Possible mechanism

Group I

Figure 1(a)

Unilateral right-sided single preureteric vena cava. Persistence of the right postcardinal vein, disappearance or failure of development of the right supracardinal vein.

Group II

Figure 1(b)

Unilateral right-sided double Inferior Vena Cava. Ureter between the two veins. Persistence of the right supracardinal vein and of the right postcardinal vein.

Group III

Figure 1(c)

Bilateral, single Inferior Vena Cava, the right being preureteric and the left postureteric. Persistence of the right postcardinal vein and the left supracardinal vein.

Group IV

Figure 1(d)

Bilateral single preureteric Inferior Vena Cava. Persistence of the right and left postcardinal veins.

Group V

Figure 1(e)

Double right vena cava, ureter between the two veins, single postureteric left vena cava. Persistence of the right supracardinal and postcardinal veins as well as of the left supracardinal vein.