Biological Effects of Penetrating Radiations 95 



are often at variance when the response of one material is com- 

 pared with that of another. Each result has to be considered by 

 itself. The contrast is most marked when the results of irradiat- 

 ing independent biological units, such as bacteria or insect eggs, 

 are compared with those of an organized colony of cells which 

 make up a body tissue. This is hardly surprising, since in the 

 one case radiation acts on single units without any biological 

 spread of effect to adjacent units, and in the other it acts upon 

 cells capable of being further influenced by changes brought a])out 

 in adjacent cells. However nearly the radiosensitivity of the 

 indicator approaches that of the body cells (one of Holthusen's 

 stipulations for the ideal test object) it is unlikely to give the 

 same information as would be obtained from direct observations 

 on the body cell. This is the limitation which restricts the use- 

 fulness of most of the indicators listed above. Tissue cultures 

 constitute a special case, since the technique enables samples 

 to be taken from the body (before or after radiation), and obser- 

 vations or experiments to be made under the relatively simple 

 conditions of growth in vitro for direct comparison with changes 

 seen in similar tissue in vivo after similar irradiation treat- 

 ment.^-* An intermediate step is thus provided between the 

 simplicity which is the essence of laboratory experiment, and the 

 complexity of irradiation of organized tissues in vivo, which is 

 a very useful guide in comparative investigations. 



Genetic Effects of Radiation 



The demonstration by Muller ^^' ^- and shortly after by Stad- 

 ler ^^^ that X-rays could produce ^ene mutations in Drosophila 

 and barley excited geneticists throughout the world to take the 

 keenest interest in this property of radiation ; X-rays immediately 

 became their most important tool for producing mutations. An 

 extensive literature bears witness to the enthusiasm aroused by 

 this discovery, which has opened up a new and large field of 

 research.^' i^' ^^' '^•* The sterilizing effects of X-rays were dis- 

 covered nearly a generation earlier,^ and much fundamental work 

 on the results of irradiating genetical material was completed 



