THE BODY CAVITIES, DIAPHRAGM AND MESENTERIES 185 



and 193). In embryos of 7 mm., each vein, with the overlying mesoderm, 

 forms a ridge that projects from the body wall mesially into the pleural 

 canals. This ridge, the pulmonary ridge (of Mall) , is the anlage of both 

 the pleuro-pericardial and pleuro-peritoneal 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. 



Pericardial cavity 



Common cardinal vein 



Pleuro-pericardial membrane 



Pleuro- 

 peritoneal 

 membrane 



Vein to limb 

 bud 



Stomach 



Mesonephros 



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. An arrow 

 passes from pericardial to peritoneal cavity through the pleuro-pericardial canal. 



embryos, reduces the opening between the pleural and pericardial cavities 

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

 which eventually separates dorsally the pleural from the peritoneal cav- 

 ity (Fig. 195). 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 u mm. the dorsal 

 attachment of the septum transversum is carried caudally more rapidly 



