II52 THE METABOLISM OF NUCLEIN AND [pt. m 



But a definite step forward was made by LeBreton & Schaeffer when 

 they translated the older conception of N.P.R. into physico-chemical 

 terms, and defined it as the expression: 



Purine nitrogen x lOO 

 Total nitrogen — Purine nitrogen' 



Two separate ideas seem to have been in the minds of the cytologists 

 who dealt with N.P.R., firstly, that they were measuring simply cell 

 and nuclear volume, and, secondly, that they were measuring some- 

 how the amount of nuclear substance, or even nucleoprotein, present 

 in the cell. LeBreton & Schaeffer's advance did not affect the first 

 of these ideas, for microscopical measurement of N.P.R. still remains 

 valuable cytologically, but it did supersede the second notion with 

 a real hope of exactitude. It had always been obvious that nucleo- 

 protein might be dispersed to some extent in the cytoplasm, and might 

 not be all in the nucleus.^ 



One paper existed already in the literature in which definite infor- 

 mation had been given about the "chemical" N.P.R., although its 

 author did not perceive the full significance of his experiments. 

 This was the memoir of Masing who estimated the nucleoprotein 

 phosphorus in rabbit embryos according to the method of Plimmer 

 & Scott, and obtained the following figures, which it is impossible 

 to plot, in view of the imperfect characterisation of the material. 



Table 165. 



From this it was evident that the nucleoprotein as related to the total 

 protein of the body or organ diminished during development. The 



^ In recent times it has been convincingly shown that the greater part of the purine 

 of adult muscle is present as (probably cytoplasmic) nucleotide and not as nucleoprotein. 

 Studies on the N.P.R. must in future take this into account. 



