176 R. VENDRELY 



His mitotic counts showed that there is no visible correlation between DNA 

 content of nuclei and mitotic activity. Moreover, the well-differentiated 

 tissues of the forebrain of the 11-day embryo show a much narrower range 

 of DNA values per nucleus than do those of 7-day embryos which are under- 

 going differentiation. In the adult cells of Rana pipiens studied by Swift,'^ 

 when differentiation is quite finished, the results of DNA determination per 

 nucleus are very close to each other and show a real constancy. Marinone,^^ 

 studying the differentiation of erythroblasts and granuloblasts in man, 

 observed a marked decrease in nuclear DNA in the course of differentiation. 

 This decrease was not related to pycnosis for it appeared abruptly at the 

 stage of the disappearance of the nucleoli. A further and more gradual 

 decrease occurred subsequently, and the nucleus finally contained the dip- 

 loid amount of DNA. At the end of this process any further decrease in 

 DNA was due to pycnosis and the final amount of DNA might be lower than 

 in lymphocytes. Reisner and Korson®* also showed a decrease of the DNA 

 content of erythroblasts in the course of differentiation. All these results 

 suggest that differentiation is associated A\ith a variation in the DNA con- 

 tent of the nucleus. In conclusion, the hypothesis of Moore, ^^ who interprets 

 the variations of DNA in differentiating cells as a sign of the morphoge- 

 netic activity of genes, should be mentioned. This would explain how cells 

 with the same chromosome complement differentiate into quite different 

 tissues. The genes controlling certain morphogenetic processes may produce 

 different amounts and kinds of DNA at different times with the result 

 that differentiation takes place. 



IV. Conclusion 



With the exception of the few particular cases that have just been re- 

 ported, the DNA content of the resting nucleus appears as a constant value 

 related to the number of chromosomes, and this striking fact is of great 

 interest for the biologist from the theoretical as well as the practical point 

 of view. From the theoretical point of view the discovery of the constancy 

 of the amount of DNA per nucleus in all tissues of the same animal and the 

 fact that the sperm contains half the DNA content of somatic cells is con- 

 firmation of the theory that DNA is an important component of the gene. 

 From the practical point of view Davidson and Leslie^^' ^^ have stressed the 

 importance of the constant amount of the DNA per nucleus as a measure 

 of cell multiplication and as a standard of reference in the expression of 

 biochemical changes in tissues. The results of tissue analysis are thus re- 



94 G. Marinone, Le Sang. 22, 89 (1951). 



95 E. H. Reisner and R. Korson, Blood 6, 344 (1951). 

 9« J. N. Davidson and I. Leslie, Nature 165, 49 (1950). 



" J. N. Davidson and I. Leslie, Cancer Research 10, 587 (1951). 



