THE METANEPHROS 



20I 



nation appears, dorsal and somewhat median in position (Fig. 216 B, C). The 

 bud grows at iirst 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 (Fig. 209), which, in 4.6 mm. embryos, is 

 separated from the cranial end of the cord at the twenty-seventh segment. The 

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

 grows cranially, is carried along with it. In embryos of 9 to 13 mm. the pelvis, 

 having advanced cephalad through three segments, attains a position in the 

 retroperitoneal tissue dorsal to the mesonephros and opposite the second lumbar 

 segment. Thereafter, the kidney enlarges both cranially and 

 caudally without shifting its position. The ureter elongates 

 as the embryo grows in length. The cranial growth of the 

 kidney takes place dorsal to the suprarenal gland (Fig. 232). 



Primary collecting 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 there are usu- 

 ally two others (Fig. 210 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. 210 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 develop- 

 ment. The first two primary tubules become the major 

 calyces, and the secondary tubules opening into them form the 

 minor calyces (Fig. 211). The tubules of the third and fourth 

 orders are taken up into the walls of the enlarged secondary 

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

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

 collecting tubules which form the greater part of the medulla of the adult kidney. 



When the four to six primary tubules develop, the nephrogenic cap about the 

 primitive pelvis is subdivided and its four to six parts cover the end of each pri- 

 mary tubule. As new orders of tubules arise, each mass of nephrogenic tissue 

 increases in amount and is again subdivided until finally it forms a peripheral 

 layer about the ends of the branches tributary to a primary tubule. The con- 

 verging branqhes of such a tubular "tree" constitute a primary renal unit, or 

 pyramid, with its base at the periphery of the kidney and its apex projecting into 



Fig. 211.— Re- 

 construction of the 

 ureter, pelvis, caly- 

 ces and their branch- 

 es from the meta- 

 nephros of a 16 mm. 

 human embryo (Hu- 

 ber). X 50. 



