116 PHYSIOLOGY 



this to the purine nucleus with its contained ring, the pyrimidine nucleus, 

 found in the bases cytosine, uracil, &*., which occur in the nucleins. 



With regard to the formation of the aromatic constituents of the protein 

 molecule, i.e. those containing the benzene and indol rings, we have at 

 present very little indication even of the lines along which it might be 

 possible to prosecute our researches. It has been suggested that inosite may 

 represent some stage in the formation of the benzene ring from the open chain 

 found in the carbohydrates. Inosite has the same formula as glucose, 

 namely, C 6 H 12 0' 6 , but is a saturated ring compound : 



CHOH 

 CHOHi^CHOH 



CHOH \/ CHOH 

 CHOH 



and may be expected to be formed as a result of polymerisation of formalde- 

 hyde. We have no evidence however of the possibility of such a formation, 

 and the relations of this substance with the benzene compounds are by no 

 means intimate. It is of such universal occurrence, both in plants and 

 animals, that it is difficult to refrain from the suspicion that it may play some 

 part as an intermediate stage between the fatty and the aromatic series. 

 Since plants are able to manufacture all these varied substances out of 

 the products of assimilation of carbon and ammonia or nitrates, they must 

 also find no difficulty in transforming one amino-acid into another, and we 

 know that most plants can procure their nitrogen from a solution of a single 

 amino-acid as well as from a nutrient fluid containing the nitrogen in the form 

 of ammonia. In animals the power of transforming one amino-acid into 

 another, of one group into another, is probably strictly limited. So far as 

 we know, nearly all the amino-acids utilised in the building up of the animal 

 proteins are derived directly from those contained in the food. On the 

 other hand, we have evidence in the animal body of synthesis of the purine 

 bodies, and therefore of the pyrimidine and iminazol rings. The hen's egg 

 at the beginning of incubation contains very little nuclein, nearly the whole 

 of its phosphorus being present in the form of phosphoproteins and lecithin. 

 As incubation proceeds these substances disappear, their place being taken 

 by the nucleins which form the chief constituent of the nuclei of the developing 

 chiek. In the same way the ovaries and testes of the salmon are formed 

 during their sojourn in fresh water at the expense of the skeletal muscles, 

 especially those of the back. Here again there is a transformation of a tissue 

 poor in purine bases into a tissue which consists almost exclusively of nucleins 

 and protamines. Whether in this case there is a direct conversion of the 

 mono-amino-acids of the muscle proteins into the diamino-acids and bases 

 typical of protamines, we do not know. It is more probable that only 

 diamino-acids and bases previously existing in the muscle are utilised for the 



