Pathways of Radiobiological Damage 257 



side remote from the nucleus, but only by the use of doses at 

 least twenty times as great as when the nucleus was included 

 in the field of irradiation. Zirkle drew attention to the interest- 

 ing fact that the cracking of the cell wall, which is a function 

 related to water imbibition and to chlorophyll development, 

 which might not normally be thought of as under nuclear 

 control, was evidently initiated by moderate doses of radiation 

 through an injury originating in the nucleus. 



About the same time Henshaw and Henshaw (1933) 

 exposed Drosophila eggs to polonium alpha particles at dif- 

 ferent times after the eggs were laid and found a strong 

 positive correlation between the proportion of eggs prevented 

 from hatching by a given exposure to alpha radiation and the 

 inclusion of nuclei in the irradiation field. The correlation was 

 the more striking because the stage of development which 

 brought the largest number of nuclei into the field of irradia- 

 tion happened to be one of minimum sensitivity to X-rays, 

 which, of course, irradiate the whole egg uniformly. 



Within recent years this type of study has been extended 

 by Pollard (1955) and his colleagues to smaller cells by the use 

 of very slow electrons. In this case specimens have to be 

 irradiated in vacuo. This automatically excludes any form of 

 radiation damage which may proceed from ionization of the 

 aqueous phase and does not measure the inactivating effect 

 of radiation under physiological conditions. On the other 

 hand, the fact that the Yale Group have obtained results with 

 biologically active molecules and viruses under precisely these 

 conditions, which in general accord rather well with studies 

 of the same molecules and viruses irradiated under more 

 natural conditions, may be considered to justify a cautious 

 acceptance of the results obtained with slow electrons. These 

 indicate in the case oi Bacillus suhtilis spores (Hutchinson, 1955) 

 that from the standpoint of viability (colony-forming ability) 

 after irradiation, the spore has a completely insensitive coat 

 about 230 A thick, a body of intermediate sensitivity which is 

 of smaller size but approaches to within 20-30 A units of the 

 surface at one point, and a comparatively sensitive core. 



RAD. 10 



