G. Carl Hubcr • 45 



the neck of the iiriniferous tubule. At this stage of development this 

 portion of the tube is not clearly defined and is not fully dift'erentiated 

 until the epithelium of the portion which will form the proximal con- 

 voluted tubule shows structural differentiation. The region of the 

 junction of the S-middle piece with the lower part of the upper S-curve 

 forms the anlage for the loop of Henle, that portion of the S-middle 

 piece which does not participate in the formation of the proximal con- 

 voluted tubule forming the proximal arm of the loop, the lower part of 

 the upper S-curve to the region where appears the secondary curvature, 

 about its middle, as above stated, forming the distal arm of the loop, 

 the junction of the two parts, the loop itself. The secondary curvature 

 of the upper S-curve first to appear forms the anlage of the distal con- 

 voluted portion. The upper portion of the upper S-curve from its 

 attachment to the collecting tubule to the region where the distal con- 

 voluted portion has its anlage may be recognized as the beginning of 

 the junctional tubule. The epithelium of the entire tubular portion 

 of the uriniferous tubule presents at this stage of development essen- 

 tially the same structural appearances. The cells throughout are of 

 columnar shape, with a relatively small amount of protoplasm, which, 

 unless the bleaching has been very thorough, retains to a certain extent 

 the nuclear stain, for instance staining lightly in hsemotoxylin if this 

 stain has been used. The nuclei are relatively large, of round or oval 

 shape, and stain relatively deeply. The lumen throughout is of about 

 the same size and is narrow. The statements here made as to genesis 

 of the different parts of a uriniferous tubule are based on observations 

 made on reconstructions of developing uriniferous tubules, of stages 

 beginning with the S-stage to a time when the metamorphosis has pro- 

 gressed until the different parts of a uriniferous tubule may be recog- 

 nized. Attention sliould, however, be again called to the fact that, 

 while models which show these stages of development present a certain 

 similarity of form and arrangement of parts, they differ when studied 

 in detail, as was stated in discussing earlier stages of development. 



In the further elongation of the tubular portions of the uriniferous 

 tubules, as these proceed in their development, beginning with a stage 

 as above discussed, the portion which was above mentioned as formino- 

 the anlage of the loop of Henle needs first consideration. This portion 

 of the tubule is found in the S-stage and some little time after, imme- 

 diately above the concavity of the lower S-curve, the anlage of Bow- 

 man's capsule, as may be seen in reproductions of reconstructions show- 

 ing this stage. As this loop elongates, it either grows a little to one 

 side (forward) and then elongates downwards toward the pelvis of the 



