204 



NORMAL HISTOLOGY. 



of the urethra are liberally supplied with blood-vessels, which form 

 rich capillary net-works beneath the epithelium. The larger lym- 

 phatics lie in the submucosa, where they receive the radicles accom- 

 panying the blood-vessels within the mucosa. The nerves which 

 bear the blood-vessels company find their endings within the super- 

 ficial sub-epithelial layer of the mucosa. 



The Development of the Urinary Organs. In tracing the 

 history of the formation of these structures the genesis of three 

 distinct divisions must be considered the development of the 

 kidney and ureter, that of the bladder, and that of the urethra. 



The permanent kidney is preceded in the embryo by an important 

 although transient excretory organ, the Wolffian body ; the prod- 

 ucts of this organ are carried off and emptied into the primitive intes- 

 tinal canal by its excretory tube, the Wolffian duct. All parts of 

 the Wolffian body and the duct consist of mesodermic tissue alone, 

 these structures arising essentially as outgrowths from the primitive 

 peritoneal lining into the surrounding mesoderm. The mesothelial 

 evaginations so originating constitute the primary tubules of the 

 Wolffian body, from which numerous secondary canals are derived ; 

 the subsequent development of blood-vessels in intimate relation 

 with the tubules produces the primitive Mal- 

 pighian corpuscles of the fcetal organ. 



The first step in the development of the 

 kidney consists in a dorsal outgrowth from 

 the Wolffian duct near its cloacal end ; this diver- 

 ticulum grows forward and parallel with the 

 Wolffian duct until its extremity reaches a 

 position behind and somewhat above the caudal 

 end of the Wolffian body. The primary kidney- 

 tube now expands at its upper end, the dilated 

 portion subsequently undergoing peripheral 

 cleavage into a number of tubular compart- 

 ments. Coincidently with the growth and dif- 

 ferentiation of the epithelial evagination from 

 the Wolffian duct, the mesodermic tissue into 

 which the expanded extremity of the diverticu- 

 lum makes its way becomes greatly condensed. 

 The fundamental structures in the development 

 of the kidney and the ureter are now distinctly 

 defined. The narrow, elongated portion of the 

 outgrowth from the Wolffian duct becomes the 

 epithelial lining of the ureter, while the ex- 

 panded terminal part forms that of the pelvis and of the urinifer- 

 ous tubules. The connective and vascular tissues are derived from 



FIG. 246. 



Sagittal section of eleven- 

 day rabbit embryo, show- 

 ing earliest stage of de- 

 velopment of kidney as 

 outgrowth (k) fromWolffian 

 duct (jc) into surrounding 

 mesoderm (m). 



