THE KIDNEY 39 



(Myxine)y for example, the archinephric duct continues 

 to drain a single pair of nephrons in each segment of 

 the body, while the lamprey (Petromyzon) has simply 

 gone one step further by greatly multiplying the number 

 of nephrons draining into the archinephric duct. In the 

 higher fishes, the appropriation of the anterior part of 

 the archinephric duct by the testis has forced the kidney 

 to increase the number of nephrons attached to the pos- 

 terior segments of the archinephric duct, thus producing 

 a more or less compact organ, the *mesonephros/ While 

 in the reptiles, birds, and mammals, in which the kidney 

 has completely abandoned the archinephric duct to the 

 testis, the kidney must develop its own ureter. In the 

 mammals, both the pronephros and mesonephros are 

 only passing embryonic episodes, and the adult kidney, 

 or metanephros,' is formed by the local multiplication 

 of nephrons in the most posterior segments of the body 

 —as far away from the embryonic testis as it can get. 



Biological patents carry no automatic expiration date 

 but yield only to new inventions that supersede them 

 because of greater effectiveness, and old and new may 

 long overlap in simultaneous operation or until the old 

 is put to some new use. Thus it was with the processes 

 of tubular reabsorption and excretion. It may be as- 

 sumed that the coelomic tubules of the protovertebrate 

 reabsorbed some substances from the fluid draining 

 through them, in order to conserve them for the body; 

 but we can do little more than speculate about the na- 

 ture of these reabsorptive operations. With the invention 

 of the glomerulus, however, it was necessary to speed 

 up all reabsoi-ptive processes to a high rate, and prob- 

 ably to add new ones: because practically all the osmoti- 

 cally active constituents of the plasma had to pass, along 

 with water, through the semipermeable glomerular 

 membrane, since otherwise the limited pressure in the 

 glomerular capillaries would be insufficient to filter any 

 significant quantity of water. Thus increased tubular re- 



