00 Developnu'iit aiul Sh;ipc of Uriniferous Tubules 



tubule show as yet no specific differentiation, but present tlie appearances 

 shown by the epithelium lining the entire tubule at an earlier stage of 

 development. In their further development, such tubules increase 

 greatly in length. This growth in length affects all parts of the tubule, 

 though not to the same extent, the proximal convoluted portion elongat- 

 ing more than the distal convoluted portion, the loops of the former 

 becoming, more pronounced and new ones forming. The chief increase in 

 length is, however, observed in the two arms of Henle's loop; this, in 

 elongating, grows toward the pelvis of the kidney. While thus elongat- 

 ing, a characteristic cellular differentiation is observed in the proximal 

 or descending limb of Henle's loop, beginning with that region of this 

 portion of the tubule which is marked by the termination of the cells with 

 clear protoplasm and extending to near the loop itself — the region where 

 the descending limb becomes continuous with the ascending limb — though 

 never involving the loop itself. The epithelium lining this portion of the 

 descending limb of the loop, differentiates into a flattened epithelium with 

 flattened elliptical nuclei. The protoplasm of these cells shows very little 

 granulation. The tubule obtains in this portion a smaller diameter 

 than in other parts ; this is more clearly seen at a later stage of develop- 

 ment, some little time after the epithelial differentiation here ijientioned 

 may be clearly recognized. The size of the lumen of this portion of the 

 tubule remains about the same as that of other parts. Coincident with 

 this change of cell structure, as observed in the descending limb of 

 Henle's loop, the cells lining the loop itself, the ascending limb, the distal 

 convoluted portion and for a variable distance in the junctional tubule, 

 undergo a change in structure. The protoplasm of the cells lining these 

 portions of the tubule increases in quantity and acquires a granular, 

 often faintly striated appearance and shows an affinity for protoplasmic 

 stains (eosin and erythrosin), these portions then staining more deeply 

 than the proximal convoluted tubule and the proximal arm of Henle's 

 loop. The cells may be described as being of cubical or low columnar 

 shape, with nuclei of spherical or slightly oval form and placed centrally 

 in the cells. The diameter of these portions of the uriniferous tubules 

 is greater than that of the proximal arm of Henle's loop, showing the 

 flattened epithelium, though less than in the proximal convoluted portion. 

 The epithelium lining the different parts of a uriniferous tubule will 

 receive further consideration in discussing more fully developed urinifer- 

 ous tubules. In Fig. 13, A, is shown a model of a uriniferous tubule in 

 which the epithelial differentiation of the different parts of the tubule, as 

 here described, may be clearly recognized. The tubule shown represents 

 one which measures 8.5 cm., of which just about one-half falls to the loop 



