G. Carl Huber 61 



of Henle. (This measurement and others giving the length of urinifer- 

 ous tubules are obtained from models, the length of the tubule as repre- 

 sented in the model being ' divided by the magnification at which it was 

 made — 400). In B of this figure is shown one of the sections of the 

 series of sections of this tubule from which the model was made; it 

 presents with other parts, a section of the distal part of the proximal 

 convoluted portion and its continuity with the proximal arm of Henle's 

 loop, showing the fiattened epithelium, which is cut through its entire 

 length to the immediate vicinity of the loop itself. The transition from 

 the clear epithelium with basal nuclei of the proximal convoluted portion 

 and the first part of the descending limb of Henle's loop to the flattened 

 epithelium of the remainder of the descending limb of Henle's loop, is 

 here clearly shown. The ascending limb of Henle's loop shown in the 

 figure does not belong to the tubule shown in the reconstruction, but to an 

 adjacent tubule also in part reconstructed and of about the same shape 

 and stage of development as the one shown in A of this figure, here 

 added to enable a comparison betwen the epithelial lining of the two arms 

 of Henle's loop as observed at this stage of development. It may be 

 emphasized for purposes of further discussion that the parts here desig- 

 nated as representing the proximal and distal arm of Henle's loop were 

 thus designated after reconstruction of the two tubules of which they 

 form a part. 



The account here given of the differentiation of the epithelium of the 

 two arms of Henle's loop differs very materially from that given by 

 Stoerk in his description of the development and cellular differentiation 

 of this portion of the uriniferous tubule. Stoerk states that at a time 

 when the loop of Henle is as yet very short, the clear epithelium which 

 characterizes that portion of the uriniferous tubule destined to form the 

 proximal convoluted portion extends to about the middle of the bend, 

 which is to form the loop of Henle, lining therefore its proximal half, 

 while the other half of the loop anlage is lined by the darker epithelium, 

 which lines the remainder of the tubule. (The terms clear and dark 

 epithelium are here used in the sense given them by Stoerk who speaks of 

 " hellem und dunklem Epithel"). As the loop elongates, the part lined 

 by dark epithelium grows more rapidly than the part lined by clear 

 epithelium, so that tlie dark epithelium takes in the loop and extends for 

 a variable distance into its proximal arm the clear epithelium ending, 

 therefore, a little alxjve tbc loop. He further states that " The descending 

 arm of Henle's loop is tliorefore to be regarded as representing genetically 

 and morphologically the end segment of the tubulus contortus of the first 

 order, while the ascending limb represents genetically and morphologically 



