MECHANISM OF RENAL SECRETION 59 



of charcoal is universally known, and colloids, also having tre- 

 mendous surface areas, similarly are able to adsorb substances 

 and so control their distribution. The adsorptive power of 

 such colloids is greatly altered by environmental condition, as, 

 for example, the hydrogen ion concentration, and therefore a 

 given cell may adsorb, retain, or secrete depending upon the 

 various influences at work upon it. Similarly, chemical com- 

 binations may bind a substance locally within a cell, but this is 

 perhaps the least essential of the three factors. 



Such a conception is entirely in agreement with that of cell 

 polarity, both morphologic and physiologic. Direct evidence of 

 such polar differentiation as seen in this work is the evident 

 tendency for iron to accumulate under the brush border in the 

 convoluted tubules. The idea of cell polarity is by no means a 

 new one, and has long been recognized as an important factor 

 in any epithelial secretory mechanism (69). The excellent work 

 of Child (70) on metabolic gradients in unicellular and other 

 low forms of animal life is an important contribution in this con- 

 nection. In the higher forms Tashiro (71) demonstrated quan- 

 titatively different metabolic rates in different levels of a nerve 

 fiber and changes in rate with the passage of an impulse from the 

 higher to the lower level of activity. Alvarez (72, 73, 74) in a 

 grosser way demonstrated a functional gradient of the alimentary 

 tract which is independent of its anatomic units. And so it is 

 not without precedent to suppose that within the individual 

 cell the two poles may behave quite differently. 



In the routine of the retention experiments the specific gravity 

 of each sample was determined among the other tests. In thir- 

 teen retention experiments twenty-nine examples of the effect 

 of iron secretion on the specific gravity of the urine were studied 

 and in every instance the specific gravity was greatly lowered 

 during the iron elimination. The average reading in fifty-seven 

 observations on urine containing iron was 1.012 and the average 

 for seventy-one readings of normal, iron-free urine, 1.0183. In 

 many instances the difference was much more marked, as in 

 K72: a reading of 1.006 when iron was present as contrasted to 

 1.018, 1.020, 1.022, when absent. This characteristic of lowered 



