II ORGANOGENESIS 741 



adult and probably of the embryo as well. They point further to a possible concept 

 of the evolution of the excretory mechanism. Excretion of ammonia and urea 

 by amphibia is well established. It would appear that, genetically, the simplest 

 explanation for the reptilian and avian devices would be transmission from 

 primitive amphibia to reptiles and modern amphibia of a urea-synthesizing 

 mechanism. Superimposed upon this device the mechanism for uric acid synthesis 

 would be added; whether the latter appeared independently, or in response to 

 the development of urease, must remain a moot question. 



The presence of urease during a portion of the incubation period of the snapping 

 turtle has been observed in our laboratory. It was not, however, detectable in the 

 contents of the alligator egg. In the chick it was followed throughout development, 

 increasing regularly but not so rapidly as arginase. 



Goldie (1959) has followed the distribution of arginase in the chick embryo and 

 extra embryonic membranes, its localization within the embryo, and its distri- 

 bution between nucleus and cytoplasm. Although Needham, Brachet and Brown 

 (1935) deny the existence of the ornithine cycle in the chick, the basis for their 

 conclusion seems inadequate. This question, as well as the role of urease, is 

 currently under investigation. 



If then, we have the demonstrated fact of maximal protein combustion early in 

 development, what of the "advisability" of such a procedure from the point of 

 view of the embryo? In the chick, for example, the question of toxicity is illusory. 

 The total nitrogen excreted up to the eighth day in the chick requires the com- 

 bustion of only 12 mg protein; the available protein from the yolk alone is 8.0 g. 

 Since the accumulated waste is diffusible, and is indeed recoverable in the yolk, 

 the concentration of either resultant ammonia or urea is of the order of 3.0 mg%, 

 obviously a harmless range. 



In summary, it appears that reptiles have inherited the necessity of producing 

 ammonia and the capacity of synthesizing urea from early amphibian ancestors. 

 These capacities have been passed on to the chick (birds?), and superimposed 

 upon urea synthesis has been the accessory production of uric acid, which may 

 or may not have been accompanied by the evolutionary appearance of urease. 

 Protein utilization as an energy source is highest very early in development 

 and gradually is replaced by fat. Carbohydrate utilization may occur to a limited 

 extent (certainly, even early in development, the capacity to burn carbohydrate 

 is present), and might, indeed, be a requirement for protein utilization. There is 

 no rational physiological impediment to the use of protein as an energy source 

 early in development, when the absolute quantities are small. In the chick, uric 

 acid is produced systematically from the beginning; evidence either from the 

 chick, or from a study of the total performance of the several reptiles studied, of a 

 biochemical recapitulation of excretory mechanism or products is wanting. 



III. GENERAL SUMMARY 



Progress in any field of science is determined by its conceptual orientation and 

 the degree to which its concepts can be evaluated by the tools at hand. The 



Literature p, 744 



