Biological Effects oj Ionising Radiations 129 



to the square of the dose when the exposure time is constant, 

 and decrease with increasing duration of exposure for all types 

 of X-radiation ; they increase in proportion to the dose and are 

 independent of the duration of exposure (except in so far as this 

 affects the state of development of the cells irradiated) for 

 neutrons and alpha particles. The more densely ionizing radia- 

 tions produce more structural changes of this type per unit dose 

 than X-rays when the dose is small, and fewer when it is large. 



It is interesting to note that we have here an exception to the 

 general rule that, from the biological standpoint, a radiation 

 may be characterized by its ion density. Very soft X-rays, 

 on account of the limited range of the secondary electrons, do 

 not exactly parallel neutrons, even when the ionizing particles 

 generated by these two radiations have the same average ion 

 density as was demonstrated experimentally by Catcheside and 

 Lea.^ 



c. The ratio of the number of certain types of structural 

 change produced by X-rays to the number produced by an equal 

 dose of neutrons varies with the stage of development of the 

 cell at the time of irradiation. 



Comparative Studies with Other Biological Material 



Lethal Ejfect on Drosophila Eggs 



Many experiments have been made to determine the propor- 

 tion of fertilized eggs which hatch after receiving varying doses 

 of radiation. The eggs are usually irradiated about 2 hours 

 after laying, when about 8 mitotic cycles have been completed 

 and the &gg contains above a hundred nuclei. The careful 

 observation of Packard -^ showed that a given dose produced 

 the same degree of mortality whatever the quality of the radia- 

 tion within the X-ray therapeutic range, but this, as we have 

 seen, throws little light on the question of a possible dependence 

 of the efficiency of the radiation on ion density. Packard,-*" Hen- 

 shaw and Francis,^-'' and others, extended the investigations to 

 supervoltage X-rays and gamma rays. It appeared at first that 



