KIDNEY 



parts are mesodermal. The part derived from the Wolffian duct may be 

 considered first. 



Each Wolffian duct, near the place where it enters the cloaca, forms a 

 knob-like outpocketing which elongates rapidly, becoming the ureter. 

 The distal end of the outpocketing expands and becomes lobular, thus 

 producing the pelvis of the kidney. After the ventral part of the cloaca 



Wd ; Md. Md . 



FIG. 306. THE DEVELOPMENT OF THE RENAL PELVIS AND URETER. (Keibel.) 



A, Human embryo of 11.5 mm. UJ weeks); B, 25 mm. (SJ-p weeks), a., Anus; al. d., allantoic. duct 

 bl., bladder; cl., cloaca; M. d., Mullerian duct; p., pelvis of the kidney; r., rectum; ur., ureter; u. s., 

 urogenital sinus; W. d. Wolffiian duct. 



has been split off to form the bladder, the ureter and Wolffian duct, on 

 either side, open into it by a common outlet (Fig. 306, A). Later, the 

 terminal portion of each Wolffian duct is taken up into the wall of the 

 expanding bladder, so that the ureters acquire openings separate from 



FIG. 307. RECONSTRUC- 

 TION OF THE URETER, 

 RENAL PELVIS. AND 

 ITS BRANCHES IN A 20- 

 MM. HUMAN EMBRYO. 

 (Huber.) 



FIG. 308. FROM A SECTION OF A KIDNEY OF AN 18- 

 MM. HUMAN EMBRYO. X 233. (Huber.) 



a., Primary collecting tubule, with dilated extremity; 

 b,b'., inner layer, and c., outer layer of dense mes- 

 enchyma; d., loose mesenchyma; e., vesicle, the 

 beginning of a renal tubule. 



those of the ducts. With further growth the orifices of the Wolffian ducts 

 are carried toward the median line and downward toward the outlet of 

 the bladder (Fig. 306, B), and this position is permanently retained. 

 Meanwhile the lobes of the renal pelvis have become deeper and formed 



