1 84 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



membranes. Later it broadens and thickens cranio-caudally (Fig. 193), forming 

 a triangular structure whose apex is continuous with the septum transversum 

 (Fig. 194). Its cranial side forms the pleuro-pericardial membrane and in 9 to 10 

 mm. embryos reduces the opening between the pleural and pericardial cavities 

 to a mere sHt. Its caudal side becomes the pleuro-peritoneal membrane, which 

 eventually separates dorsally the pleural from the peritoneal cavity (Fig. 195). 



Pericardial cavity 



Common cardinal vein 



Pleuro-pericdidial memli in 



Lung I lid 



Pleuro- 



periloneal 



membrane 



Vein to lifnb 

 bud 



Stomach 



HI soiicpkros 



Fig. 193. — Reconstruction of a 7 mm. human embryo showing from the left side the pleuro-pericardial 

 membrane, the pleuro-peritoneal membrane and the septum transversum (after Mall). X 20. The 

 phrenic nerve courses in the pleuro-pericardial membrane. Arrow passes from pericardial to peritoneal 

 cavity through the pleuro-pericardial canal. 



The two sets of membranes at first lie nearly in the sagittal plane and a portion 

 of the lung is caudal to the pleuro-peritoneal membranes (Fig. 193). Between 

 the stages of 7 and 1 1 mm. the dorsal attachment of the septum transversum is 

 carried caudally more rapidly than its ventral portion and its primary ventral 

 surface becomes its dorsal side (Figs. 192 to 194). The pleuro-peritoneal mem- 

 brane is carried caudad with the septum transversum until the lung lies in the 

 angle between the pleuro-peritoneal and pleuro-pericardial raembranes and is 



