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Fig.4. —Proposed embryogenesis of four passages draining venous blood from left upper body to heart.

A, Four-millimeter embryo: Left anterior and posterior cardinal veins join to form left common cardinal vein (black), entering left sinus horn and then primitive sinus venosus, eventually becoming coronary sinus.

B, Ten-millimeter embryo: Development of intersubcardinal anastomosis is illustrated. Analogously, we postulate formation of intersupracardinal anastomosis (gray) around this period. This anastomosis should have vertical connections oriented along cephalocaudal axis and different ventral-dorsal planes. Large defect in illustrated intersupracardinal anastomosis leaves space for development of primitive aortopulmonary system (not shown).

C, Seventeen-millimeter embryos: These diagrams show regression of different parts of previous intersupracardinal anastomosis as result of failure to successfully compete for available space taken by growth of truncoaortic sac and primitive aortic arch system (not shown).

D, Twenty-four-millimeter embryos: Primitive models of major venous channels between bilateral cardinal veins are illustrated. Vessel shown in D1 will become left brachiocephalic vein. Figure reveals nearly total regression of middle and distal portions of left common cardinal vein to become fibrous ligament. Vessel in D2 will become anomalous brachiocephalic vein. Greater preservation of distal portion of left common cardinal vein is constituted as forming proximal part of this vessel. Middle portion of left common cardinal vein is totally obliterated and becomes a fibrous ligament. This ligament is shorter in D2 than in D1. Total preservation of left common cardinal vein results in a persistent left superior vena cava with either presence (D3) or absence (D4) of normal left brachiocephalic vein.