GENERAL EMBRYOLOGY 1645 



either side of the fore-gut and its dorsal mesentery there is a pro- 

 longation of the peritoneal cavity into the thorax. These thoracic 

 prolongations are narrow cleft-like passages, known as the thoracic 

 or pleural recesses, and they represent the primiiive pleural cavities. 

 They communicate ventralwards with the dorsal aspect of the peri- 

 cardial cavity by the pleuro-pericardial openings, and caitdalwards 

 with the peritoneal cavity by the pleuro-peritoneal openings. 



The lung-buds of the pulmonary diverticula abut against these 

 dorsally placed thoracic or pleural recesses in a ventro-lateral 

 direction. As the lung-buds grow, the primitive pleural cavities 

 (thoracic recesses) extend ventro-laterally, gradually displacing the 

 pjirietal pericardium from the body-wall in a mesial direction. 

 Subsequently they surround the pericardial ca\'ity, except on its 

 ventral and cauda.! aspects. 



The coelom, or body-cavity, originally single, is now divided into 

 four compartments — pericardial, two pleural, and peritoneal — all of 

 which communicate with one another. 



The pericardial cavity is shut off from the pleural cavities by the 

 formation of the pleuro-pericardial membrane. This membrane is 

 derived from (i) the right and left lateral mesocardia, and (2) a 

 portion of the dorsal mesocardiimi. The lateral mesocardia (pleuro- 

 pericardial folds) grow in the form of two curtains towards the 

 mesial line, and unite with a portion of the dorsal mesentery to form 

 the pleuro-pericardial membrane. The pericardial cavity is now 

 completely isolated from the pleural cavities. At an earlier date 

 it is shut off from, the peritoneal cavity by the septum transversum. 



The parietal pericardium is derived from (i) the somatic mesoderm 

 of the somatopleure of the body-wall, which has been displaced 

 mesially by the growth of the lungs ; (2) the cephalic or pericardial 

 layer of the septum transversum; and (3) the pleuro-pericardial 

 membrane. The visceral pericardium is derived from the splanchnic 

 mesoderm of the primitive cardiac tube. 



Pleural Cavities. — ^These cavities are formed by the thoracic or 

 pleural recesses of the peritoneal cavity, on either side of the fore- 

 gut and its dorsal mesentery. They originally communicate ven- 

 trally with the dorsal aspect of the pericardial cavity by the pleuro- 

 pericardial openings, and with the peritoneal cav'ity by the pleuro- 

 peritoneal openings. As the lung -buds of the pulmonary diverticula 

 grow, each carries along with it splanchnic mesoderm continuous 

 with that of its diverticulum, and the superficial part of this 

 splanchnic mesoderm ultimately forms the visceral pleura. This 

 explains why the lungs lie behind the pleural sac. The parietal 

 pleura is derived from the somatic mesoderm of the somatopleure of 

 the body- wall. 



Closure of Pleuro-Peritoneal Openings. — The closure of these 

 openings is effected by the pleuro-peritoneal membranes, each of 

 which is formed by the growth ventralwards of the mesoderm of 

 the dorsal body- wall. When this mesoderm reaches the ventral 

 boundary of the pleuro-peritoneal opening on either side, it joins 



