THE URINARY ORGANS. 



205 



the surrounding mesoderm, the epithelium of the kidney and of its 

 duct alone being the immediate product of the evagination. The 

 formation of the collecting tubes and the uriniferous tubules fol- 

 lows the division and subdivision of the compartments into which the 

 primitive pelvis and calices separate, the entire elaborate system of 

 tubules resulting from the extension and branching of the primary 

 canals. The surrounding mesodermic tissue early differentiates a 

 limiting zone or primitive capsule, which defines the form of the 

 developing kidney and opposes the growth of the tubules in a straight 

 direction, thereby inducing the marked tortuosity accompanying the 

 subsequent increase in the length 



of the uriniferous canals. The in- FIG. 248. 



vagination of the termination of the 

 tubule and the simultaneous devel- 



FIG. 247. 





Sagittal section of fifteen-day rabbit em- 

 bryo : the developing kidney presents an 

 oval mass of condensed mesoderm (m) into 

 which the tubular compartments (t) of the 

 divided primitive renal pelvis (k) extend. 



Sagittal section of 30 mm. cow embryo: K, 

 developing kidney, containing Malpighian 

 bodies (M) and tubules; u, part of renal 

 pelvis ; W ', atrophic Wolffian body ; m, glo 

 merulus of primary Malpighian body ; t, de- 

 generating tubules of the organ. 



opment of groups of capillary blood-vessels in intimate relation with 

 them give origin to the characteristic Malpighian bodies. The 

 epithelium of all parts of the uriniferous tubules, of the renal 

 pelvis, and of the ureter is derived directly from the outgrowth 

 from the WolfHan duct ; the interstitial connective tissue, the 

 blood-vessels, and other structures are contributed by the surround- 

 ing condensed mesoderm. Since, as has been already stated, the 

 entire Wolffian body, including its duct, is a product of the meso- 

 derm, the epithelial evagination and its derivatives must be referred 

 likewise to the middle blastodermic layer, all parts of the urinary 



