THE NUCLEIC ACIDS 



first it was not clear whether nuclein or protamine was the source of 

 these bases. Kossel^" had by 1881 inclined more to the former opinion, 

 and continued to work on these substances from this point of view. In 

 1889, however, Altmann^^ first described a method for preparing 

 nucleic acids which were free from protein, from both animal tissues 

 and yeast, and within a few years Kossel and Neumann^^ were able to 

 isolate thymonucleic acid from the thymus gland. These advances in 

 the study of the structure of the nucleic acids did not lead to any 

 immediate comparable progress in histochemistry. By means of double 

 staining methods, Lilienfeld^^ attempted to distinguish between 

 nucleoproteins and free nucleic acid in the nuclei of resting and dividing 

 cells, but similar efforts by Heine^* led this author to the conclusion 

 that such a distinction could not be made by the staining methods then 

 available. When Fischer^ ^ wrote his classical monograph on the 

 Fixierung, Fdrbung und Bau des Protoplasmas the subject of biological 

 staining was already in the complex state in which it has largely 

 remained. In this work, he discussed the influence on the results 

 obtained of such factors as the nature of the fixative, and the order in 

 which stains are applied, both with tissue sections and with films of 

 proteins and nucleic acids. It was recognized that methyl green has a 

 special affinity for thymonucleic acid, although Fischer pointed out 

 that proteins, when fixed by heavy metal salts, can also absorb this 

 stain. In a series of papers at this time (Mann^^), the question was 

 debated whether the affinity for dyes depends upon chemical factors 

 or whether physical effects are alone involved. In the end, future 

 progress depended not only on the further development of micro- 

 chemical staining methods, but also on the use of specific enzyme 

 preparations on tissue sections under the microscope. The first histo- 

 chemical researches which related to the nucleoclastic enzymes were 

 studies on autolysis. Such experiments indicate the presence of tissue 

 ferments only when conducted in the presence of antiseptics, which 

 Salkowski^' first used for this purpose. Zaleski^^ demonstrated that 

 purine bases were liberated on prolonged autolysis of plant material at 

 30° C. while Oes^^ ^^ showed that a few hours' incubation under these 

 conditions was sufficient to break up and dissolve the chromosomes in 

 dividing cells of both plants and animals. 



The investigations of van Herwerden^^ ^^ differed in both methods 

 and results. By means of the method described by Sachs^^ a nuclease 

 was prepared from beef pancreas or spleen. The expressed juice of these 

 organs was precipitated with saturated ammonium sulphate, and the 

 washed product was subsequently dissolved in water and dialysed. 

 Sections of echinoderm eggs were incubated for 24 hours in this 

 preparation, and were then stained in haemalum. On comparing them 

 with control sections incubated in water, van Herwerden found that 





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