THE MITOTIC CYCLE 



a known breakdown product of these sugars. This was beheved for a. 

 number of years ahhough certain observations did not accord therewith, 

 notably the unstable nature of the sugar of thymonucleic acid, and the 

 fact that elementary analysis gave an empirical formula for this acid 

 which did not suggest that the molecule contained a hexose. In 1914^ 

 Feulgen^^ ^* drew attention to these discrepancies; three years later 

 he showed that among the products of gentle hydrolysis is a substance 

 which gives the Schiff 's test for an aldehyde, namely the re-appearance 

 of coloured fuchsin in a solution of dye decolorized by sulphur dioxide. 

 This is the basis of the now famous Feulgen reaction, the application of 

 which to microscopical preparations was subsequently described 

 (Feulgen and Rossenbeck^^). In this classical paper these authors 

 apply the test to a number of tissues and organisms, including the wheat 

 germ, from which triticonucleic acid had been prepared (Osborne and 

 Harris^^) similar in nature to that of yeast. They did not therefore 

 expect to find, as they did, that sections of the wheat embryo gave a 

 positive reaction with the test: 



This gave us a great surprise,, for the nuclei of the wheat embryo gave the nucleal 

 reaction more intensively than we have ever seen in any animal tissues. This was 

 confirmed in other plants, and so it was demonstrated that the plant nucleus 

 contains nucleal bodies. The old dualism of yeast and thymonucleic acids is thus 

 set aside. . . . 



It now seems remarkable that it took several years from the publica- 

 tion of this paper before biologists began to make use of the means 

 which had been put into their hands to follow up this demonstration 

 of a fundamental property common to both plant and animal nuclei. 

 At the time, however, it was uncertain whether Feulgen's test related 

 to the sugar of thymonucleic acid or was due to traces of furfural 

 formed by hydrolysis of pentose nucleic acid (Steudel and Peiser^'). 

 Possibly, also there was confusion between Feulgen's nucleal reaction, 

 and the 'plasmal' test for cytoplasmic aldehydes, liberated after a 

 prolonged hydrolysis (Feulgen and Voit^^), which also uses de- 

 colorized fuchsin as a reagent. 



The proof that the carbohydrate of thymonucleic acid is indeed 

 responsible for the Feulgen reaction was given by Levene and London.^* 

 They were able to separate the constituent nucleosides, units consisting 

 of base plus sugar, and later to isolate the latter component itself. 

 Levene et alii^^ proved it to be fi?-2-deoxyribose. By this time, biologists 

 had begun to use the Feulgen reaction (e.g. Cowdry^^). The growth 

 of the ensuing literature followed a sigmoidal curve; in 1938, Milovi- 

 DOW*^ could list over 450 papers in this field. In recent years, discussion 

 of the Feulgen reaction has centred round two points ; one, whether the 

 distribution of the liberated dye within a microscopical preparation 

 indicates the precise localization of the DNA when the cell was alive; 



6 



