Tiny tubules start to form within intermediate mesoderm; the definitive kidney develops from the metanephros and its collecting system from the ureteric bud which sprouts from the mesonephric duct. The bladder develops from the urogenital sinus - the anterior portion of the cloaca at the end of the embryo’s hindgut. The urinary and genital systems are closely related in adult anatomy - and very closely related in embryonic development:

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