Biological Effects of Penetrating Radiations 97 



gated in insects and plant cells which have a small number of 

 chromosomes of large size, and they are most easily recognized 

 in the metaphase and anaphase of division, at whatever point 

 in the life cycle of the cell the irradiation is given. The practice 

 of scoring abnormal anaphases as a measure of radiation ef- 

 fect ^^' ^^ has the limitation, however, that cells irradiated in 

 premitotic or early mitotic stages may break down altogether in 

 late prophase or early metaphase. Such cells are, therefore, 

 missed in the anaphase count. 



The total number of breaks produced is proportional to the 

 dose and independent of intensity, but neutrons are more efficient 

 in producing breaks than are X-rays.^-- "^^^ ^^^ These observa- 

 tions can be explained on the hypothesis that a chromosome is 

 broken by the passage through it of a single ionizing particle, 

 but that it is necessary for the ionizing particle to be sufficiently 

 densely ionizing for several ionizations to be produced within 

 (or very near) the chromosome. A proton (from neutron ir- 

 radiation) is sufficient; only the "tail" of a fast electron track 

 gives a sufficient number of ionizations in the given volume. 

 On this hypothesis, X-rays of long wave length should be more 

 efifective than those of short wave length, and this has been 

 found to be the case with an optimum at 4A." Longer wave 

 length X-rays produce too short an electron track to span a 

 chromosome, and so their efficiency is diminished. 



Changes in the composition of hereditary particles which lead 

 to gene mutations occur in germ cells of all types, but have been 

 studied most extensively in the case of the fruit-fly, Droso- 

 phila}^' '^^' "'^^ ^^ A dose of 3,000r of X-rays produces a mutation 

 rate of about 12%. This is about one hundred times the natural 

 mutation rate, but qualitatively is indistinguishable from spon- 

 taneously-occurring mutations. The yield of radiation-produced 

 mutations is proportional to dose, independent of intensity, and 

 diminishes for equal doses of different radiations in the order : 

 X-rays, neutrons, alpha rays. It is considered that a mutation 

 in Drosophila is the result of a single ionization. 



All cells are not equally susceptible to the mutational effects 

 of radiation, and other factors, e.g., temperature, anesthesia, 



