BODY CAVITIES, DIAPHRAGM AND MESENTERIES 



195 



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, is shifted to a horizontal position and gradually 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. 



The Diaphragm and Pericardial Membrane. The lungs grow and expand, 

 not only cranially and caudally, but also laterally and ventrally (Fig. 190 A, B). 

 Room is made for them by the obliteration of the very loose, spongy mesenchyme 



Vena caya 

 inferior 



Pleural Cavity 



Pleuro-pfribneaJ. 

 membrane 



Phrenic nerve in 

 septum transversum 



Liver 



FlG. 189. Transverse section through a 10 mm. embryo showing the pleuro-peritoneal membranes, 

 X 1 6 (from an embryo loaned by Dr. H. C. Tracy). 



of the body wall (Fig. 189). As the lungs grow laterally and ventrally in the 

 body wall around the pericardial cavity, they split off from 'the body wall the 

 pericardial membrane and more and more the heart comes to lies in a mesial 

 position between the lungs (Fig. 190 B). The pleural cavities thus increase 

 rapidly in size. At the same time, the liver grows enormously and on either side 

 a portion of the body wall is taken up into the septum transversum and pleuro- 

 peritoneal membranes. The diaphragm, according to Broman, is thus derived 

 from four sources (Fig. 191): (i) its ventral pericardial portion from the septum 

 transversum; its lateral portions from (2) the pleuro-peritoneal membranes plus 

 (3) derivatives from the body wall; lastly, a median dorsal portion is formed 



