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vein (c. [right] Fig. 2 C). The left posterior cardinal vein remains 

 practically unchanged throughout development (c. [left] Fig. 2C). 



A series of stages in the formation of the vena cava may be 

 arranged as follows. The most primitive condition is seen in Cera- 

 todus (Fig. 2 A, B, C) where the vena cava is in part formed from a 

 single posterior cardinal vein, the anterior portion of that vein re- 

 maining present; the other cardinal vein is unmodified , and the hepatic 



Fig. 2. Diagrams illustrating the fonuation of the vena cava. A young Cera- 

 todus. B young Ceratodus. C adult Ceratodus (schematized from Spencer). D Proto- 

 pterus (schematized from Parker). E Salamandra (schematized from Wiedersheim). 

 F Rana (schematized from Ecker), c. posterior cardinal vein. h. hepatic vein. v.c. 

 vena cava. 



and caval veins are entirely separate. The next stage is seen in 

 Protopterus and Lepidosiren (Fig. 2 D) where, although only one 

 cardinal vein shares in the formation of the vena cava, its anterior 

 portion disappears completely and the hepatic veins open into the vena 

 cava instead of into the heart directly. A third stage appears in the 

 Amphibia where the vena cava is formed partially by the fusion of 



