CHOICE OF METHODS XXI 



fresh. Neutral red picks out the Islets of Langerhans of the pancreas, pyronin 

 the duct system of the pancreas, naphthol blue the parietal cells of the Stomach 

 and so on. In the same way Nerve Fibers can be marked for subsequent study 

 by vascular perfusion with methylene blue and degenerating nerve fibers in 

 poUomyelitis (and presumably in other conditions) can be sharply differentiated 

 from uninjured ones by the fact that they take up neutral red (Covell, W. P. 

 and O'Leary, J. L., Arch. Neurol. & Psych., 1932, 27, 518-524). It has long 

 been known that the best way to mark renal glomeruli is to perfuse in the same 

 fashion with a dilute solution of janus blue. The glomeruli stand out clearly in 

 the fresh kidney by their deep blue color in a red background (Cowdry, E. V., 

 Contrib. to EmbryoL, Carnegie Institution of Washington, 1918, No. 25, 39- 

 160). A similar selective staining in less brilhant colors is obtainable with janus 

 green. Relatively permanent preparations can be made of some of these 

 specimens. 



The same dyes, and many others, can also be applied in dilute solutions to 

 cells freshly removed from the body and which are still living. Such methods 

 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 they 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. 



