THE EMBRYOLOGY OF THE BIRD S LUNG 



469 



ular sac connected with the transverse branch of the first ento- 

 bronchus. The mesial moiety {Mes.moi.) arises in connection 

 with the anterior intermediate air-sac, and these two have a 

 common opening into the third entobronchus. The distal end 

 of the mesial moiety is forked and partly encircles the main 

 bronchus. 



Fig. 19 J^i^so( lion cxpoMiiii, (lu i iglit lung and adjacent viscera of an embryo 

 at the close of the tenth day (jt incubation. Shows indentations of the ribs and 

 recurrent bronchi from the abdominal and posterior intermediate air-sacs. 



In embryos 10| days old the air-sacs exhibit the very interest- 

 ing condition (figs. 40 and 47) of recurrent bronchi springing 

 from all the air-sacs except the cervical (which never has any), 

 and, further, that the interclavicular sac is derived from two 

 moieties on each lung. These are the lateral and mesial moieties 

 already referred to. Details in reference to the union of these 



