BODY CAVITIES, DIAPHRAGM AND MESENTERIES 



193 



cardinal veins (ducts of Cuvier) on their way to the heart curve around the pleural 

 cavities laterally in the body wall (Figs. 184 and 186). In embryos of 7 mm. 

 each vein forms a ridge which projects from the body wall mesially into the pleu- 

 ral canals. This ridge, the pulmonary ridge of Mall, later broadens and thickens 

 cranio-caudally (Fig. 186). Its cranial and caudal margins form two sides of a 

 spherical triangle, the third side or base of which is the line of attachment of the 

 dorsal mesentery to the body wall (Fig. 187). At its ventral angle the sides of 



Pleuro-perica.rdi<t,l membrane. 

 Phrenic nerve 



Perlcardial 



Septum 't 



Pleuro- 



peritonea,! 



membrane 



Mesonep/ir 



tomach 



FIG. 187. Reconstruction of an n mm. embryo to show the same structures as in Fig. 186 (after Mall). 



this triangle are continuous with the septum transversum. 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 slit. Its caudal side be- 

 comes the pleuro-peritoneal membrane, which eventually separates dorsally the 

 pleural from the peritoneal cavity. The membranes at first lie nearly in the sagit- 

 tal plane and a portion of the lung is caudal to the pleuro-peritoneal membranes 

 (Fig. 186). Between the stages of 7 and n mm. the dorsal attachment of the 

 13 



