But, as MacLeod, discussing these experimental and clinical findings, observes, there are to hand even more direct proofs that _purin synthesis_ occurs in mammals. Thus, as McCallum has pointed out, we cannot escape the admission that young mammals are able to synthetise the purins essential for their growth, and this from food containing no purin, _e.g._, milk. Again, prior to incubation, a hen’s egg contains practically no nucleic acid, whereas after development its content in the same increases by great strides. The eggs of insects, too, with the progress of development, amass purin very rapidly. Again, Miescher noted long since that salmon, on leaving the sea to ascend rivers for the object of spawning, have at that time well-developed muscles; but on arriving at the upper reaches, marked muscular wasting ensues, while the testes undergo enormous enlargement. MacLeod, reflecting on these observations, argues that, “as the fish takes no food during the migration, there must be conversion of the protein of the muscles into the cellular tissue of the sexual glands, and _nucleic acid_ must be produced.” MacLeod’s conclusion is that “Purin synthesis undoubtedly occurs in the mammalian body, but it is difficult to recognise in metabolism investigation, because it is a slow continuous process ... whether or not changes in the activity of purin synthesis occur in conditions of disease, is a question which awaits investigation.” Lastly, the opinion of most authorities is that, while they concede the possibility of synthetic formation, the amount of uric acid produced in this manner is negligible, and that by far the most important mode of formation in mammals is by the _splitting_ and _oxidation_ of _nucleins_; in other words, that uric acid in the main is derived from the _amino-purins_ by _deaminisation_ and subsequent oxidation, and from the _oxy-purins_ directly by oxidation. CHAPTER VIII FORMATION AND DESTRUCTION OF URIC ACID The chemical structure and sources of uric acid having been dealt with, we are now in a position to resume our narrative, and to take up the thread at the point when Horbaczewski revealed the derivation of uric acid from nucleic acid. It now devolves upon us to scrutinise more narrowly the process by which the formation of _uric acid_ from _nucleic acid_ is achieved. Incidentally, it will not be unprofitable to note, if only briefly, the steps by which the necessary expansion of our chemical and physiological knowledge of nucleic acids has been acquired. As may be imagined, the primary difficulty was to prepare nucleic acids of such purity as admitted of their elementary chemical analysis. The necessary researches were to a large extent confined to two types of nucleic acid, one derived from _yeast_, and the other from the _thymus gland_; in other words, to representatives of the only two nucleic acids in nature, one derived from the nuclei of _animal_ cells, the other from the nuclei of _vegetable_ cells.