INTRODUCTION 5 



the well-known Feulgen reaction which is commonly said to 

 give localization of deoxyribonucleic acid, we have in fact a 

 method which probably indicates a linkage between deoxy sugar 

 and any other group which one can split off from the glycoside 

 linkage at roughly the same rate as are the purines. Thus the 

 Feulgen reaction, in fact, gives us information which is limited 

 to telling us that deoxy sugar is present in the specimen and 

 that it is in glycosidic linkage with a substance which can be 

 split away by acid hydrolysis at the same rate as are certain 

 purines. The Feulgen reaction does not tell us whether the 

 substance which is split off is a purine, nor does it tell us 

 whether the sugar is linked through phosphate bonds to other 

 similar units so as to complete a nucleic acid. In a similar way, 

 when the ultraviolet spectrum of a specimen is studied, one 

 can readily detect the presence of a substance absorbing in the 

 same region of the spectrum as the purines and pyrimidines. 

 There are, of course, other substances which absorb in this 

 region. A notable example, recently reported by Chayen (1952), 

 is ascorbic acid. Thus, in a material in which ascorbic acid 

 may occur, it is necessary to take steps to differentiate between 

 ascorbic acid and purine groups. When all the necessary elimi- 

 nation of this type has been done, we can perhaps be certain 

 that in a particular part of a specimen there is purine or 

 pyrimidine. But it has not so far proved possible from spectro- 

 photometric studies to determine whether the purine or pyrimi- 

 dine is present as part of a nucleic acid molecule, or whether it 

 is present in some other form, e.g., linked directly to a protein. 

 It is notable in this connection that Panijel has recently found 

 a protein in Ascaris sperm which has the same absorption spec- 

 trum as nucleic acid, containing a considerable proportion of 

 purine, but no phosphorus. 



The problems which arise from the biologist's point of view 

 are somewhat different in nature and are less readily defined. 

 They must, however, involve a constant awareness of the fact 

 that an animal, and a cell, cannot be dissociated from its en- 

 vironment. Due respect must be paid to such principles as those 

 of homology and analogy, and at the same time it is necessary 

 to maintain a more rigid guard against the acceptance of generali- 

 sations than is usually necessary in the fields of physics and 

 chemistry, owing to the fact that variables on the purely biologi- 



