THE EMBRYOLOGY OF THE OPOSSUM 131 



A conspicuous bud running more vent rally from the original 

 liver diverticulum is the anlage of the gall bladder (fig. 42, 

 reconstruction, just above the omphalomesenteric vein). And 

 finally, at the junction of the liver diverticulum and the duo- 

 denum a small ventrocaudal bud (not shown) represents the 

 ventral pancreatic diverticulum. 



The septum transversum and the pleuropericardial mem- 

 brane. In the opossum the omphalomesenteric veins play no 

 part in the formation of the septum transversum. In figure 26 

 it was shown how the two heart tubes and the veins which are 

 their posterior extensions ran in the splanchnopleure before 

 there was a foregut; how the splanchnopleure folded ven- 

 trally to form the floor of the foregut, and the heart tubes 

 came together to form a single median heart with only a 

 dorsal mesocardium no ventral one. The dorsal meso- 

 cardium forks posteriorly, descends around the edges of the 

 anterior intestinal portal (fig. 33, stage 27, d.m.l.), and con- 

 tinues into the extra-embryonic splanchnopleure as the at- 

 tached face of the omphalomesenteric vein. In the region of 

 the heart proper the line of attachment of the tube is dorsal ; 

 at the level of the anterior intestinal portal it spirals around 

 the omphalomesenteric vein on its medial side; and in the 

 extra-embryonic region it is ventral. The peritoneal cavity 

 is thus continuous with the pericardial cavity by a passage 

 which in stages 27, 28 and 29 is lateral to the omphalomesen- 

 teric veins at the level of the anterior intestinal portal. 



The closure of this passage begins, however, back in stage 

 25 (fig. 25, H) when the sinus venosus expands until it touches 

 the opposite somatopleuric wall at the level of the anterior 

 end of the umbilical veins. In stages 26 and 27 the umbilical 

 gains access to the sinus through the resorption of the walls 

 at this point; and the anterior cardinal also joins in the 

 anastomosis. 



The point at which the sinus meets the somatopleure is well 

 shown in figure 36, section D. Dorsocaudal to the sinus and 

 on either side of the mesentery are the pleural canals the 

 dorsal remnants of the original wide communication between 



