1922. ] ‘* Renal Portal’’ System. 87 
merely the “ result of the penetration of the nephridia [kidney 
tubules] into the cardinal vein’ (38)—the penetrating acti- 
If then, the common assumption that the “‘ renal portal” 
system is of some use in connection with renal excretion be 
more recently confirmed this observation for the kidneys of 
sharks. Further, as I shall prove, the evidence on which 
physiologists base the assumption that venous blood traverses 
the intertubular capillary plexus is extremely faulty. I may 
here remark that the solution of this problem as to whether 
or not the venous blood traverses the intertubular plexus of 
the frog or other “ portal” kidney is of supreme importance, 
because, as the following pages will show, the demonstration 
of the fact that venous blood does not traverse the minute 
capillary-like sinusoids which alone supply the tubules, provides 
evidence which enables us to state definitely that in the 
kidney it is the tubules alone which secrete the urine, the 
encapsulated glomeruli taking no direct part (see Part IV). 
The Current Opinion concerning the Construction of the 
‘“* Portal”? Kidney is ill-founded. 
The assumption (almost universally adopted by physio- 
logists) that the venous blood in the renal afferent vein mixes 
with the arterial blood derived from the efferent glomerular 
arteries and therefore traverses the intertubular plexus is 
based on the results of oft-repeated dye injection experiments. 
It has been stated on many occasions that whether dye be 
