872 THE DEVELOPMENT OF THE COELOMIC CAVITIES 



4. Separation of the Parietal Pericardial Wall in Mammals 



On the other hand, in the mammals, it is the pleural cavities, i.e., the pleural 

 divisions of the splanchnocoelic coelom, which extend ventrally around the 

 heart and thus separate the parietal pericardium from the thoracic body wall 

 (fig. 364A and B). Posteriorly, they separate the pericardium from the anterior 

 face of the developing diaphragm (fig. 363B). The secondary condition of 

 the mediastinum thus is established which extends dorsoventrally between the 

 two pleural sacs (fig. 364B-2). It is to be observed that the medial walls of 

 the pleural sacs fuse with the lateral walls of the pericardium by means of 

 the connective tissue which forms between these two layers. 



F. The Mammalian Diaphragm 



The mammalian diaphragm is a musculotendinous structure, innervated by 

 the phrenic nerve and developed from tissues around the gut, primary septum 

 transversum, the two pleuroperitoneal membranes, and possibly also by con- 

 tributions from the body wall. Study figure 364C. The exact origin of the 

 voluntary musculature of the diaphragm is in doubt, but it is assumed to 

 come from the cervical myotomes in the region of origin of the phrenic nerve, 

 together with some invasion of muscle substance from the lateral body wall 

 posterior to the cervical area. Successive caudal positions of the septum trans- 

 versum and developing diaphragm, assumed during its recession in the body, 

 are shown in figure 364D. 



G. The Pulmonary Diaphragm or Aponeurosis of the Chick 



The pulmonary diaphragm in the chick is a composite structure formed 

 of two membranes which develop in a horizontal position in the dorsal region 

 of the thoracic area below the lungs. Each of these two membranes fuses 

 with the median mesentery and the lateral body wall and thus forms a par- 

 tition separating the pleural cavities above from the peritoneal cavity below 

 (fig. 362J). The development of this partitioning membrane is as follows: 



In the four- to five-day chick as the lung buds grow out dorso-posteriad 

 each lung bud pushes into a mass of mesenchyme which is continuous from 

 the splanchnopleure around the esophagus to the dorsal region of the liver. 



This connecting bridge of mesenchyme is the pleuro-peritoneal membrane 

 and it extends from the region of the esophagus across the lower part of the 

 lung bud tissue to the liver lobe on each side. The mesenchymal connection 

 of this membrane with the liver then spreads laterally to unite with the lateral 

 body wall. As a result, the pleural cavity above is shut off from the peritoneal 

 cavity below. A continual growth dorsoposteriad of the pleuro-peritoneal 

 membrane, and subsequent fusion with the dorsal body wall tissues, separates 

 the pleural cavity completely from the peritoneal cavity. However, certain 

 canals remain in this membrane for the passage of the air sacs (see Chapter 

 14) of the lungs. Striated musculature from the lateral body wall grows into 



