1156 THE METABOLISM OF NUCLEIN AND [pt. iii 



As for the fall in N.P.R. itself, they mentioned the obvious relation 

 between it and the fall in percentage growth-rate (see Section 2). 

 The higher the growth-rate, the higher the N.P.R. , i.e. the more 

 purine nitrogen present per cent, of total nitrogen — but the connec- 

 tion between the two remains enigmatic. Defining further their 

 notion, LeBreton & Schaeffer went on to say that with senescence 

 the total nitrogen "grows" more rapidly than the purine nitrogen. 

 They considered, further, that the N.P.R. would probably be found 

 to rise from fertilisation to the blastula stage, and then to descend, 

 but so far no determinations have been made which support or 

 throw doubt on this view. LeBreton & Schaeffer themselves found 

 the ripe eggs of the mackerel [Scomber scombrus) to have an N.P.R. of 

 1-5 and the adult liver cells one of 4-4, so that, if the latter had 

 decreased from a much higher value, the former must have increased 

 to it. More work along these lines would be very desirable. 



10-3. Nuclein Synthesis in Developing Eggs 



It has already been said that purine bases are undoubtedly 

 synthesised by the developing chick embryo. The same holds for 

 the embryo of the silkworm, if we may accept the early data of 

 Tichomirov, who isolated 0-02 per cent, wet weight of total purine 

 bases from the hibernating eggs, but over ten times as much, 0-23 per 

 cent., from the fully developed embryos, of which about half was 

 xanthine, and the rest hypoxanthine, guanine and adenine. 



Similar results were obtained for the egg of the cod, Gadus morrhua, 

 by Levene, although his methods were of doubtful validity. 



Perhaps the increasing basic nitrogen found by Gortner in his trout 

 and salamander eggs may indicate an increase in purine nitrogen. 

 In the latter case especially there was a gain of nitrogen in the ether- 

 insoluble-but-alcohol-soluble fraction of 4-1 mgm. nitrogen, or 0-7 

 per cent, of the total nitrogen, which he ascribed to the synthesis 

 of purine and pyrimidine bases. 



The only investigator who actually isolated purine bases from echi- 

 noderm eggs was Masing. In the unfertilised eggs of Arbacia pustulosa 



