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DISCUSSION 



Davson: As far as I can make out, Dr. Thaysen, you postulate that 

 there is a region through which the urea can pass quite easily, and later 

 on in the ducts there is a relative impermeability to urea. This is rather 

 in conflict with what people have thought in the past, because, on the 

 assumption that it penetrates into all cells very rapidly, urea has been 

 used to determine cell water. Your view certainly does fit in with what 

 is found with the cerebrospinal fluid and the aqueous humour; urea does 

 not penetrate those barriers easily. If one confined oneself to these in- 

 stances, then, one would say that urea did not penetrate cells easily at all. 



Thaysen : Yes, I believe that the cells in the region of water reabsorp- 

 tion are less permeable to urea than the cells at the site of precursor 

 formation. In all probability the difference in permeability is, however, 

 relative rather than absolute. In other words, I do not think that the cells 

 at the site of precursor formation are so freely permeable to urea that the 

 concentration of urea in the precursor is equal to that of the plasma at all 



