46 S. S. COHEN 



in the haploid gametes. The possible vahdity of proposition (1) will be 

 discussed in a later portion of this section. 



Boivin and the Vendrelys (1948) and Mirsky and Ris (1949) first estimated 

 the DNA content of nuclei. It was shown that the DNA content of nuclei 

 of different bovine tissues in the same animal w^ere nearly identical and twice 

 that of the sperm (Boivm et al., 1948). Some data on the DNA contents of 

 the nuclei of various fowl tissues are summarized in Table IV. Many workers 



TABLE IV 

 Average DNA Content per Nucleus of Fowl Tissue "- ^ 



Tissue DNA 



« Allfrey e« a/. (1955b) 



have obtained similar results, e.g., Swift (1950) for plant nuclei, and it has 

 generally been concluded that the amount of DNA was constant for each 

 chromosome set. Indeed, it is now known that the base composition of DNA 

 remains the same from tissue to tissue in the same animal. 



Leuchtenberger and the Vendrelys (1951) apphed both cytochemical and 

 chemical methods to the analysis of isolated nuclei and concluded that the 

 thesis of Boivin was essentially correct, at least with respect to adult tissues. 

 In addition, they were able to distinguish three classes of liver nuclei whose 

 DNA contents w^ere in the ratio of 1 : 2 : 4- — a phenomenon ascribed to the 

 existence of diploid, tetraploid, and octaploid nuclei in hver. 



The constancy of DNA per cell has been a very useful base line for many 

 researches. Kurnick (1951) has followed both the average increase in mass 

 and increase in DNA during normal growth of kidney tissue. Since the two 

 maintained a constant ratio, it was concluded that the increase in mass was 

 due primarily to increase in cell number rather than to the growth of indi- 

 vidual cells. Davidson and his collaborators (Leshe, 1955) have similarly used 

 DNA content as a standard in investigating the change of cell composition 

 in tissue cultures, a type of material in which suitable base lines had been 

 unusually difficult to establish, prior to the development of suspended cell 

 cultures. 



