THE EMBRYOLOGY OF THE BIRD S LUNG 



463 



the cephalic end aiul another at the caudal extremity. These 

 mark the points of emergence of the cervical and of the abdomi- 

 nal air-sacs. Figure 15 A and B, from a specimen of the last 

 half of the seventh daj^ show the surface of the left lung and of 

 the right lung of the same embryo. Early on the seventh day 

 the abdominal air-sac projects beyond the border of the lung 

 proper, but the cervical lags behind the abdominal in its devel- 

 opment. These are the first two sacs to develop and the others 

 follow shortly except that the posterior intermediate sac is the 

 last to emerge outside the lung wall. The course of the pul- 

 monary artery and of the pulmonary vein is shown in both lungs. 



Pul.At 



Pul.Vn. 



Fig. 1-4 Lung territorj' of a well injected specimen of the early seventh day 

 of development. Note e.specially the laryngo-tracheal artery and its capillary 

 network succeeded hy a vessel that connects with the trunk of the united jjulmo- 

 uary veins. 



The net work of blood vessels of the anterior dorsal surface of 

 the lung was not easily seen in this specimen, partly on account 

 of imperfect injection and partly because the outer covering is 

 thickened, but in heavily injected specimens, the network is 

 seen (as in fig. 14) to occupy the dorsal anterior half of the lung. 



The eighth and ninth days are important periods in the em- 

 bryonic development of the lung, not only on account of inter- 

 nal changes, but also because the air-sacs emerge, and on the 

 ninth day of development, project beyond the surface of the 

 lung, and thus one of the characteristic structural features of the 

 bird lung is established. 



Figure 16, A and B, shows the surface appearance of the lungs 

 on the ninth day of development. The lung has increased in 



