256 L. H. Gray 



symposium), but direct observation of the living cell im- 

 mediately after irradiation has not so far yielded any definitive 

 information as to the relative importance of these changes for 

 the initiation of radiobiological damage. Burns (1954) has 

 set limits of —1-7 cc./mole to +2-4 cc./mole to the early 

 chemical changes associated with lethality in haploid yeast, 

 which are probably recessive lethal mutations. This figure 

 may be compared, for example, with changes of — 20 to 

 — 70 cc./mole for the volume change associated with each 

 peptide bond broken in the enzymic hydrolysis of protein. 

 Moreover, positive results were obtained by McElroy (1952) 

 and McElroy and Swanson (1951) for u.v. and nitrogen 

 mustard mutations in Neurospora and Aspergillus. The inter- 

 pretation of the negative results with ionizing radiation should, 

 however, be accepted with caution as far as reactions which 

 take place along the track of the ionizing particle are concerned, 

 since they involve the application of thermodynamic consider- 

 ations to a highly transient and irreversible system. 



The yield of information by the chemical approach is thus 

 very small. It may be that we have not yet examined radiation 

 response in chemical systems at the right level of organization 

 for the important physicochemical changes to be revealed. 



Anatomical Evidence 



The anatomical approach has proved more profitable. A 

 variety of experimental techniques have been brought to 

 bear on this problem. 



1. Tiie use of radiations of limited penetrating power 



Many years ago Zirkle (1932) took advantage of the hmited 

 range of the polonium alpha particle (up to 30 y.) and of the 

 fact that in the fern spore the nucleus (diameter 10 (j.) lies to 

 one side of the protoplast (diameter 38 pi), to compare the 

 results of irradiations which included or excluded the nucleus. 

 The biological criteria were inhibition of cell division. All 

 these effects could be brought about by irradiations from the 



