410 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



Fig. 138. 



Diagram of the Development of the Paired Venous 

 System of Mammals (Man). (From Gegenbau:,-.) 



j. jugular vein ; cs. vena cava superior ; s. subclavian veins ; c. 

 posterior cardinal vein ; v. vertebral vein ; az. azygos \ein ; 

 cor. coronary vein. 



A. Stage in whicli the cardinal veins have already disap- 

 peared. Their position is indicated by dotted lines. 



B. Later stage when the blood from the left jugular vein is 

 carried into the right to form the single vena cava superior ; a 

 remnant of the left superior cava being however still left. 



C. Stage after the left vertebral vein has disappeared ; the 

 right vertebral remaining as the azygos vein. The coronary vein 

 remains as the last remnant of the left sux3erior vena cava. 



developed, and gradually carries off a larger and larger 

 portion of the blood originally returned by the posterior 

 cardinals. It unites with the common stem of the 

 allantoic and vitelline veins in front of the liver. 



At a later period a pair of trunks is established 

 bringing the blood from the posterior part of the cardinal 

 veins and the crural veins directly into the vena cava 



