132 EDWARD MCCRADY, JR. 



the pericardium and the peritoneal cavity. Section C shows 

 that these pleural canals pass the ducts of Cuvier on their 

 medial sides. This is also particularly well shown in section B 

 of figure 39. The central reconstruction in figure 36 shows 

 that the pericardium also still communicates with the peri- 

 toneal cavity ventrolaterally by the pericardioperitoneal 

 canal which passes over the omphalomesenteric vein. On 

 each side of the body the original, single, wide communication 

 between the anterior and posterior portions of the splanchno- 

 coele has thus been divided in two by the developing sinus, 

 leaving a narrow dorsal passage (the pleural canal) and a 

 narrow ventral passage (the pericardioperitoneal canal). 



The latter of these two is the first to close, and its closure 

 is accomplished by the continued lateral and ventral expan- 

 sion of the sinus venosus. Figure 39, reconstruction and 

 sections C and D, shows the fusion of the sinus with the 

 lateral body wall continued almost to the point of closure 

 ventrally. In stage 30 (fig. 42) this closure is complete, and 

 the septum transversum is thus formed by the expansion 

 .of the sinus venosus not by the omphalomesenteric veins. 



The closure of the pleural canals, however, is like the 

 same process in other mammals. The central reconstruc- 

 tion shows the one on the left passing medially behind the 

 duct of Cuvier and emerging between the duct and the lung 

 buds. On the right (not figured) this great vein has pressed 

 medially until its wall has fused with the mesentery and thus 

 obliterated the pleural canal on that side. By the same 

 process the thin slit which remains on the left side will be 

 closed in stage 32. The membrane formed by the inward 

 migration of the ducts of Cuvier is the pleuropericardial 

 membrane. 



The septum transversum in the opossum is thus formed by 

 the lateral and ventral expansion of the sinus venosus. The 

 pleuropericardial membrane, on the other hand is formed 

 by the medial migration of the ducts of Cuvier, as in other 

 mammals. 



