338 THE DEVELOPMENT OF THE CHICK 
ventral posterior (Fig. 193); the ducts of Cuvier thus become 
oblique also, and the lateral mesocardia become converted into 
an oblique septum between 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 
body-wall, and the bronchi which project laterally beneath the 
cesophagus. 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 and pericardial 
cavities is known as the pleuropericardial 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 (ef. 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 oceupy the peritoneal spaces thus established 
(Fig. 194). In the mammals, on the other hand, it is the ex- 
