The definitive kidney or metanephros originates from two mesodermal rudiments: the ureteric bud (metanephric diverticulum) and the metanephric mesoderm (metanephrogenic blastema).
The collecting system develops from the ureteric bud, a dorsal outgrowth of the mesonephric (Wolffian) duct close to its entrance into the cloaca. The bud penetrates the metanephric mesoderm forming the ampulla, which undergoes a series of repeated bifurcations. The ureteric bud gives rise to the ureter, renal pelvis, major and minor calyces and collecting tubules within the kidney.

1. Metanephric tissue cups
2. Ampulla
3. Collecting tubule
4. Metanephric mesoderm

1. Metanephric tissue cups
2. Cell clusters
3. Renal vesicle
4. Collecting tubule

1. Nephron
2. Bowman's capsule
3. Metanephric mesoderm
4. Collecting tubule

1. Bowman's capsule
2. Distal convoluted tubule
3. Collecting tubules

1. Distal convoluted tubule
2. Glomerulus
3. Collecting tubules
4. Proximal convoluted tubule
5. Henle's loop
6. Bowman's capsule

Metanephric mesoderm develops from the sacral portion of the nephrogenic cord. Under the inductive influence of the ampulla, the metanephric mesoderm differentiates into renal vesicles and than into S-shaped tubules. The tubules give rise to the excretory parts of the metanephros, called nephrons. A nephron consists of the Bowman's capsule, the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.

1 . Metanephric mesoderm
2 . Ureteric bud

1. Metanephric mesoderm
2. Ureteric bud
3. Pelvis
4. Major calyx

1. Metanephric mesoderm
2. Ureteric bud
3. Collecting tubules
4. Pelvis

The metanephros originates in the pelvic region. Between the 6th to 8th week of the development, the metanephros ascends to the lumbar region, because of the disproportionate growth of the lumbar and sacral region. During the kidney ascent to the abdominal level it rotates medially almost 90º. Initially, the kidney hilum faces ventrally, but is finally directed anteromedially after the ascent and rotation.

1. Urogenital ridge
2. Genital ridge
3. Mesonephric duct
4. Mesonephros
5. Gonad
6. Metanephric tissue
7. Ureter
8. Cloacal membrane
9. Cloaca
10. Allantois
11. Urorectal septum
12. Tail

1. Mesonephric duct
2. Mesonephros
3. Gonad
4. Metanephros
5. Ureter
6. Cloacal membrane
7. Cloaca
8. Urorectal septum
9. Urogenital sinus
10. Anorectal canal
11. Tail

1 . Ureter
2 . Metanephros
3 . Gonad
4 . Mesonephric duct
5 . Anal membrane
6 . Urogenital membrane
7 . Perineal body
8 . Bladder
9 . Rectum