THE EMBRYOLOGY OF THE BIRD's LUNG 459 



stage the pulmonary artery is usually established and, in injected 

 specimens forms one of the external anatomical landmarks. In 

 the subsequent descriptions the pulmonary artery will be included 

 as a feature of external anatomy but the details of its formation 

 are separately considered under another heading. 



In the specimen sketched in figure 8, the proximal end of the 

 pulmonary artery shows as a spur from the sixth aortic arch, but 

 owing to imperfect injection, the distal part that is formed in the 

 lung wall is not seen. The rudimentary fifth arch is present in 

 this specimen as a short vessel arising from the truncus arteriosus 

 and joining the lower half of the sixth arch. Much variation 

 exists as to the presence or absence of the rudimentary fifth 

 arch and as to its dimensions when present. The degree of de- 

 velopment of the pulmonary artery also varies in different speci- 

 mens of this age. 



Figure 9 represents a side view, and figure 10 a ventral view of 

 the lung in the last half of the fifth day of development. The 

 lobe at the posterior end of the lung pouch is shown in figiu-e 9 

 and the trunk of the pulmonary is fully established. When 

 viewed from the ventral surface (fig. 10) the right lung forms a 

 somewhat greater angle (fig. 6) with the oesophagus than the left, 

 producing an appearance of asymmetry. This asymmetry, how- 

 ever, is not owing to a difference in size of the lung (as in mam- 

 mals and some reptiles) but to the pressure of the stomach (ven- 

 triculus) enlargement which begins at about this period. The 

 greatest asymmetry comes about the middle of the fifth day; it 

 is gradually rectified with the change in relative position of the 

 viscera and the symmetry is restored by the eighth day. In 

 figure 10 the pulmonary veins and the laryngo-tracheal branch 

 are also shown on the ventral surface of specimen. 



Near the close of the fifth day of development well injected 

 specimens (fig, 11) show a network of blood vessels near the sur- 

 face occupying a small area on the anterior dorsal part of the 

 lungs. Sections and transparencies of this stage show that the 

 network of vessels within the lungs occupies chiefly the dorsal 

 region and is more extensive than appears from the surface. 

 This figure shows also a branch from the pulmonary artery ex- 



