THE EMBKYOLOGY OF THE BIRD S LUNG 5 



side, and in subsequent references we should follow with care the 

 development of a mesial and of a lateral moiety from different 

 sources. 



The ninth day stage. In the interval between the seventh and 

 the ninth day the entire bronchial tree grows rapidly and the 

 air-sacs enlarge. 



Early on the ninth day of incubation carefully prepared air in- 

 jections show important advances. The primordia of all five air- 

 sacs now project beyond the lung surface. 



The cervical sac (fig. 36, Cerv.sc.) is the forward prolongation 

 of the cranial lobe of the first entobronchus and is little changed 

 from the former stage. 



In the meantime the first entobronchus has divided into sev- 

 eral branches and from one of them (the transverse branch) may 

 now be seen the beginning of the lateral moiety {Lat. inoi.) of the 

 interclavicular sac. At this stage it is small and does not pro- 

 ject beyond the lung wall. As shown in subsequent develop- 

 ment this lateral moiety fuses with the mesial moiety to form a 

 part of the interclavicular sac of the adult. 



The mesial moiety is well developed at this stage. It arises 

 on an anterior branch of the third entobronchus. This branch 

 bifurcates early on the seventh day of development (not figured) . 

 The smaller, and more cephalad, division becomes the mesial 

 moiety of the interclavicular air-sac, the larger, and more cau- 

 dad, division the anterior intermediate air-sac. As shown in fig- 

 ure 37, the mesial moiety, although very slender on the ninth 

 day, is nevertheless sufficiently elongated to project beyond the 

 lung wall. 



Exceptionally the mesial moiety arises on a branch of the sec- 

 ond entobronchus, in which case the third entobronchus gives 

 origin only to the anterior intermediate sac. This condition is 

 illustrated in figure 38 which represents a slightly earlier stage 

 than the one sketched in figure 37. 



When the development follows the usual rule the mesial moiety 

 of the interclavicular sac is an offshoot of the anterior intermedi- 

 ate air-sac and the two are connected with the third entobron- 

 chus by a single orifice (the interclavicular canal). 



