13 



THE STUDY OP SIX AND TEN MILLIMETER PIG EMBRYOS 



The Veins. The cardinal veins have been reconstructed by Lewis in a 12 

 mm. pig (Fig. 121). The veins of the head drain into the anterior cardinal vein, 

 which becomes the internal jugular vein of the adult. After receiving the ex- 

 ternal jugular veins and the subclavian veins from the upper limb buds the anterior 

 cardinals open into the common cardinal veins (duct of Cuvier). 



The posterior cardinal veins arise in the caudal region, course dorsal to the 



FIG. 121 A. Reconstruction of a 12 mm. pig embryo to show the veins and heart from the left side. 

 For names of parts see Fig. 121 B on opposite page (F. T. Lewis). X 13.5. 



mesonephroi, and drain the mesonephric sinusoids. The subcardinal veins 

 anastomose just caudal to the origin of the superior mesenteric artery and the 

 posterior cardinals are interrupted at this level. The caudal portion of the 

 right posterior cardinal vein now anastomoses with the right subcardinal vein 

 and with it forms a part of the inferior vena cava. The proximal portions of the 

 posterior cardinals open into the common cardinal veins as in the 6 mm. embryo. 



