202 



THE UROGENITAL SYSTEM 



distal end expands and forms the primitive pelvis. Its proximal elongated 

 portion is the ureter. The anlage grows into the lower end of the nephro- 

 genic cord (Fig. 209), which, in 46 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 usually 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 development. 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 primary 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 tribu- 

 tary to a primary tubule. The converging branches 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 the pelvis. The 

 apices of the pyramids are termed renal papilla, and through them the 



FIG. 211. Recon- 

 struction of the ureter, 

 pelvis, calyces and 

 their branches from 

 the metanephros of a 

 1 6 mm. human embryo 

 (Huber). X 50. 



