208 



UROGENITAL SYSTEM 



hinfl-g 



outer zone 

 nner zont 



In embryos of 4.5 to 5.5 mm. the mesonephric duct makes a sharp bend just 

 before it joins the cloaca and it is at the angle of this bend that the ureteric anlage 

 of the metanephros appears, dorsal and somewhat median in position (Fig. 209 

 B, C). The bud grows at first dorsally, then cranially. Its distal end expands 

 and forms the primitive pelvis. Its proximal elongated portion is the ureter. 

 The anlage grows into the lower end of the nephrogenic cord which, in 4.6 mm. em- 

 bryos, is separated from the cranial end of the cord at the twenty-seventh seg- 

 ment. The nephrogenic tissue forms a cap about the primitive pelvis and, as the 

 pelvis grows cranially, is carried along with it (Fig. 202). In embryos of 9 to 13 

 mm. the pelvis has reached a position in the retroperitoneal tissue dorsal to the 



mesonephros and opposite 

 the second lumbar segment. 

 Thereafter, / the kidney grows 

 both cranially and caudally 

 without shifting its position. 

 The ureter lengthens as the 

 embryo grows in length. The 

 cranial growth of the kidney 

 takes place dorsal to the 

 suprarenal gland (Fig. 225). 



Primary renal tubules 

 grow out from the primitive 

 pelvis in 10 mm. embryos. 

 Of the first two, one is cranial, 

 the other caudal in position, 

 and between these are two to 

 four others (Fig. 203 B, C). 



From an enlargement, the ampulla, at the end of each primary tubule grow out 

 two, three or four secondary tubules. These in turn give rise to tertiary tubules 

 (Fig. 203 D) and the process is repeated until the fifth month of fetal life, when 

 it is estimated that twelve generations of tubules have been developed. The 

 pelvis and both primary and secondary tubules enlarge during development. 

 The first two primary tubules become the major calyces, and the secondary 

 tubules opening into them form the minor calyces (Fig. 204). The tubules of 

 the third and fourth orders are taken up into the walls of the enlarged second- 

 ary tubules so that the tubules of the fifth order, 20 to 30 in number, open 

 into the calyces minor as papillary ducts. The remaining orders of tubules 



cloacal membrane 



FIG. 202. Reconstruction of the anlages of the meta- 

 nephros (after Schreiner). The layers lettered inner and 

 outer zones constitute the nephrogenic tissue of the 

 metanephros. 



