THE EMBRYOLOGY OF THE BIRD's LUNG 461 



tending towards the trachea and i)assing through a network of 

 capillaries whicli coniniiinicate with the vein, mentioned above, 

 as running on the ventral surface of the laryngo-tracheal groove. 



The immediately following external features of development 

 may be rapidly passed over since, for some time, there is no sig- 

 nificant change in the external appearance of the lung. 



Figure 12 shows a dissection of the lung tei-ritory in an embryo 

 of 5^ days incubation and figure 13 a similar dissection of an 

 embryo during the last half of the sixth day. In both these fig- 

 ures the anterior dorsal (cephalic) part of the lung is protuberant 

 and the hook-hke process at the caudal extremity is more evi- 

 dent than in earher stages. They both exhibit the course of the 

 pulmonary artery and pulmonary vein as seen in surface views, 

 and figure 12 also shows in addition, a short ventral spur from 

 the pulmonary artery. 



A more comprehensive \'iew of the superficial blood \'essels of 

 the lung is shown in figure 14, sketched from a specimen in the 

 early part of the seventh day. The shape of the lung and the 

 external appearance of both pulmonary artery and pulmonary 

 vein are well shown. Especially to be noted is the trunk of the 

 vein on its way to enter the left atrium of the heart, and the 

 juncture with this trunk of the pulmonary vein and of the vein 

 (laryngo-tracheal) running along the ventral surface of the 

 trachea. A short arterial branch lea^'esthe pulmonarj^ artery 

 on its ventral border in front of the linig and, passing thi'ough a 

 capillary network, connects with the laryngo-tracheal vein. The 

 anterior ])art of the lung abo^x the pulmonary artery shows a 

 superficial network of blood capillaries. 



Although there is relatively little change in the sml'ace appear- 

 ance on the sixth day, it is to be understood that internal changes 

 of great significance are taking place. The first branches of the 

 bronchial tree arise on the sixth day of development and the 

 network of internal capillaries is moulded over them. These 

 internal changes are described in later divisions of this paper. 



The seventh day stages, as seen from the side, show the lung 

 approximating a I'ectangular outline with a protuberance^ from 



