Frank A. Stromsten 465 



sists still longer, and is present for a short time after the posterior ring 

 is formed. 



The rapid development of the liver gradually forces the stomach, to- 

 gether with the left omphalo-mesenteric vein, from its original median 

 position. At the same time this vein l)ecomes more and more hroken 

 up into a rete mirabile. This change in position, as well as the breaking 

 up of the left omphalo-mesenteric vein, causes it to be a less favorable 

 route for the blood to reach the heart than the sinusoids of the right half 

 of the liver. As a result, we find in an 8 mm. embryo (Fig. 3) that a 

 new path has been formed for the right vein which soon serves to carry 

 most of the blood from the omphalo-mesenteric veins to the heart. The 

 left omphalo-mesenteric vein then begins to degenerate and eventually 

 loses its connection with the sinus venosus. In the meantime, the anas- 

 tomosing branch through the liver connecting the two omphalo-mesen- 

 teric veins has been gradually breaking up into sinusoids so that later it 

 becomes entirely lost in the hepatic network. All of the blood from the 

 liver must now enter the sinus venosus through the newly formed hepatic 

 portion of the right omphalo-mesenteric vein. 



The omphalo-mesenteric veins of an emhryo of about 10 mm. — The 

 two omphalo-mesenteric veins now fuse ventral to the intestine some 

 distance caudad of the dorsal anastomosis to form the posterior venous 

 ring around the intestine (Fig. 4). The left vein craniad of the dorsal 

 anastomosis is now represented by a very slender branch which passes 

 forward and downward in the wall of the intestine without entering the 

 liver. The right omphalo-mesenteric vein splits soon after entering the 

 liver into two branches of nearly equal size which, after passing forward 

 a short distance, unite again to form a single vessel. Since these two 

 branches are to form the right hepatic revehent and the mesenteric portal 

 veins, while the common stem formed by their union cramially is to be 

 the common hepatic vein, they have been designated Ijy these names in 

 Fig. 4. 



Just as the right omph.alo-mesenteric vein enters the liver, it receives 

 a small vein from the ventral surface of the stomach, the gastric vein 

 of Fig. 4. As this vein is to play a somewhat important role in the 

 formation of the portal vein of Bojanus, it is advisable at this point to 

 notice briefly its earlier development. 



The gastric vein is first definitely seen at the time when the ventral 

 anastomosis between the two omphalo-mesenteric veins is formed through 

 the liver. It then is a very short l)ranch, situated in th.e ventral mesen- 

 tery of the stomach anlage and opens into the anterior ventral anasto- 

 mosis. Later, as the liver increases in size, the vein becomes longer, and 



