LATENT PERIOD 35 



for the initiation of mutations. The hit may be supposed to take place 

 in some sensitive region in the chromosome. In the case of Escherichia 

 coli (B. coli) the volume essential to life as estimated from the area in 

 which the hit took place was less than 6 per cent of the bacterium itself 

 (Wyckoff [1930]). This volume is about 0.75 m 3 - It was concluded 

 from this work that the absorption of a single x-ray photon was sufficient 

 to kill an organism, although on the average only about one in twenty 

 of the absorbed photons was effective. 



The biophysical effects of x-rays are due to energy actually absorbed 

 by the tissues. This energy absorption, as previously indicated, does 

 not take place directly, but through secondary processes, i.e., through 

 the production within the tissues of high-velocity electrons and some 

 of lower velocity due to Compton scattering. These electrons give up 

 their energy in producing ionization along their paths (Table 1-8). At 

 this point the study must be taken over by the biochemist, who should 

 explain why, as a result of sufficient ionization thus produced, living 

 tissue of all kinds disintegrates. 



From the experimental evidence available, one may conclude that 

 absorbed photons in the form of x-rays and 7-rays for equal ionization 

 doses produce equal effects. Genetic changes occur in direct proportion 

 to the dose of radiation and are independent of its wavelength. 



The biological effects of radiation are intimately related to ionization 

 phenomena produced in a specific region of the living organism. The 

 same number of ions may, however, be produced in the same region and 

 during the same time interval by ionizing radiations of very different 

 frequencies. In general, the biological effect is probably the same in 

 kind but not in degree. 



The receptor in which this ionization takes place is fundamentally the 

 living cell. A cell, whether it lives as an isolated entity or as a part of an 

 organism, is enclosed in a membrane which permits the passage into the 

 cell of substances necessary to maintain its normal life and excludes 

 others. The adjustment of the semi-permeability of the cell's mem- 

 brane is so delicate that it can differentiate between two such very 

 similar elements as sodium and potassium. The living cell is equipped 

 to maintain the physical and chemical characteristics of its cytoplasm 

 in a very constant state. The nucleus, located inside the cell and acting 

 as the controlling center of the cell, is well protected by its environment. 

 The x-ray energy passing through the cell wall, contents, and nucleus, if 

 absorbed, will start a train of well-developed changes. The changes 

 that occur depend on many factors, but primarily on the magnitude of 

 the dose of radiation. 



The nucleus contains a certain definite number of threadlike structures 



