336 THE GENITOURINARY ORGANS. 



mention especially the careful researches of Cushny, who brings 

 forth strong proof to show that with the fluid passing through the 

 glomerular epithelium there are carried certain salts and urea, the 

 salts and urea in the proportion in which they occur in the blood- 

 plasma, and that in passage through the uriniferous tubules a cer- 

 tain percentage of the fluids and certain salts are again absorbed, 

 the salts in proportion to their diffusibility or their permeability of 

 the renal cells. 



The permanent kidney is developed as early as the fifth week of 

 embryonic life. The renal anlagen, from which the epithelium of 

 the ureter, renal pelvis, and a portion of the uriniferous tubules is 

 formed, originate from the median portion of the posterior wall of 

 the Wolfrian duct. These buds grow with their blind ends ex- 

 tending anteriorly, and are soon surrounded by cellular areas, the 

 blastema of the kidneys. After the renal bud has become differ- 

 entiated into a narrow tube (the ureter) and a wider central cavity 

 (the renal pelvis) hollow epithelial buds are developed from the 

 latter. These extend radially toward the surface of the renal 

 anlagen, where they undergo a T-shaped division. These latter are 

 the first traces of the papillary ducts and collecting tubules. The 

 ends of these T-shaped divisions are surrounded by a cellular tissue, 

 derived from the mesoderm, which is known as the renal blastema 

 or the nephrogenic tissue. In this tissue there are differentiated 

 spheric masses of cells, which in their further growth differentiate 

 into S-shaped structures one end of which unites with the ends 

 of the epithelial buds, developed as above described. The S-shaped 

 structures acquire a lumen and form the anlagen of the uriniferous 

 tubules, from the arched collecting tubules to and including 

 Bowman's capsule. The ducts of the kidneys, from the papillary 

 ducts to the collecting tubules of the medullary rays, have their 

 origin from the epithelial buds which develop from the side of the 

 Wolfrian ducts, while the uriniferous tubules proper have their 

 origin in the nephrogenic tissues. 



2. THE PELVIS OF THE KIDNEY, URETER, AND BLADDER. 



The renal pelvis, ureter, and urinary bladder are lined by strati- 

 fied transitional epithelium. Its basal cells are nearly cubical ; 

 these support from two to five rows of cells of varying shape. They 

 may be spindle-shaped, irregularly polygonal, conical, or sharply 

 angular, and provided with processes. Their variation in form is 

 probably due to mutual pressure. The superficial cells are large 

 and cylindric, a condition characteristic of the ureter and bladder. 

 Their free ends and lateral surfaces are smooth, but their bases pre- 

 sent indentations and projections due to the irregular outlines of the 

 underlying cells. The superficial cells often possess two or more 

 nuclei. 



