BODY CAVITIES, DIAPHRAGM AND MESENTERIES 



187 



sagittal plane (Fig. 193), is shifted to a horizontal position (Fig. 194), and gradu- 

 ally its free margin unites with the dorsal pillars of the diaphragm and with the 

 dorsal mesentery. The opening between the pleural and peritoneal cavities is 

 thus narrowed and finally closed in embryos of 19 to 20 mm. 



B 



A 

 Esophagiis 



Common cardinal vein 



Pleuro-pericardial canal 

 Lung 



Pericardial cavity 



Pleural cavity 



^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 mem- 

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



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 



Fig. 198. — ^Diagram showing the origin of the diaphragm (after Broman): i, Septum transver- 

 sum; 2, 3, derivatives of mesentery; 4, 4, derivatives of pleuro-peritoneal membrane; St 5t parts derived 

 from the body walls; ^, aorta; Oe, esophagus; FC, inferior vena cava. 



into the body wall around the pericardial cavity, the pleuro-pericardial mem- 

 branes 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. 



