336 



TEXT-BOOK OF EMBRYOLOGY. 



to the pulmonary artery. No lobe develops on the left side corresponding to 

 the upper (eparterial) on the right. There is a possibility that it is absent in 

 order to allow the arch of the aorta to migrate caudally as it normally does 

 (see p. 254). One of the larger ventral bronchial rami of the left lung is ab- 

 sent, owing to the inclination of the heart toward the left side; but as a compensa- 

 tion the corresponding ramus of the right lung develops more extensively 

 and projects into the space between the pericardium and diaphragm as the 

 infracardiac ramus. 



From the fact that the anlage of the respiratory system is enclosed within 

 the mesentery between the gut and the pericardial cavity, and that its caudal end 

 becomes enclosed within the dorsal edge of the septum transversum, it is obvious 



Pulmonary artery 



Right bronchus 



Upper right 

 bronch. ramus 



Middle right 

 Tbronch. ramus 



Lower right 

 bronch. ramus 



Mesoderm 

 (mesenchyme) 



Trachea 



Left bronchus 



Upper left 

 bronch. ramus 



Lower left branch 

 pulmonary vein 



Lower left 

 bronch. ramus 



FIG. 289. Anlage of lungs of a human embryo of 10.5 mm. His, 



that the lungs will push their way into the dorsal parietal recesses or pleural 

 cavities (Figs. 290 and 295). The way in which the lungs and pleural cavities 

 enlarge and separate the pericardium from the body wall on each side and from 

 the diaphragm is described on page 346 (see Figs. 296 and 297). The mesoder- 

 mal tissue that surrounds the primary lung buds is in part pushed before the 

 numerous outgrowths and in part remains among them (Figs. 287, 288, 289). 

 The part around the lungs, with its covering of mesothelium, comes to form the 

 visceral layer of the pleura which closely invests the entire surface of the lungs 

 and dips down between the lobes. At the roots of the lungs it is continuous 

 with the parietal layer of the pleura lining the inner surface of the pleural cavi- 

 ties. The mesodermal tissue among the bronchi and their terminations gives 

 rise to the connective tissue that separates the lobes and lobules and invests all 

 the structures in the interior of the lungs. This connective tissue at first con- 



