STUDIES UTILIZING DISINTEGRATION 103 



sufficient for the emitted rays to have darkened the film adequately. 

 The film is then developed, and the material is studied with the micro- 

 scope. If developed grains occur over a particular part of the cell, it 

 follows that the isotope is located in that part. It is necessary to focus 

 alternately on the material and on the film to make the identification. 



This technique was used by Taylor in a study of the fate of the 

 chromosome materials during cell division. Taylor fed onion root tips 

 with tritium-labeled thymidine for a while, and then stopped further 

 detectable labeling by adding a large excess of unlabeled thymidine. The 

 cells were allowed to go through controlled numbers of divisions, and 

 after each division, the roots were sectioned and radioautographs ob- 

 tained. It was found that grains appeared over both chromatids at 

 the beginning of the experiment. After all rounds of cell division, 

 chromosomes usually contained either one labeled and one unlabeled chro- 

 matid, or else completely unlabeled chromatids. There was some chromo- 

 some breakage, and in such instances the part of one chromatid which 

 was unlabeled was matched precisely with an equal length of labeled 

 sister chromatid. This experiment shows directly the doubled nature of 

 the chromosome and that the chromatids are retained intact during 

 mitosis except for some occasional breakage and reunion. 



By similar methods it has been demonstrated that strontium is de- 

 posited almost entirely in bone, while other atoms, such as phosphorus, 

 are much more generally deposited. The very high localization of iodine 

 in the thyroid has been amply confirmed by these techniques. Localiza- 

 tions of this kind are extremely useful clinically. For example, the 

 administration of radioactive strontium could be of use in treatment of 

 bone diseases, and treatment of thyroid disorders by giving radioactive 

 iodine is so standard that thyroidectomy is now very infrequent. 



Radioautography is limited for several reasons, the most important of 

 which is the limitation on the resolution due to the finite size of the 

 grains in the photographic emulsion and to the fact that the radiations 

 are emitted in all directions, so that to each source of radiation there is 

 a much larger zone of blackening of the film. 



3. Studies utilizing the disintegration itself 



We have already mentioned the use of localization of various atoms in 

 the clinical treatment of disease. There has been important use of dis- 

 integration in fundamental biological studies. One of the most significant 

 series of studies has been on the effect of P 32 decay on the course of 

 virus infection. It was first shown that P 32 -labeled phages, kept in a 

 refrigerator, exhibited a progressive decrease of viable phages. By plac- 

 ing unlabeled phages in a solution of P 32 which would give them about 



