THE DEOXYRIBONUCLEIC ACID CONTENT OF THE NUCLEUS 165 



to express their measurements in arbitrary units. Their results confirmed 

 the homogeneity of the nuclear population of the kidney and the presence 

 of at least two classes of nuclei in liver. 



These early investigations show that cytophotometry in ultraviolet light 

 can be used successfully for the study of the DNA content of nuclei. 



2. The DNA Content of the Nucleus in Different Species of Animals 



Since the DNA content of the nucleus appears as a constant value charac- 

 teristic of the species, a comparison of the amounts of DNA per nucleus 

 throughout the animal kingdom is of some interest. Numerous results are 

 already available (Tables VI and VII), but their interpretation is not easy. 

 Nevertheless, a few points can be stressed. Mammals and birds so far stud- 

 ied show values of DNA per nucleus which are confined within narrow 

 limits. Mirsky and Ris" remark that the values of DNA per cell in lung 

 fish, amphibians, and reptiles suggest that in the evolution of these verte- 

 brates there has been a decline in the DNA content per cell. On the other 

 hand, in invertebrates, the amount of DNA per cell is greater in higher 

 forms than in primitive forms. Vendrely and Vendrely^* suggest that some 

 high values found in fishes could represent polyploids, but sufficient infor- 

 mation is not yet available to settle the question. 



In conclusion, can the study of the DNA content of the nucleus be of 

 some help in problems concerning evolution? The quantity of DNA does 

 not seem to be related to the number of genes, for the amount of DNA does 

 not increase unequivocally with the complexity and number of hereditary 

 characters. It should, however, be pointed out that less organized living 

 beings seem to have small amounts of DNA in their nuclei: Boivin et al.^ 

 estimated that the DNA content of a bacterial nucleus represented about 

 one-hundredth that of the beef nucleus, and Whitfeld"*^ recently found a 

 value of the same order of magnitude (0.059 pg.) in Plasmodium herghei. 

 But, within the same species, the amount of DNA per nucleus is certainly 

 related to the number of chromosomes, and it may well be that in related 

 species the differences in the DNA content of nuclei could be correlated 

 with differences in the size and the total length of the chromosomes. The pre- 

 cise enumeration of the chromosomes is sometimes very complex on account 

 of fragmentation and other difficulties. In such cases it is readily possible to 

 obtain additional information from the measurement of DNA. Therefore 

 the DNA complement can be considered, as Hughes-Schrader^" points out, as 

 "a cytotaxonomic tool in evaluating evolutionary relationship among species 



" A. E. Mirsky and H. Ris, J. Gen. Physiol. 34, 451 (1951). 

 « R. Vendrely and C. Vendrely, Compt. rend. 230, 670 (1950). 

 ^9 P. R. Whitfeld, Nature 169, 751 (1952). 

 60S. Hughes-Schrader, Biol. Bull. 100, 178 (1951). 



