THE EMBRYOLOGY OF THE BIRD's LUNG 11 



removed from the left lung of an embryo of nine and one-half 

 days incubation. (A) represents a view upon that surface of the 

 anterior intermediate air-sac which is in contact with the lung. 

 It is notable for showing the primordia of recurrent bronchi 

 budding from the anterior lateral border of the air-sac. By 

 comparison with figure 47 it will be observed that the recurrent 

 bronchi of this sac lie close to those from the lateral moiety of the 

 interclavicular sac. 



Figure 49 shows further the mesial moiety of the interclavicular 

 sac spinging from a common canal (the interclavicular canal) 

 into which the anterior intermediate sac also opens. The inter- 

 clavicular in turn opens into the third entobronchus. No recur- 

 rent bronchi arise from the mesial moiety. (B) shows a view 

 upon the cephalad surface of the anterior intermediate air-sac 

 and brings into prominence the forked extremity of the mesial 

 moiety of the interclavicular air-sac, and, also shows the common 

 origin from the third entobronchus of the mesial moiety and the 

 anterior intermediate sac. 



The twelve and fifteen day stages. The subsequent history of 

 the air-sacs presents little difficulties except as regards the forma- 

 tion of the azygous condition of the interclavicular sac of the 

 adult from the union of four parts which arise separately in the 

 embryo. The changes in this air-sac are so unusual that they 

 will be described more in detail. 



The condition of the air-sacs at the close of the twelfth day of 

 incubation is represented in figure 50. This is the camera out- 

 line of a dissection and in finishing is made only slightly diagram- 

 matic. The external aspects of lungs and air-sacs are represented 

 but the recurrent bronchi have not been sketched. By com- 

 parison with figure 47 (the eleventh day stage) it will be seen 

 that the medially directed forks of the mesial moieties of the 

 interclavicular sac have expanded and approach each other more 

 closely in the interbronchial region. The cranially directed 

 prongs have also lengthened and extend forward nearly parallel 

 to the trachea. 



In this figure the connection between the mesial moiety of the 

 interclavicular and the anterior intermediate air-sac is clearly 



