The Weigert-Meyer law describes the relationship of the upper and lower renal moieties in duplicated collecting systems to their drainage inferiorly.

Weigert-Meyer law

With duplex kidney and complete ureteral duplication, the upper renal and lower renal moieties are drained by separate ureters, each having its own ureteral orifice in the bladder.

• upper renal moiety ureter has ectopic insertion medial and inferior to the lower renal moiety ureter, and frequently ends in a ureterocele

• lower renal moiety ureter has orthotopic insertion lateral and superior to the upper renal moiety ureter, and vesicoureteral reflux can occur 

Embryologically, duplication occurs when two separate ureteric buds arise from a single Wolffian duct (mesonephric duct). Due to the future lower pole ureter separating from the Wolffian duct earlier, it migrates superiorly and laterally as the urogenital sinus grows and becomes the upper pole moiety. Despite this migration of the upper tract, the insertion inferiorly maintains the original embryologic relationship, and thus the upper pole moiety ureter drains infero-medial to the normal lower moiety ureter. 

The ectopic insertion often has a ureterocele which obstructs its own collecting system, and can distort the orthotopic lower pole moiety insertion such that it is prone to reflux.

Stephen's ectopic pathway

As always, there are exceptions to the rule. Rarely reported in the literature, Stephen's ectopic pathway postulates that an ectopic ureter may drain not only distally to the normal ureteric orifice (as per Weigert-Meyer law), but may drain medially and superiorly to it (breaking Weigert-Meyer law) ^6,7.

History and etymology

Carl Weigert (1845-1904) was a German pathologist who published his findings regarding ureteric duplication in 1877 ^8,9. Robert Meyer was an American pathologist who first published on this subject in 1907 and formulated it as a "rule" in 1946 ^5,10.