The Origin and Fate of Urea in the Developing Hen’s Egg

1. The urea formed by the chick embryo during its development is not derived from the ammonia of general protein catabolism, either directly or by way of the ornithine cycle. 2. Nor is it derived from uric acid by a complex uricase similar to that of fishes and Amphibia (Stransky's complex: uricase, allantoinase and allantoicase). The uricase discovered in the chick embryo by Przylecki probably breaks down uric acid only as far as allantoin. 3. The urea of the chick embryo is entirely derived from an active arginine-arginase system present at all stages examined from the second day of incubation onward. 4. The arginase of the embryo, quantitatively estimated, falls off very regularly as development proceeds, reaching a minimum about the 12th day. This negative heterogony is experimentally shown not to be due to the parallel diminution of activators such as glutathione and ascorbic acid, but must be regarded as a real diminution in the relative amount of the enzyme. It forms an important support for the view of Edlbacher, hitherto mainly based on observations of tumour tissues, that arginase is an enzyme associated with high growth rate and probably nuclein synthesis, as well as with the production of urea as a vehicle of waste nitrogen. 5. Arginase is also contained in the yolk-sac, but in much smaller quantity than in the embryo. Here also the low activity is not due to the absence of activators. 6. Experiments made to test the possibility that the urea, once formed, might, as would be the case on Wiener's theory, participate in the formation of uric acid, took the form of injection of possible precursors into the egg on the 12th day of development and estimation of the uric acid in the egg on the 15th. In this way it is shown that urea cannot be converted into uric acid by the chick embryo. This conclusion is in agreement with much recent work on the adult bird. 7. A preliminary test was also made of the possibility that biuret and sarcosine could be condensed to uric acid by the chick embryo. The results were negative.