II44 PROTEIN METABOLISM [pt. iir 



system may suffice in the sea, an embryonic reptile with a bag of 

 yolk almost as big as itself would have, owing to obvious difficulties 

 of locomotion, very little chance of survival on land. Such imper- 

 fectly mobile eggs would have been too tempting for adults of other 

 species. The terrestrial egg had therefore to be constructed in such a 

 way that the young organism could stay inside a long time and hatch 

 out substantially mature. It had no chance, therefore, either to 

 get rid of nitrogenous excreta at an early stage by hatching, or to 

 excrete them through a semipermeable membrane. The only solution 

 of the problem was uric acid. When, at a still later date, the proto- 

 theria and metatheria branched off from the reptiles in the mam- 

 malian direction, acquiring at last true viviparity, the need for a 

 uricotelic metabolism ceased. For it is to be noted that there is no 

 reason why an adult land animal should not excrete urea, if it drinks 

 sufficient water, or even ammonia, if it has enough acid to spare. 

 Thus Ambard found that a cat or dog on a meat diet, if left to 

 itself, drinks exactly enough water to excrete urea at its maximum 

 normal concentration, i.e. just to avoid the slightest uraemia. This 

 is Ambard's "volume obligatoire". But what the adult does will 

 depend on what it had to do as an embryo ; in other words, on what 

 its facilities then were for absorbing water and for getting rid of waste 

 nitrogen. The existence of an albuminous solution round the yolk of 

 the terrestrial egg is, as Gray says, an admirably adapted mechanism 

 for providing the growing embryo with water. The use of uric acid — 

 insoluble, non-diffiisible — instead of urea or ammonia, is an equally 

 well adapted mechanism for dealing with incombustible waste, and 

 the re-absorption of water through the allantoic wall is the mechanism 

 which unites the two. There is no need, however, to postulate any re- 

 version from uricotelic to ureotelic metabolism in the case of mammals, 

 for the palaeontological evidence admits of the possibility that they 

 arose from some early reptile ^ which laid non-cleidoic eggs. True 

 viviparity may thus have been a genuine alternative to uric acid 

 production. 



A final reference may be made to certain special cases, e.g. the 

 Prototheria and the Elasmobranchs. The Elasmobranchs are at one 

 and the same time the only marine animals which have evolved the 

 closed-box system or somiething approaching it and the only ones 

 which have found out a way to withstand high concentrations of 

 urea. There are, perhaps, two possible explanations of their behaviour. 



^ Probably one or more lines of cynodont (Therapsid) reptiles. 



