THE KIDNEY 4I 



of protein metabolism, urea, is excreted almost entirely 

 by simple diffusion through the respiratory epitheliimi 

 of the gills. Salt, the most important inorganic constitu- 

 ent in the body fluid, is actively excreted by the gills 

 of fishes in the interests of body-fluid regulation, despite 

 the fact that the renal tubules are working steadily to 

 reabsorb almost every trace of salt filtered through the 

 glomeruli in order to save it from being lost from the 

 body. In the adult Amphibia (which have no gills) the 

 skin plays an almost equally important role in both salt 

 and water balance. In short, full responsibility for body- 

 fluid regulation did not devolve on the kidney until the 

 first terrestrial animals took to living permanently on the 

 land and the participation of both gills and skin in body- 

 fluid regulation was wholly abandoned; only then did 

 the filtration-reabsorption system come into its own as 

 a highly eflBcient device serving both body-fluid regula- 

 tion and the excretion of waste products. 



And so it was with tubular excretion, a process by which 

 the tubule cells remove certain substances from the 

 plasma and deposit them directly in the tubular urine, 

 thus supplementing the process of filtration. That tubu- 

 lar excretion is an invention dating back to the proto- 

 vertebrate is plausible enough since specific secretory 

 operations of this nature are common among the inverte- 

 brates. It is conceivable that a simple filtration-reabsorp- 

 tion system could have met all the requirements of the 

 aquatic vertebrates— but, as it came about, the mecha- 

 nism of tubular excretion was carried over into the glo- 

 merular nephron, and at a later day made possible the 

 survival of three large groups, the marine fishes, the arid- 

 living reptiles, and the birds, and it continues to con- 

 tribute importantly to the excretion of waste products in 

 man. 



And so, also, with the kidney's blood supply, where 

 old and new inventions overlapped and for a long time 



