92 Development and Shape of Uriniferous Tubules 



It is not my purpose to enter here on a discussion of the epithelium 

 lining the different parts of a uriniferous tubule. For this the reader 

 is referred to Disse's quite recent account, he himself having made note- 

 worthy contributions to this point. The observations made on the sec- 

 tions from which the models were made lead me to recognize three dis- 

 tinct varieties of epithelium in a uriniferous tubule : 



1. The epithelium lining the entire proximal convoluted portion, to 

 the region in the upper part of the descending limb, where the tubule is 

 quite rapidly reduced in size, where it assumes the shape and structure 

 of the narrow portion of the limb. The epithelium of the proximal 

 convoluted portion is sufficiently known to obviate an especial description 

 of it. 



2. The clear, flattened epithelium of the descending limb of the loop 

 in the region in which it presents a small diameter. I have gained the 

 impression that too much emphasis is put on the statement that the 

 cells of this epithelium are thicker in the region of the nucleus, thus 

 bulging into the lumen of the tubule, and as the nuclei do not lie oppo- 

 site, but alternate in the course of the tubule, its lumen is not of the 

 cylindrical form, but irregular — wavy as seen in sections. In sections 

 from material which I have regarded as showing good fixation, the 

 lumen of this portion of the uriniferous tubule presents a much more 

 regular outline than in sections from material not so well fixed, the cells 

 presenting throughout a more uniform thickness. The suggestion is 

 therefore made that the description, as generally given, refers to prepa- 

 rations which do not present the correct structure of this portioti of the 

 uriniferous tubules. This point is reserved for further investigation. 



3. The last portion of the descending limb, the loop itself, the ascend- 

 ing limb, the distal convoluted portion, and the junctional tubule to 

 near the collecting tube present an epithelium which shows great simi- 

 larity of form as well as structure. The lining cells show a form which 

 is irregularly cubical or short columnar. Disse describes the epithe- 

 lium of the ascending limb as showing indistinct cell boundaries, while 

 these are more distinct in the epithelium of the distal convoluted portion ; 

 the protoplasm of the latter, although granular, is also described as 

 being clearer. In both regions the cells show an outer dark zone which 

 is finely striated and an inner zone which is lighter, the nuclei being 

 placed at the junction of the two zones. In my own preparations the 

 epithelium lining the ascending limb and the distal convoluted portion 

 do not respectively present a structure which differs to a degree making 

 it necessary to group them separately, and I have, therefore, placed them 

 under one head. The uriniferous tubule, for a short though variable 



