THE DEVELOPMENT OF THE VASCULAR SYSTEM 



257 



is collected by the segmental vessels and poured into the cardinal veins and is then dis- 

 tributed in the mesonephros by smaller channels or sinusoids (Minot), whence it is 

 collected and carried off by the subcardinal veins. This passage of blood through purely 

 venous channels simulates the conditions in the liver where there is a true hepatic portal 

 system. 



From this time on, the changes are largely regressions in the cardinal and 

 subcardinal systems, corresponding to the atrophy of the mesonephroi, and 

 rapid increase in the vena cava and its branches. The cranial end of each 

 cardinal becomes smaller; the left loses its connection with both the vena cava 

 and the duct of Cuvier, the right its connection with the vena cava only (Fig. 



Aorta 



Post, cardinal vein 

 Mesonephric duct' 



Omphalomesenteric artery 

 Right umbilical vei 



Intestine 



Post, cardinal vein 



Dorsal mesentery 

 Ccelom 



Left umbilical vein 



FIG. 235. From a transverse section of a 5 mm. human embryo, at the level of the 

 omphalomesenteric (vitelline, superior mesenteric) artery. 



234;. Subsequent changes in these parts of the cardinals will be considered 

 in the following paragraph. For a time the caudal ends of the two cardinals 

 are of equal importance. Later, however, the right becomes larger, while the 

 left atrophies. The right thus becomes a direct continuation and really a 

 part of the vena cava (Figs. 233 and 236). This is brought about, of course, 

 by the original anastomosis between the vena cava and the subcardinal and 

 cardinal. On the left side the anastomosis persists simply as the proximal 

 part of the renal vein (Fig. 236) ; on the right side the renal vein is a new 

 structure which develops after the kidney has attained practically its final 

 position, and opens into the vena cava secondarily. The inferior vena cava 



