338 THE DEVELOPMENT OF THE CHICK 



ventral posterior (Fig. 193); the ducts of Ciivier thus become 

 obHque also, and the lateral mesocardia become converted into 

 an oblique septum l^etween the posterior parts of the incipient 

 pleural cavities and the pericardial cavity (pleuro-pericardial 

 membrane). In front of the sinus venosus, however, the pleural 

 and pericardial cavities communicate with one another between 

 the ducts of Cuvier, which form a projection from the lateral 

 bod3^-wall, and the bronchi which project laterally beneath the 

 oesophagus. These apertures are gradually closed by fusion of 

 the walls of the bronchi with the projecting duct of Cuvier, begin- 

 ning in front and extending back to the sinus venosus. Thus the 

 incipient pleural cavities come to end blindly in front, though 

 they still communicate widely behind with the peritoneal cavity. 

 The membrane thus established between pleural alid pericardial 

 cavities is known as the pleuro-pericardial membrane. 



Establishment of Independent Pericardial Walls. With the 

 formation of the ventral body-wall the precardial plate (a portion 

 of the splanchnopleure, which at first forms part of the floor of 

 the pericardial cavity) is gradually replaced by the ventral body- 

 wall. The pericardial cavity is thus bounded ventrally and 

 laterally by the body-wall and posteriorly by the median mass 

 of the septum transversum. It has no independent walls at 

 first. The definitive pericardium is, however, a membranous 

 sac, and this is formed by two main processes: in the first place 

 the membrane of the anterior face of the liver (median mass of 

 the septum transversum) which forms the posterior boundary 

 of the pericardium becomes much thickened, and gradually 

 splits off from the liver (cf. Figs. 148 and 150), the peritoneal 

 cavity extending pari passu between the liver and the membrana 

 pericardiaco-peritoneale thus formed. The suspensory ligament 

 of the liver, however, remains in the middle line, and the mem- 

 brane is also directly continuous with the liver dorsally around 

 the roots of the great veins. Thus a membranous wall is estab- 

 lished for the posterior part of the pericardium. In the second 

 place the peritoneal cavity extends secondarily into the body- 

 wall bounding the pericardium ventrally and laterally, and thus 

 splits a membranous pericardial sac off from the body-wall. In 

 this process the liver appears to play an active role. At least 

 its anterior lobes occupy the peritoneal spaces thus established 

 (Fig. 194). In the mammals, on the other hand, it is the ex- 



