THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHTME. 577 



now designated as the iliaca communis. However, two connective- 

 tissue cords result from the degenerating vessels, the ligamenta 

 vesico-umbilicalia lateralia, which run to the navel on the right and 

 left of the bladder. 



(d) Metamorphoses of the Venous System. 



The older excellent works of RATHKE and the more recent meri- 

 torious investigations of His and HOCHSTETTER constitute the 

 foundation of our knowledge in the difficult field with which we are 

 now concerned. They show us that originally all of the chief trunks 

 of the venous system, with the exception of the inferior vena cava, are 

 established in pairs and symmetrically. This holds true not only for 

 the vessels which collect the blood from the walls of the trunk and 

 from the head, but also for the veins of the intestinal tube and the 

 embryonic appendages which arise from it. 



In the first place, so far as regards the veins of the body, the 

 venous blood is collected from the head into the two jugular veins 

 (fig. 320 vj and fig. 321 A je, ji), which run downwards along the 

 dorsal side of the visceral clefts and unite in the vicinity of 

 the heart with the cardinal veins (fig. 320 vca and fig. 321 A ca). 

 The latter advance in the opposite direction, from below upwards, 

 in the dorsal wall of the trunk, and collect the blood especially 

 from the mesonephros. There arise from the confluence of the 

 two veins the Cuvierian ducts (figs. 320, 321 A dc), from which 

 are subsequently developed the two superior venae cavse. The 

 veins of the trunk in Fishes exhibit a symmetrical arrangement 

 like this throughout life. 



In the earliest stages the Cuvierian ducts lie for some distance in 

 the lateral wall of the pericardio-pleural cavity, where they run 

 downwards from the dorsum to the front [ventral] wall of the trunk 

 (fig. 320). On arriving at this point, they enter into the septum 

 transversum, KOLLIKER'S mesocardium laterale, in order to reach the 

 atrium of the heart. This important embryonic structure forms a 

 point of collection for all the venous trunks emptying into the heart. 

 In it there are joined to the Cuvierian ducts the veins from the 

 viscera (fig. 313 V.om and Vu, fig. 320 dv and nv), the paired yolk 

 veins and umbilical veins, all of which are joined into the common 

 sinus venosus, which was previously (p. 558) mentioned apropos of 

 the development of the heart, and which is situated directly between 

 atrium and septum transversum. 



The two vitelline veins (v. omphalomesentericae) return the blood 



