THE EMBRYOLOGY OF THE BIRD'S LUNG 



13 



at which particular morphological changes occur one must, as 

 previously indicated, take into account that individual variation 

 is verj^ common in embryonic development. 



The lateral moieties (fig. 51) have greatly expanded so as to 

 unite in the median line. At this period they are the most promi- 

 nent part of the interclavicular sac. The stalks connecting this 

 portion_of the interclavicular with the first entobronchus are 



A.lntSc. 



P.lnt.Sc. 



Mg. 50 Ventral view of the lungs and air-sacs of a twelve day embryo. 



clearly indicated but the recurrent bronchi have not been shown. 

 At the extreme antero-lateral margin of this division of the sac 

 there is a narrow neck opening into the axillary sac. 



The method of formation of the single interclavicular sac of 

 the adult is now clearly foreshadowed. The four parts from 

 which it is formed (two moieties from each lung) are in contact 

 but still separated by partition walls. 



The sixteenth day stage. The recurrent bronchi are now suf- 

 ficiently advanced to observe the main features of their distribu- 

 tion. Figure 52 represents a partly diagrammatic camera trac- 



