200 FKEDERICK TILNEY 



ing components may be distinguished in the omphalomesenteric 

 drainage line. 



a. The omphalic or vitelline element, a single vein which 

 drains the blood from the yolk sac into the sub-hepatic sinus. 



b. The intra-hepatic vitelline element, appearing as a right 

 and left channel, of which the right is to persist as the hepatic 

 portion of the post-cava, receiving the hepatic revehent veins 

 and, during intra-uterine life, the ductus venosus; the left chan- 

 nel takes part in the development of the hepatic revehent veins 

 and sinusoids. By its early association with the mesenteric 

 veins, this vessel aids in the formation of the hepatic portal 

 system of post-natal life. 



c. The mesenteric elements, appearing in the gut wall as 

 right and left veins, the left communicating with the left intra- 

 hepatic channel, while the right vessel gives its drainage to the 

 left mesenteric by a semicircular anastomosis behind the gut. 

 These latter elements give rise to the definitive portal system. 



2. The umbilical veins (Jig. 5, 6), These vessels, in greater 

 part, still retain their symmetrical arrangement, but the general 

 venous symmetry of the earlier stage is now to some degree 

 disturbed in the region in which the omphalic portion of the 

 vitelline veins become confluent with the umbilical veins to 

 form the sub-hepatic sinus. The significance of the umbilical 

 veins below the sinus appears to be so different from that above 

 it as to justify the distinction of infra-sin al and supra-sinal por- 

 tions in either vein. The capacity of the infra-sinal portion 

 is several times that of the supra-sinal. The left infra-sinal 

 segment has increased greatly in size and, in places, is a double 

 channel. The right infra-sinal segment is also double, but for 

 a shorter distance. The supra-sinal segment of the umbilical 

 vein is a relatively slender vessel, extending from the outer side 

 of the sub-hepatic sinus to the sinus venosus. In its entire 

 course cephalad it receives numerous anastomotic branches from 

 the umbilico-cardinal plexus and is thus still largely concerned 

 in drainage of the body wall. It aids in the return of blood 

 from the sub-hepatic sinus and so affords two direct passages 



