1076 PROTEIN METABOLISM [pt. hi 



as have already been described in the Section on general metabolism. 

 It would appear, therefore, that within the embryo there is a distinctly 

 greater population of amino-acid molecules in the free state at some 

 times than at others. 



9-4. The Accumulation of Nitrogenous Waste-products 



It is now time to turn to the engine room of the embryo, and to 

 deal with the breakdown of the protein molecules which by their 

 combustion furnish a supply of energy. The amount and intensity 

 of combustion can be far more easily gauged in the case of protein 

 than in the case of carbohydrate and fat, because of the incom- 

 bustible residues left behind. 



Urea may be taken first among the end products of protein com- 

 bustion, although, as Fourcroy & VauqueUn found as long ago as 

 1805, by far the preponderant nitrogenous end product in the fowl 

 is uric acid. Goindet was the first to find urea in avian urine, and 

 in 1825 Prevost & Le Royer isolated from the allantoic fluid of a 

 chick on the 1 7th day of development a substance which gave an 

 insoluble compound with nitric acid, and which they identified with 

 urea. In 19 12 Fridericia carried out some experiments in which he 

 estimated the quantity of urea in the allantoic fluid of the chick by 

 the Schondorff method. Thus on the 17th day of development 

 Fridericia obtained 4-5 mgm. urea per embryo, or 12 per cent, 

 of the total excreted nitrogen (urea + uric acid). On the 20th day 

 he got much less, only 1-7 mgm. urea per embryo, or 1-9 per cent, 

 of the total excreted nitrogen. He concluded that only small and 

 variable amounts of urea were present, and that the chick, like the 

 adult hen, excreted all its nitrogen in the form of uric acid. 



The subject merited re-examination however, and in 1925 I went 

 into it anew. The method used was that of Folin & Wu, which, 

 involving as it does the use of the specific enzyme urease, was more 

 satisfactory than Schondorff's. The results obtained, applying the 

 method to embryo + allantoic fluid + amniotic fluid, are shown 

 graphically in Figs. 313 and 314. In Fig. 313 is seen the milligrams 

 per cent, wet weight plotted against the time and also the milligrams 

 per embryo. The latter rise steadily, as might be expected. The 

 former rises steadily also, until the 9th day, at which point it becomes 

 stationary for the rest of development. In other words, as far as 

 the wet weight is concerned, the rate of production or excretion of urea 



