SIZE AND MORPHOLOGY OF BACTERIOPHAGES 45 



of various radiations are known, it is possible to calculate from the 

 37 per cent survival dose the size of the target in which ioniza- 

 tions must be produced in order that inactivation results. A 

 description of the target theory and of the methods of calculating 

 the target size from the 37 per cent survival dose of ionizing 

 radiations is given by Lea (1946). For small viruses the calcu- 

 lated target size is within experimental error the same as the 

 virus size, indicating that a cluster of ionizations anywhere within 

 the virus particle may be lethal. However for larger viruses the 

 target size has been found to be smaller than the size of the virus 

 particle. The larger viruses are heterogeneous in sensitivity 

 containing considerable amounts of material that is relatively 

 insensitive to irradiation. The target size will be a minimum 

 estimate of the virus particle size. When the target size is de- 

 termined using two different kinds of radiation such as X-rays 

 and alpha particles, the results are usually in good agreement in 

 the case of small viruses. However with larger viruses the target 

 size determined from alpha particle data is larger than that 

 calculated from X-ray data. This has been taken to mean that 

 with the larger viruses the target is not a uniformly sensitive 

 sphere as assumed in the theory but is structurally complex. 

 That the particles of the larger viruses are structurally complex is 

 evident from electron micrographs, and the inactivation data 

 extend this complexity even into that portion of the particle which 

 is radiation sensitive. Further applications of the target theory 

 to the study of size and structure of viruses are discussed by Pol- 

 lard (1953, 1954). 



Size of bacteriophages determined with the aid of target 

 theory are included in Table H for comparison with sizes de- 

 termined by other methods. These have been derived from 

 graphs given by Lea (1946) and more recent data. It should be 

 realized that the theory involves arbitrary assumptions that 

 render interpretation doubtful (Watson, 1950). In particular 

 it is assumed that two or more ion pairs are wasted for each 

 lethal ionization within the phage particle, the number depend- 

 ing on target size and kind of radiation. 



