THE EMBRYOLOGY OF THE BIRD's LUNG 15 



imitiiig into a single trunk there is a single orifice (do.) opening 

 from the abdominal sac into the recurrent bronchi. 



The recurrent bronchi of the posterior intermediate air-sac do 

 not extend so far forward as do the branches of the preceding 

 group. They occupy the extreme ventral part of the lung. 

 Their anastomoses (not sketched) are principally with parabron- 

 chi of the first and second laterobronchus. 



The recurrent bronchi of the two other air-sacs anastomose 

 during the sixteenth day with parabronchi in parts of the lung 

 adjacent to them. The distal ends of the recurrents from the 

 interclavicular anastomose chiefly with the more ventral para- 

 bronchi of the first entobronchus and the recurrents of the an- 

 terior intermediate sac unite with parabronchi of the latero- 

 ventral part of the lung. It results that the anterior intermedi- 

 ate sac comes into communication with the air circuits in the 

 latero-ventral lung region and the interclavicular sac comes into 

 communication with passages in the anterior part of the lung. 



As already pointed out, the anastomosing twigs are at first 

 very slender, but, by the eighteenth day of incubation have in- 

 creased in diameter so as to be practically the same size as the 

 branches which they connect. The recurrent bronchi have by 

 the eighteenth day of development assumed the relations to other 

 parts of the bronchial tree which they bear in the adult lung. 



Transition to the adult. In showing how the adult condition 

 is reached it will be advantageous to summarize the principal 

 changes subsequent to the eleventh day after which period the 

 sacs grow more rapidly. 



The abdominal sacs expand so as to fill the abdominal cavity, 

 and partly surround the viscera therein contained. About the 

 fourteenth day the walls of these sacs begin to fuse with the peri- 

 toneum and this fusion is apparently completed sometime be- 

 fore the eighteenth day of development. The left abdominal sac 

 is somewhat larger than is the right. 



The history of the posterior intermediate sacs after the eleventh 

 day is closely parallel to that of the abdominals and does not 

 require detailed description. 



