Il62 



THE METABOLISM OF NUCLEIN 



[PT. Ill 



off until the end of development. The formation of creatine cannot 

 account for the decline in the curve during the latter part of 

 development, for creatine and its anhydride were readily oxidised 

 by the process used, and guanidine coming from them would be in- 

 cluded in the figures for total guanidine. The total guanidine would 

 have been expected to remain constant. Burns thought there were 

 two possibilities, ( i ) that the configuration of the protein molecule 

 of the chick might be more resistant to permanganate oxidation than 

 the protein molecule of the yolk and white, or (2) that the guanidine 

 precursor of creatine in the protein molecule may be partially 

 destroyed by the oxidation pro- 

 cess, just as enzyme action 

 may sometimes split a protein 

 through the guanidine group. 

 Neither of these explanations is 

 very satisfactory, for they both 

 tend to throw doubt on the 

 accuracy of the method as a test 

 of the total guanidine present, 

 and so to remove significance 

 from the curve. A third pos- 

 sibility might be that after the 

 mid-point of development the 

 missing guanidine may be transformed into resistant rings such as 

 pyrimidine or iminazol compounds. It is to be hoped that further 

 work will be done on these obscure subjects. 



The best determinations of creatinine in developing organs of a 

 mammalian foetus are those of Beker given in Fig. 358. The increase 

 seems to be exceedingly regular. 



Hunter has found 546 mgm. per cent, creatine in the muscle tissue 

 of the ovoviviparous dogfish, Squalus sucklii, at a time when the muscles 

 of its foetuses (7 cm. long) gave 460 mgm. per cent. Probably, there- 

 fore the elasmobranch foetuses accumulate the substance in much 

 the same way as mammalian ones. 



3 4 5 6 7 



Months, development (cow) 



Fig. 358. 



