THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 591 



19. The umbilical arteries give off the iliaca interna to the cavity 

 of the pelvis, the iliaca externa to the lower limbs. 



20. After birth the umbilical artery degenerates into the ligamen- 

 tuni vesico-unibilicale laterale, with the exception of its proximal part, 

 which persists as the iliaca comniunis. 



Development of the Chief Venous Trunks. 



21. With the exception of the inferior vena cava, all venous trunks 

 are established in pairs. 



22. The two jugulars collect the blood from the head, the two 

 cardinals from the trunk, but especially from the primitive kidneys. 



23. The jugular and cardinal veins of either side unite to form the 

 Cuvierian ducts, which pass transversely from, the lateral wall of the 

 trunk to the posterior end of the heart, imbedded in a transverse fold 

 of the front wall of the trunk, the septum transversum. 



24. The two vitelline veins collect the blood from the yolk-sac ; 

 from the navel onward they run in the ventral mesentery to the 

 septum transversum. 



25. The two umbilical veins collect the blood from the placenta ; 

 from the attachment of the umbilical cord they run at first in the 

 abdominal wall to the transverse septum. 



26. In the septum transversum the Cuvierian ducts and the 

 vitelline and umbilical veins unite to form the sinus reunions, which 

 subsequently disappears as an independent structure and is in- 

 corporated in the atrium. 



27. The cardinal veins diminish in importance (1) in consequence 

 of the degeneration of the primitive kidneys, and (2) from the fact 

 that the blood from the lower half of the body is conveyed back to 

 the heart by the inferior vena cava. 



28. The upper part of the inferior vena cava arises as an unpaired, 

 independent vessel between the two cardinal veins, and then, at the 

 place where the renal veins empty in, unites with the right cardinal 

 vein. The latter is in this way converted into the lower portion of 

 the inferior cava. 



29. The Cuvierian ducts with the beginning of the jugular veins 

 are designated as the venae cavse superiores. 



30. An asymmetry in the embryonic venous trunks, which are 

 established in pairs, is brought about by the fact that the two 

 superior vena3 cavse, and also at their middle the remnants of the 

 two cardinal veins, are joined together by transverse trunks: 



