RESULTS OF FIXED TISSUE STUDIES 117 



granules. To call them mitochondrial secretion products offhand is open 

 to criticism, since we have not accepted it as an absolute fact that the 

 mitochondria of the kidney enter into the secretory role. 



When we now compare the various groups of the kidneys studied in 

 reference to the appearance of their brush borders, it again must be ad- 

 mitted that there is no proof that this structure is influenced in any way 

 by urea feeding. Such variations as may be found can be demonstrated 

 within each individual group, and therefore must be considered as a nor- 

 mal variation of the brush-border picture. 



In the studies of the mitochondrial apparatus of the kidney of the 

 white rat certain generalities have been agreed upon by all investigators. 

 Upon general survey one is always impressed with the intense staining of 

 the proximal and to a lesser degree of the distal convoluted tubules. In 

 the proximal tubules the mitochondrial apparatus is very distinct and con- 

 sists of true rods or of varying degrees of granular dissolution of these 

 rods. The mitochondrial numerical concentration is higher here than in 

 other segments of the tubule. Also the affinity of the individual granule 

 for certain stains is greater because the granule is larger here than in any 

 other segment (see Figs. 4 and 5). Both conditions explain the color 

 prominence of the proximal and, to a lesser degree, of the distal convo- 

 luted tubules. The greater staining affinity is aparently of purely physical 

 and not chemical origin, this assumption being based on a greater concen- 

 tration of the mitochondrial material in one unit. In the medial and dis- 

 tal parts of the proximal tubules the mitochondrial structure is less dense 

 and the granular dissolution more pronounced. The descending limb of 

 the loop of Henle is very poor in granules and occasionally appears free 

 from them, so that it is hard to recognize them with mitochondrial stains, 

 the chainlike or rodlike arrangement of the mitochondria, which is so 

 typical of the proximal convoluted tubules, is suggested again in the mito- 

 chondrial arrangement of the ascending limb of Henle's loop. But here 

 the granules are very much more scarce, smaller and quite irregularly 

 arranged. Especially in the cell cupola this irregular arrangement is ap- 

 parent and gives to it a peculiar, loose appearance. The distal convoluted 

 tubules are similar to the proximal convoluted tubules, except that the 

 mitochondrial apparatus is less dense and the cell is lower in general. 

 The epithelium of the collecting tubule is very low and the granular struc- 

 tures fine and well scattered (see Figs. 13 and 14). 



Studying on this general basis the kidneys of the rats which had 

 undergone prolonged urea feeding, and comparing the results with those 

 of the rats of the other three groups, no essential difference in the mito- 

 chondrial apparatus can be detected. Also in this group the intensely 

 staining mitochondria of the proximal convoluted tubules dominate the 

 picture (see Figs. 4, 5, 9, 10, 12). But on careful study one is at once 



