254 FROM FISH TO PHILOSOPHER 



gen excretion by marine fish {79}, we concluded that 

 the branchially excreted ammonia was derived directly 

 from blood ammonia, presmnably by diffusion; but sub- 

 sequent studies in the lungfish {80} led us to abandon 

 this view in favor of peripheral formation, and in the 

 study of the fresh-water elasmobranch, Pristis microdon 

 {69, 70}, evidence was obtained that the branchial ex- 

 cretion of ammonia is under physiological control. 



The statement that ammonia excretion is characteristic 

 of all aquatic organisms {75}, and the imphcation that 

 this ammonia is the ultimate, hepatic product of protein 

 metaboHsm in the fresh-water and marine teleosts and 

 the lungfish {76}, ignore both the abundant branchial 

 excretion of ammonia and virea, as compared with uri- 

 nary excretion, and the possibility of peripheral am- 

 monia formation. 



Leon Goldstein and R. P. Forster, working at Salis- 

 bury Cove, have recently (1959) shown that the am- 

 monia excreted by the gills of the marine shorthorn 

 sculpin, Myoxocephalus scorpius, is formed de novo. 

 (Only small amounts of urea are excreted by the gills in 

 this species, and urea excretion in the urine is probably 

 negligible.) The gill is rich in glutaminase and glutamic 

 dehydrogenase, the total maximal activity of which is 

 about equivalent to the observed ammonia excretion. 

 The presence of these enzymes indicates that glutamine is 

 a major precursor of the branchial ammonia in this fish, 

 as it is in the mammahan kidney. 



In the frog and mud puppy ammonia is practically 

 absent from the blood and glomerular filtrate, but is 

 secreted into the urine by the distal tubule {62}, and 

 it may confidently be accepted that nearly all urinary 

 ammonia in the Amphibia is of peripheral origin. 



The invertebrates aside, until cogent evidence to the 

 contrary is presented, we propose, on the evidence sup- 

 phed by the elasmobranch and teleost fishes, the lungfish, 

 the frog, and the mud puppy, that the primary nitroge- 

 nous end-product of protein metabolism in the verte- 



