The Weigert-Meyer law describes the relationship of the upper and lower renal moieties in duplicated collecting systems to their drainage inferiorly.
Embryologically, duplication occurs when two separate ureteric buds arise from a single Wolffian duct. Due to the future lower pole ureter separating from 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 ureterocoele which obstructs its own collecting system, and can distort the orthotopic lower pole moiety insertion such that it is prone to reflux.
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