468 JOHN W. G O W E N 



clusters, as, for instance, 7 rays, as contrasted with densely spaced 

 ion clusters found in a rays. This method may be used to supplement 

 other means of estimating the number of the sensitive volumes within 

 an organism. The estimate is subject to considerable error; both 

 square and cube terms are introduced into the calculations. 



Another means of producing biological effects by X rays is that of 

 raising the energy level of an electron in an atom or molecule. In this 

 unstable state chemical reactions may occur that would not occur 

 otherwise. With small molecules excitation is often as effective as 

 ionization, but with large molecules excitation appears to be much 

 less effective. Biological material offers possibilities for both types of 

 reactions. 



X-ray energy differs from ultraviolet energy in the manner in 

 which it is absorbed as well as in the type of effects produced. Ultra- 

 violet energy is absorbed selectively by certain cell constituents, 

 whereas the X-ray energy is distributed at random along ionization 

 tracks but in a more homogeneous manner over the tissue irradiated. 

 The X-ray energy may produce either ionization or excitation, 

 whereas the effect of ultraviolet probably is confined to excitation. 



D. MATERIALS FOR STUDY OF IRRADIATION EFFECTS 



Experience has shown that for the adequate interpretation of X- 

 ray effects it is frequently necessary to follow the exposed cell and 

 its progeny through many cell generations so that various delayed 

 effects of the X rays may appear. For critical quantitative experi- 

 mentation the irradiated material should be composed of single self- 

 reproducing entities. With the wealth of genetic information and 

 the development of the different techniques, Drosophila sperm has 

 become one of the most useful materials for quantitative irradiation 

 work in which the real mechanism of irradiation effects is sought. 

 This material allows observation on the lethal effect of the X rays 

 and also on carry-over effects of radiation to succeeding generations. 

 These effects have been sho^vn to be due to the capacity of the X rays 

 to alter the inheritance units, the genes, and to break and rearrange 

 chromosomes. These effects in Drosophila may be studied by both 

 genetic and cytological methods {26,41). Other material as, for in- 

 stance, corn pollen has equal advantages in that the cell structure is 

 well understood from a cytological viewpoint and each pollen grain is 

 separate and can be followed through as a separate entity. But 



