THE EMKHYOLOGY OF THE BIRD S LUNG / 



In an embryo of 9| days incubation (figs. 39 and 40) the cer- 

 vical sac shows only an increase in size proportioned to the gen- 

 eral growth of the king. 



The lateral moiety of the interclavicular sac (fig. 40, Lat.moi.) 

 now shows a well defined extension outside the lung wall. The 

 subsequent history of this sac indicates that some of the intra- 

 pulmonary part of the transverse branch of the first entobron- 

 chus, from which the sac arises, must be considered as a part of 

 the air-sac primordium. The line of separation betwen the ento- 

 bronchus and the lateral moiety is where the more dorsal buds of 

 recurrent bronchi arise. Two groups of recurrent bronchi be- 

 longing to this division of the interclavicular have appeared. 

 The bud which represents the more ventral group of these bronchi 

 extends caudally and ventrally and is as yet undivided. The 

 more dorsal group is represented by an already bifurcated bud 

 projecting caudally and somewhat mesially. These buds develop 

 into the only recurrent bronchi arising from the interclavicular 

 air-sac and it is to be noted that they arise only on its lateral 

 moiety. 



The mesial moiety of this stage, a part of which is shown in 

 figure 40, is scarcely changed from the condition described in the 

 eight day embryo, except that it has increased in size. 



The anterior intermediate air-sac {A.int.sc.) at the close of the 

 tenth day of incubation has enlarged considerably, as compared 

 with the preceding stage. The recurrent bronchi have not 

 greatly changed, but the stem thereof has elongated to some 

 extent. 



The posterior intermediate sac now projects beyond the lung, 

 and its distal end forms a flask-like swelling. The proximal part 

 remains still constricted and lies within the lung. From this 

 part two buds are seen projecting dorsad and cephalad. The 

 more anterior of these buds is already divided at its tip. These 

 branches indicate the beginnings of the recurrent bronchi of the 

 posterior intermediate air-sac. 



The abdominal sac (figs. 39 and 40) has expanded greatly since 

 the eighth day stage, and now lies almost entirely outside the 

 lung. Its recurrent bronchi, of which there are two sets, have 



