1 88 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



(Lewis) (Fig. 196). Between the free margins of the membranes and the 

 mesentery a temporary opening is left on each side, through which the 

 pleural and peritoneal cavities communicate (Figs. 175, 194 and 200). 



A 



Esophagus 



Common cardinal vein \ Pleuro-pericardial canal 

 Lung 



Pericardia! cavity 



Pleural cavity N<^_ "S#3U^7" 'Lung 



'Pleural cavity 



Septum transversum Pleura- peritoneal membrane Heart Pericardial membrane 



FIG. 197. Diagrams showing the development of the lungs and the formation of the pericardial 

 membrane (modified after Robinson). A, Coronal section; B, transverse section. 



Owing to the caudal migration of the septum transversum and the 

 growth of the lungs and liver, the pleuro-peritoneal membrane, at first 

 lying in a nearly sagittal plane (Fig. 193), is shifted to a horizontal position 



(Fig. 194), and gradually its free 

 margin unites with the dorsal pillars 

 of the diaphragm and with the 

 dorsal mesentery. The opening be- 

 tween the pleural and peritoneal 

 cavities is thus narrowed and finally 

 closed in embryos of 19 to 20 mm. 

 The Diaphragm and Pericardial 

 Membrane. The lungs grow and 

 expand not only cranially and 

 caudally but also laterally and 

 ventrally (Fig. 197). Room is made 

 for them by the obliteration of the 

 very loose, spongy mesenchyme of 

 the adjacent body wall (Fig. 196). 

 As the lungs burrow laterally and 

 ventrally into the body wall around 

 the pericardial cavity, the pleuro- 

 pericardial membranes enlarge at the expense of this tissue and more 

 and more the heart comes to lie in a mesial position between the lungs 

 (Fig. 197 B). The pleural cavities thus increase rapidly in size. 



FIG. 198. Diagram showing the origin 

 of the diaphragm (after Broman). i, 

 Septum transversum; 2, 3, derivatives of 

 mesentery; 4, 4, derivatives of pleuro-peri- 

 toneal membrane; 5, 5, parts derived from 

 the body wall; A, aorta; Oe, esophagus; VC, 

 inferior vena cava. 



