160 R- VENDRELY 



But, as some authors have pointed out''"^^ the chemical method of ap- 

 proach must be applied to a large number of nuclei, and the DNA content 

 is calculated from the gross analysis and from the enumeration of the nuclei. 

 The result thus obtained is exact if we assume that all the nuclei of the tissue 

 under consideration have approximately the same DNA content. This con- 

 clusion seems to be justified, since nuclei as widely different as those of 

 erythrocytes, liver, pancreas, heart, and so on, have been found to contain 

 the same amount of DNA, an amount double that in the sperm. This strik- 

 ing result cannot be obtained by chance but must reveal a fundamental 

 property of the DNA of the nucleus. 



Nevertheless, in certain cases the nuclei of a tissue may not represent a 

 homogeneous population but a mixture of different types. The livers of 

 rodents and some mammals, for instance, are known to contain three kinds 

 of nuclei — diploid, tetraploid, and octoploid — whereas kidney tissue con- 

 tains only diploid nuclei. In such cases of polyploidy, results for the mean 

 amount of DNA calculated for a single nucleus are markedly higher than in 

 a normal tissue ; the ratio between these values for rat liver is 1 .4 : 1 to 1.5:1. 

 These results are mean values and yield, of course, no significant infor- 

 mation concerning the real DNA content of the individual nucleus. In such 

 cases, as well as in rapidly growing and differentiating tissues and abnormal 

 or pathological tissues, the techniques of cytophotometry are valuable 

 when chemical methods alone are unable to provide the answer. Cyto- 

 photometric methods allow an estimation of the DNA content of individual 

 nuclei, and with their aid the question of the complications due to poly- 

 ploidy has been clarified. 



Before considering the results of photometric methods we must first 

 examine the particular problem of the chemical determination of the DNA 

 content of the egg nucleus. It might be expected that the egg would contain 

 the same amount of DNA as the sperm of the same species. Chemical anal- 

 yses have usually been carried out on whole eggs, not on isolated nuclei, 

 and the results seem to be obviously wrong; for instance, Schmidt et al.,^^ 

 by estimating DNA phosphorus, and Vendrely and Vendrely,-" measuring 



" A. W. PoUister, H. Swift, and M. Alfert, J. Cellular Comp. Physiol. 38, Suppl. 1, 



101 (1950). 

 12 L. Lison and J. Pasteels, Arch. biol. {Liege) 62, 1 (1951). 

 »' C. Vendrely, Bull. biol. France et Belg. 86, 1 (1952). 

 1^ R. Vendrely and C. Vendrely, Experientia 5, 327 (1949) . 

 '*M. F. Harrison, Nature 168, 248 (1951). 

 '« C. Leuchtenberger, R. Vendrely, and C. Vendrely, .Proc. Natl. Acad. Sci. U. S. 37, 



33 (1951). 

 '' R. Y. Thomson, F. C. Heagy, W. C. Hutchison, and J. X. Davidson, Biochem. J. 



53, 460 (1953). 

 '8 R. Vendrely and C. Vendrely, Compt. rend. 235, 444 (1952). 

 '8 G. Schmidt, L. Hecht, and S. J. Thannhauser, J. Gen. Physiol. 31, 203 (1948). 



