Interestingly, and explaining the Weigert-Meyer rule
, the future lower pole ureter separates from the Wolffian duct earlier and migrates superiorly and laterally as the urogenital sinus grows.
If the supernumerary kidney is located cranially in relation to the normal kidney, ureter is usually completely separate and may enter bladder ectopically, in which case the Weigert-Meyer rule
may be followed and ureter may insert medially and inferiorly into bladder.
The Weigert-Meyer rule
describes the inverse relationship of the duplex ureteral orifices, in which the ectopic ureter or ureterocele associated with the upper pole is caudal to the lower pole ureteral orifice.