THE BIOLOGICAL ROENTGEN 



85 



x-radiation measured in roentgens, in terms of the percentage of eggs 

 which survived and were subsequently hatched. 



The form of the graph is that of a typical biophysical sigmoid-shaped 

 response curve with its approximate constant slope at the point of inflec- 

 tion. In this restricted region the percentage of eggs surviving per dose 

 administered was nearly constant. The response to the radiation appar- 

 ently differed greatly among similar individuals. Some were killed by 

 small doses yet others remained alive even after exposure to large doses. 

 These and other results also indicated that damage caused by ionizing 



50 100 150 200 250 300 350 400 

 Dose in roentgens 



450 



Fig. II— 1 1 . A typical survival curve of Drosophila eggs after eggs have been sub- 

 jected to the lethal effect of 120-kv x-rays. Fifty per cent of the eggs survive when 

 exposed to 190 roentgens. (By courtesy of C. Packard.) 



radiations was probably traceable to chemical changes in the cell or 

 in its surface structure which manifested themselves by changes in 

 osmotic pressure, because expanded cells were frequently observed 

 after irradiation both of tissue and of microorganisms. 



The experiments were then repeated with gamma-ray doses measured 

 in millicurie-hours. Such an experiment is difficult because the eggs 

 must be exposed at very short distances from the source and the intensity 

 must be uniform over the field occupied by the eggs. Exner and Packard 

 [1935] used highly compressed radon gas in a sufficiently small tube, 

 with platinum walls 0.5 mm thick, so that it could be considered a point 

 source. This was placed at the center of a spherical shell of Bakelite 

 4 mm thick. The eggs were placed on the outer surface of the shell, so 



