566 FRED M. UBER 



that the radiation itself may be a distinctly harmful factor biologi- 

 cally. This may well be true of genetic experiments or where one is 

 dealing with cells that are actively dividing and therefore relatively 

 sensitive to radiation. In many studies with tracers, the concentra- 

 tion of radioactive material need not be so high that great concern 

 must be felt about the biological effects of the radiation emitted. 

 Nevertheless, the matter must always be considered in relation to 

 each individual experiment and a decision made on the merits of the 

 several factors involved. 



The technical skills involved in concentration measurements of 

 both stable and radioactive isotopes have been discussed elsewhere. 

 It will be noted that measurements on stable isotopes normally re- 

 quire a qualitative extraction of the biological material so that test 

 samples of the various constituents can be obtained in a high degree 

 of purity. While this is often true of radioactive isotopes also, there 

 are some instances in which the path of transportation or the site of 

 deposition of radioactive isotopes may be studied in a living organism 

 without resorting to any chemical segregation of the material. Ob- 

 viously this represents a time-saving simplification, which may well 

 weight a decision in favor of a radioactive isotope for a given type of 

 study. 



Permissible dilution factors for the various stable isotopes have a 

 limiting value in the neighborhood of 10,000. This is adequate, of 

 course, for many investigations. However, the principal advantage of 

 the radioactive over the stable isotopes lies in the fact that a much 

 greater dilution of radioactive isotopes can be tolerated. Dilution 

 factors as great as 1,000,000 or even 10,000,000 lie within the realm 

 of possibility. Clearly this will extend the use of radioactive tracers 

 to the investigation of compounds that occur in minute quantities 

 only. Before arriving at a decision as to which type of isotope to em- 

 ploy in a given experiment, a careful estimate should be made of the 

 dilution to be expected. 



B. POTENTIAL RESEARCH APPLICATIONS 



1. Diffusion and Transport Studies 



The transport of nutrient materials, both organic and inorganic, 

 in living organisms has been studied for many years by a great many 

 ingenious methods. Most of these earlier techniques have been out- 



