THE DEVELOPMENT OF THE VASCULAR SYSTEM. 229 



cardinals and cardinals, that is, just cranial to the renal veins, disappear. 

 The more cranial portion of the right cardinal persists as the azygos vein 

 which receives the intercostal (segmental) branches and opens into the 

 superior vena cava. An oblique anastomosis is formed, dorsal to the aorta, 

 between the two cardinals (Fig. 195). This anastomosis and the portion of 

 the left cardinal caudal to it together form the hemiazygos vein. The por- 

 tion of the left cardinal cranial to the anastomosis loses its connection 

 with the duct of Cuvier (or coronary sinus) and becomes the accessory 

 hemiazygos vein (Fig. 196). The ascending lumbar veins, which join the 

 azygos and hemiazygos, probably do not represent persistent parts of the 

 caudal ends of the cardinals, but are formed by longitudinal anastomoses 

 between the original segmental lumbar veins. 



The changes which occur in the region of the liver are of much im- 

 portance and result in conditions which bear no resemblance to the primary 

 ones. As has already been noted, the omphalomesenteric veins enter the 

 body at the umbilicus, pass cranially along the intestine and open into the 

 caudal end of the heart. The umbilical veins, which appear soon after, 

 enter the body at the umbilicus and pass cranially, one on each side, in the 

 ventro-lateral part of the body wall; at the level of the heart they turn 

 mesially through the septum transversum and join the corresponding 

 omphalomesenteric veins to form a common trunk on each side, into which 

 the duct of Cuvier then opens (Fig. 193). When the liver grows out as an 

 evagination from the intestine, it comes in contact with the proximal ends 

 of the omphalomesenteric veins and, as it enlarges, breaks them up into 

 numerous smaller channels (Fig. 199). 



The blood then, instead of having a direct channel, is forced to flow 

 through these smaller channels which have been termed sinusoids. When 

 the liver has attained a considerable size a more direct and definite channel 

 is formed, which extends through the substance of the liver from the proximal 

 end of the right omphalomesenteric vein obliquely caudally to the left 

 omphalomesenteric vein. This newly formed channel is the ductus venosus 

 (Figs. 199 and 200). In the meantime, three transverse anastomoses develop 

 between the omphalomesenteric veins just caudal to the liver. The middle 

 one is dorsal to the intestine, the other two ventral, so that the intestine is 

 surrounded by two venous loops or rings (Figs. 199 and 200). At the same 

 time a cross-anastomosis develops between the left umbilical vein, which is 

 primarily the smaller, and the corresponding omphalomesenteric. This 

 anastomosis joins the omphalomesenteric at about the point where the latter 

 joins the ductus venosus, so that it seems to be a continuation of the ductus 

 venosus. A similar cross-anastomosis also develops between the right um- 

 bilical and right omphalomesenteric (Figs. 199 and 200). Thus the blood 



