42 



DIAPHRAGM 



which become the pleural spaces, and lateral recesses which belong to the peritoneal 

 cavity. These peritoneal (antero-lateral) recesses of course overlap the pericardium. 

 Thus, while the anterior edge of the septum transversum internal to the 

 pleuro-peritoneal membrane separates on each side the pleural space from the 

 pericardium, the portion lateral to the pleuro-peritoneal fold separates the peritoneal 

 recess from the pericardium (peritoneo-pericardial membrane) (fig. 297). The 

 pleuro-peritoneal membranes soon become altered in position owing to the growth 

 of the body- wall and the extension of the pleural cavities. Their dorsal attach- 

 ments are shifted laterally until the membrane ultimately assumes an oblique 

 position, lying in a caudo-dorsal and cranio-ventral direction. At the same 

 time another fold has formed on the right side of the mesenteric septum, due to 

 the formation of a ccelomic diverticulum extending inwards and then forwards 



p leuro-peritoneal 

 membrane 



sopJiagus 



pleural space 

 lung 



pericardiu 



ventricle 



FlG. 297. -DIAGRAMMATIC SECTION OF THE TRUNK OF AN EMBBYO CALF 15 MM. LONG. (Wolfel.) 



into its substance. This cleaves the mesentery into two lamellae, of which the 

 left follows the gut and the right passes on to the dorsal aspect of the right lobe 

 of the liver (fig. 296). This light lamella (mesolateral fold of Brachet) has a 

 free edge bounding the opening (foramen of Winslow) into the diverticulum, and 

 it has a dorsal and a ventral pillar. The ventral column passes on to the liver to 

 be continuous with the ventral pillar of the pleuro-peritoneal membrane; the 

 dorsal is continued on the dorsal wall of the body-cavity in a fold in which the 

 vena cava inferior runs, and hence is known as the caval mesentery. The pleuro- 

 peritoneal openings are constricted and ultimately closed by the extension of the 

 liver trabeculse, first mesially into the caval and mesolateral folds on the right 

 side, and into the dorsal mesentery round the cardia of the stomach on the left 

 side, causing a thickening of both lamellae of the mesenteric septum ; second 



