MECHANISM OF RENAL SECRETION 37 



stances are detectable in quantity only in the convoluted tubules. 

 According to his results, the glomeruli have no share in the 

 excretion of these substances in any histochemically demon- 

 strable quantity. Leschke's work was extended and confirmed 

 by Oliver (27) in her work in connection with experimental 

 uranium nephritis. 



Pure cytologic studies of morphologic differentiation of the 

 various parts of the uriniferous system belong in this group of 

 investigations of local function, for morphologic specialization is 

 always a criterion of specific function. This correlation is so 

 universal that one can not discard such strictly anatomic data. 



Lastly comes the group of investigations concerned with the 

 altered function in conditions of renal pathology. To attempt 

 to include any form of discussion of nephritis in this paper is 

 impossible in view of the limited space, but as some of the most 

 essential evidence of the true nature of renal secretion arises in 

 the study of the experimental nephritides, a certain amount of 

 discussion of this phase of the subject will be necessary. The 

 work of Aschoff (28) and his student Suzuki (29) deserves espe- 

 cial comment, as they demonstrated the extreme specificity of 

 various levels of the tubules in their reaction toward renal 

 poisons. Their work will be taken up more in detail at a later 

 point. The works of others, such as Potter and Bell (30), Pearce 

 (31, 32, 33), Dickson (34), Lyon (35), Schneider (36), and Under- 

 bill, Wells and Goldschmidt (37), are all very valuable contribu- 

 tions to a clear understanding of renal activity. In connection 

 with the results recorded in this paper these investigations will 

 be discussed later. 



The problem presents therefore the question: In which direc- 

 tion through the cells of the convoluted tubules are the urinary 

 constituents passing; are they being secreted or reabsorbed? 

 This was the original question upon which this work has been 

 based, but the scope of the investigation broadened somewhat as 

 it progressed. Wishing to avoid the use of colloidal dyes because 

 their properties are foreign to the organism, as did Leschke (26) 

 and Oliver (27), and desiring to use an electrolytic salt normally 

 present in the body and yet detectable microchemically, we made 

 iron the substance of choice. 



