THE BONY FISHES 10/ 



rated from the blood by filtration through the glomeruli, 

 and that the function of the tubules is limited to the 

 reabsorption of water and other valuable constituents 

 from the glomerular filtrate. 



Now came the studies of Marshall and A. L. Graf- 

 flin, of Edwards and others, on the goosefish and other 

 aglomerular forms, that demonstrated that the aglo- 

 merular fish kidney could excrete nearly all the impor- 

 tant constituents of the urine, such as creatinine, crea- 

 tine, uric acid, trimethylamine oxide (a compound 

 excreted in substantial amounts only by the fishes), 

 magnesium, calcium, potassium, and sulfate; and, among 

 foreign substances, iodide, nitrate, thiosulfate, thiocya- 

 nate, and many dyes and synthetic compounds which 

 are entirely foreign to the body and which the renal 

 tubules had never encountered in their revolutionary 

 history. 



The demonstration that the aglomerular fish kidney 

 could excrete nearly all the important constituents of the 

 urine at least established one broad qualitative fact: tu- 

 bular excretion, in principle, was a reality. But to prove 

 the existence of tubular excretion in the aglomerular 

 fishes was only to prove the possibility of tubular ex- 

 cretion in other animals; the demonstration really an- 

 swered no questions at all so far as the glomerular kid- 

 ney of the other fishes, or the frog, or the dog, or man 

 was concerned, for conceivably the situation might be 

 different in every species, and certainly it would differ 

 for different substances. To answer this question in any 

 species required an adequate method of measuring the 

 filtration rate, and the aglomerular kidney served to fo- 

 cus the attention of investigators on this problem. 



But the aglomerular kidney also supplied a convenient 

 test organ for ascertaining what types of compoimds can 

 be excreted by the tubules— at least in the fishes, and, 

 inferentially, ff in the fishes, then possibly in other ani- 

 mals—and what types cannot. Most notable among the 

 substances which the aglomerular kidney cannot excrete 



