708 TEXTBOOK OF ZOOLOGY 



cules become dissociated and form new molecules on recombination. 

 This production of ionization is the basic action of roentgen radia- 

 tion on protoplasm. It is thought that the production of an ion-pair 

 in a chromosome produces the alteration or destruction of the gene 

 at that point. Chromatin, especially in stages of synthesis, is ap- 

 parently the most radiosensitive of cell structures. Some investi- 

 gators have theorized a "sensitive volume" for a cell or an or- 

 ganism. This conclusion may be derived from certain analyses of 

 dose-effect curves. Such interpretation of quantitative data also 

 yields ideas that in some instances single "hits" kill a cell whereas 

 others require many "hits" on the sensitive area before being killed. 

 This interpretation of destruction indicates that the amount of 

 radiation absorbed is the only factor involved in the killing action. 

 The "target" theories of radiation action appear to apply best to 

 the killing action on bacteria and protozoa, and to the production 

 of mutations and chromosomal abnormalities. 



Two important objections to the "target" theories, from a bio- 

 logical point of view, are (1) the shape of the dose-effect curves 

 may be explained on the basis of individual variation in sensitivity, 

 and (2) the metabolic condition of the cell at the time of the radia- 

 tion affects the amount of injury produced by the radiation. It 

 therefore appears that ionization is the fundamental action, but that 

 the full expression of the radiation injury depencffe upon the number 

 of vital processes in a susceptible condition (one of molecular inter- 

 changes of energy) at the time the ionization takes place. The 

 importance of the environmetal condition at the time of irradiation 

 is shown, for example, in Fig. 372 where the amount of injury pro- 

 duced w\as different even though the dosage applied was the same 

 for all of the animals. The lower temperature decreases the physi- 

 ological activity of cells and this apparently increases the resistance 

 to the radiation. 



BIOLOGICAL ACTION OF RADIUM 



Unfiltered radium emits three types of radiation: (1) alpha par- 

 ticles, (2) beta particles, and (3) gamma rays. The alpha particles 

 produce dense ionization paths and are very active biologically. 

 HowcA^er, they do not penetrate deeply into tissue, and may be 

 entirely absorbed by a sheet of paper. The beta particles may also 

 be filtered out relatively easily by enclosing the radium in a small 

 tube of platinum or gold. The biological action of beta particles 



