32 



Nature of the Genetic Material 



(1925) had already shown that tissue nuclei are found in sizes of 

 1:2:4, and had concluded that nuclei grow by endogenous polyploidy 

 (polyteny), that is, chromosomal division without cell division from 

 the diploid to the tetraploid and octoploid condition. Mirsky and Ris 

 and Boivin corroborated this conclusion and simultaneously demon- 

 strated the constancy of the amount of DNA per chromosome set. 

 Table 1 illustrates the fact well. 



TABLE 1 



Average Amount of DNA in Somatic Tissues of Mice, in Optical 



Measurements from Feulgen Slides Expressed in Arbitrary Units 



(From H. Swift, 1953) 



This rule has since been confirmed many times for many types 

 of cells. A very convincing example is illustrated in figure 2; the 

 measurements were made in somatic cells of maize, which are basi- 

 cally diploid, and also in basically triploid endosperm cells, so that 

 a clear series of sizes of 2:4:8:16 and 3:6:12:24 was obtained. The 

 range of variation is, by the way, a good indication of the reliability 

 of the method; another very good example is found in figure 3, a 

 and b. 



It is important for our problem to know the quantity relations 

 of DNA in dividing cells. The measurements showed that between 



