784 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



a. Chick 



1) Metanephric Duct and Metanephrogenous Tissue. The metanephric kid- 

 ney in the chick begins to arise at the end of the fourth day of incubation 

 from a diverticulum which evaginates from the caudal end of the mesonephric 

 duct as the latter enters the cloaca (fig. 259). The origin of the metanephric 

 diverticulum is similar to that of the various collecting ducts of the meso- 

 nephric kidney, i.e., it arises as an outpushing from the mesonephric duct. 

 The metanephric diverticulum enlarges as its distal end grows forward and 

 dorsad into the nephrogenous tissue of the caudal end of the nephrotomic 

 plate in trunk segments 31-33. As the metanephric diverticulum enlarges and 

 grows into the nephrogenous tissue in this area, the nephrogenous tissue 

 separates from the mesonephric tissue and, together with the metanephric 

 diverticulum, moves anteriad above the mesonephros to the anterior end of 

 the mesonephros. During this process, the distal end of the metanephric di- 

 verticulum enlarges into the future pelvic cavity of the kidney. Numerous 

 small secondary evaginations make their appearance and extend outward 

 from this cavity. The secondary evaginations from the primary pelvic cavity 

 of the kidney form the rudiments of the future collecting ducts of the kidney. 



2) Formation of the Metanephric Renal Units. The formation of the meta- 

 nephric renal units is similar to that of the mesonephric units. At about 7 to 

 8 days of incubation, the nephrogenous tissue around the terminal ends of the 

 collecting-duct evaginations from the primary pelvic cavity of the kidney forms 

 dense epithelial masses. Each of these masses of condensed nephrogenous 

 tissue assumes an S shape. One end of the S-shaped rudiment unites with the 

 distal end of the developing collecting duct, while the other end forms a 

 Malpighian body or renal corpuscle. (Comparable stages involving the devel- 

 opment of the S-shaped rudiment in the mammalian metanephric kidney are 

 shown in figure 348 A-C.) By the eleventh day, well-formed renal units are 

 found in the developing kidney. 



The outer capsule of the kidney arises from the peripheral portions of the 

 nephrogenous tissue and surrounding mesenchyme. The metanephric kidney 

 is retroperitoneal in position, that is, it lies outside the peritoneal cavity proper. 



The posterior end of the metanephric duct or ureter acquires an independent 

 opening into the cloaca as the above changes occur, for the caudal end of 

 the mesonephric duct is drawn into, merges with, and thus contributes to the 

 cloacal wall as the cloaca enlarges. 



b. Mammal (Human) 



1) Formation of the Pelvis, Calyces, Collecting Ducts, and Nephric Units. 



As in the bird, the metanephric kidney of the mammal has a dual origin. 

 One part, the metanephric diverticulum, arises as an evagination from the 

 caudal end of the mesonephric duct at the level of the twenty-eighth somite in 

 the 5- to 6-mm. human embryo (fig. 348H). This evagination extends dorsally 



