462 Anatomy and Dovolopmont of Veins of Oliclonia 



omphalo-mesenterics, {-2) the precardinals, (3) the postcardinals, (4) 

 the ■nmbilieal?, and (5) the sulieardinals of Lewis, 02. 



The Transformations of tire Omphalo-Meseni erics to form the 

 Mesenteric Portal and the Right Hepatic Vein. 



The omphnlo-niesentcric veins in an emhri/o of about 7.-4 mm. — In 

 this stage the omphalo-mesenteric veins are represented hy a pair of 

 vessels arising in a network of the yolk sac and extending forward along 

 the dorsal wall of the mid-gnt near the outer border of the intestinal 

 epithelium. Craniad, they gradually approach each other and iinally 

 fuse immediately caudad of the iirst anlage of the pancreas to form the 

 dorsal anastomosis. Both veins are about equal in size at this point, but 

 further caudad the left vein is somewdiat larger. Craniad of the dorsal 

 anastomosis, both veins enter the liver. The left vein passes around the 

 border of the liver and is not broken up by the hepatic tulniles. It opens 

 into the sinus venosus close to the umbilical of the same side. The right 

 vein, on the other band, soon after entering the liver is more or less 

 broken up into a number of smaller vessels which unite again at the 

 cranial end of the liver and open into the sinus venosus as a single vessel. 



In an embryo of nearly the same measurement, but in which the first 

 indications of the lungs appear, the right omphalo-mesenteric vein is 

 further broken up to form a rete mirabile. This breaking up continues 

 to such an extent that all of the l)lood from the right vein must pass 

 through a network of sinusoids in order to reach the heart. In the 

 meantime the left vein also begins to break up somewhat and there is 

 established a sinusoidal connection between the two halves of the liver. 

 The clogging up, as it were, of the entire hepatic portion of the right 

 omphalo-mesenteric vein by the invasion of the liver tubules causes a 

 large portion of the blood from this vein to stream through the sinusoids 

 to the much freer channel of the left vein. This eventually results in 

 the formation of an anastomosing branch through the liver connecting 

 the two omphalo-mesenteric veins (Fig. 2). The liranch thus formed 

 arises at about the point where the right vein begins to break up and 

 passes diagonally through the liver to the left vein, ventral to the intes- 

 tine. The blood of the right vein that does not pass through this anas- 

 tomosis still streams through the sinusoids and is collected at the cranial 

 end of the liver by two or three small vessels and thus carried to the 

 sinus venosus. 



Thus we find in turtles as well as in snakes and lizards that a double 

 anastomosis is very early formed between the two omphalo-mesenteric 



