568 EMBRYOLOGY. 



How does the closure of these three originally communicating 

 spaces take place, and how do they attain their altered, final position 

 in relation to one another ? 



The pericardial sac is the first to be separated off. The impulse 

 to separation is furnished by the Cuvierian ducts (fig. 314 dc). One 

 portion of the latter runs down from the dorsum, where it arises by 

 the confluence of the jugular and cardinal veins, along the lateral 

 walls of the trunk to the transverse septum (fig. 314 dc) ; it thereby 



Fig. 314. Sagittal reconstruction of a human embryo 5 mm. long, neck measurement (embryo 



R, His), to elucidate the development of the pericardio-thoracic cavity and the diaphragm, 



after His. 

 ab, Bulbus arteriosus ; brh, thoracic cavity (recessus parietalis, His) ; hh, pericardial cavity ; 



dc, ductus Cuvieri ; dv, vena omphalomesenterica ; m; umbilical vein ; vca, cardinal vein ; 



vj, jugular vein ; lg. lung ; z + I, fundament of the diaphragm and liver ; uk, mandible. 



crowds the pleura into the pericardio-thoracic cavity, and in this 

 manner produces the pleuro-pericardial fold. Since the latter is 

 carried farther and farther inward, it continues to narrow the com- 

 munication between the pericardial cavity (hh) and the two pleural 

 cavities (brfi) ; finally, it cuts off the communication entirely, when 

 its free edge has grown [medianwards] as far as, and has fused with, 

 the mediastinum posterius, in which the oesophagus lies. By this 

 migration of the Cuvierian ducts is also explained the position of the 

 superior vena cava, which later opens into the atrium from above, 

 for it is derived from the Cuvierian duct. Originally located in 



