to the destruction of the cell, still less 

 is the information yet obtained in the slighter and more subtle 

 changes which occur in the living cell when it is exposed to 

 quantities of radiation less than that required for a lethal effect. 

 The dosage may be graded so that no actual destruction of the 

 cell occurs, yet changes in its activity are induced ; this may 

 be shown by a reduced rate of growth which persists for many 

 generations, without any recognisable histological changes in 

 the cell or in the structure of which it may form a part. 



With still smaller quantities of radiation the boundary line 

 of what we may term " deterrent action " is crossed, and the 

 phenomenon of stimulation makes its appearance. 



It appears therefore that the rays whose action we are 

 considering have the power to influence the cell in various 

 ways, ranging from an actual increase in its general functions 

 to complete inhibition of such. The analogy to the action 

 of certain drugs is somewhat striking. 



In endeavours to formulate a theory of the action of these 

 radiations upon cell life, it seems to us that one fact estab- 

 lished by experiment should be taken into consideration. We 

 refer to the fact that certain cells have been found to be much 

 more vulnerable to radiation when they are in the dividing 

 stage than when they are in the growing stage of their life 

 cycle. This has been observed in ova of Ascaris megalocephala 

 and of Planorbis, two cases where it has been possible to 

 study the whole cycle of development in isolated cells. The 

 question arises as to whether the same fact holds in groups 

 of cells forming a composite structure, as for instance in a 

 malignant tumour. Here we are unable to separate the cells 

 which are in a state of division from those which are in a 

 state of growth, yet this is a question the answer to which 

 bears intimately upon the procedure to employ in the irradia- 

 tion of a malignant mass. 



If we suppose for the purpose of illustrating the point that 



