CHOICE OF METHODS 7 



have become very popular in hematology. However, the cells thus colored live 

 only for a limited time and it is important to cut short the observations before 

 they are vitiated by approaching death. 



It is feasible to employ a wide variety of Tracer Techniques, that is substances 

 can be traced through the body by the markings given to them. The largest 

 group is made up of Radioactive Isotopes. Because of their radioactivity they, 

 and substances in which thej^ are chemically combined, can be quantitatively 

 measured by a Geiger Counter. Wherever they go in the body, they are ap- 

 parently accepted by the tissues and play their roles in metabolism in the same 

 way as if they were not radioactive. Thus Radiocalcium is found to be stored 

 almost entirely in bone and the amount taken in in a given time is an indication 

 of the amount of nonradioactive calcium given out in conditions in which the 

 total amount of calcium is not changed. The turnover of calcium can therefore 

 be estimated. Radioiodine tends to be stored in the thyroid, and, again, when 

 the total amount of iodine does not change, the amount stored in a given time 

 balances the amount lost and is a measure of the iodine replacement. 



By the technique of Autoradiography the exact location of the radioelements 

 can be determined by holding a section of the tissue in contact with a photo- 

 graphic film. The images on fine grained films can then be magnified. Con- 

 sequently, by selection of radioelements based on information as to where they 

 are stored in largest amounts and by their use, heavy radiation can be brought 

 to bear upon several kinds of tissues leaving others influenced but little or not at 

 all. An excellent account of Isotopes in Nutrition Research is given in Borden's 

 Review of Nutrition Research, 1945, 6, Nos. 8 and 9. 



6. To Employ Culture Methods 



The common feature in these techniques is to plant cells, tissues or organisms 

 in new and different fluid environments and to observe their behavior therein. 



Thus cells can be grown in Tissue Cultures of chemical composition suited to 

 their requirements. Mixed cultures are those containing se^'eral types of cells 

 and pure cultures those containing but one sort. This technique affords un- 

 rivalled opportunities for experimentally changing the fluid environments of 

 cells, for the study of nutritional factors, growth stimulating and growth in- 

 hibiting factors, and the influence of cells on one another. Individual cells can 

 be observed at high magnification and the phenomena of motility, phagocytosis, 

 mitosis, cell death, etc. can be recorded by moving pictures so that the analysis 

 of form and function is possible with a high degree of accuracy. 



The limitation of the method is the obvious one that the fluid environments 

 are artificial and must be changed at intervals to keep the strains of cells alive. 

 Consequently tissue cultures are unsatisfactory for the investigation of inter- 

 cellular materials, like fibers, hyaline deposits and so on. Moreover the cells 

 cannot properly organize to form tissues and organs as they do in vivo since they 

 are isolated from normal influences by other tissues of the body. But they 



