EXCRETORY NITROGEN METABOLISM AND GLUCONEOGENESIS DURING DEVELOPMENT OF HIGHER VERTEBRATES

ABSTRACT

The concept of nitrogen “excretion” by mammalian and avian embryos is examined in light of contemporary knowledge of hepatic metabolism. The origin of “waste” nitrogen and the hepatic pathways for its conversion to either urea or uric acid for excretion are described. The probable evolution of these pathways among the higher tetrapod vertebrates and the impact of the selective advantage of uricoteley on the radiation of the archosaurs are considered. The intimate connection of hepatic gluconeogenesis with nitrogenous end product formation in birds and mammals is examined. Finally, the development of the capacities for gluconeogenesis and for either ureogenesis or urieogenesis in the perinate is discussed relative to the problem of development in a cleidoic egg.