DEVELOPMENT OF THE KIDNEY. 



249 



nephric tubules have been described. In one case a detached portion of 

 the Wolffian duct opposite the 6th, yth and 8th segments has been thought 

 to be associated with the pronephros. 



KIDNEY. 



Thejkidney develops after the Wolffian body has been formed. It 

 arises in two parts, one an outgrowth of the Wolffian duct; and the other, 

 a mass of dense mesenchyma which is said to be derived from the posterior 

 nephrotomes. In this mesenchyma tubules are formed, which have at 

 one end glomeruli similar to those of the \Volffian body, but smaller. The 

 tubules follow a contorted course and acquire their openings into the 

 outgrowth of the Wolffian duct. The kidney is a more complex organ than 

 the Wolffian body, yet it is constructed on a similar plan. 



Wd. M.d. Md 



FIG. 279. THE DEVELOPMENT OF THE RENAL PELVIS AND URETER. (After Keibel.) 

 A, Human embryo of 11.5 mm. (4^ weeks); B, 25 mm. (8^-9 weeks), a., Anus; al. d., allantoic duct; 

 bl., bladder; cl., cloaca; M. d., Mullerian duct; r., rectum; ur., ureter; u. s., urogenital sinus; W. d., 

 Wolffian duct. 



Development. An outpocketing of each Wolffian duct near its en- 

 trance into the allantois becomes elongated and dilated at its distal end 

 (Fig. 279, A). The tubular part becomes the ureter and the lobed terminal 

 expansion is the renal pelvis. As the allantois expands to become the 

 bladder, a portion of the Wolffian duct is taken up into its wall so that the 

 ureters acquire orifices independent of the Wolffian ducts; the latter are 

 carried toward the median line and the outlet of the bladder, as shown 

 in Fig. 279, B. The figure shows their permanent relation to the 

 ureters. 



In later stages the lobes of the renal pelvis become deeper and form 

 the major and minor calyces. In the adult there are usually two major 

 calyces, one at either end of the pelvis, and from these most of the minor 

 calyces grow out; the others spring directly from the main pelvic cavity. 

 There are about eight in all. From the minor calyces the collecting tubules 



