THE EMBRYOLOGY OF THE BIRD's LUNG 485 



expanded distal end attached by a slender stalk to its point of 

 origin. The stalk first grows niesiad, then enlarges and bends 

 dorsad, and finally, expands into the distal sac mentioned above. 

 This expansion, like that of the first entobronchus, lies directly 

 above the bronchus (fig. 29), but slightly towards the mesial 

 border of the lung. It gives ofT from its lateral border a slender 

 elongated branch which at six days, twenty hours, crosses the 

 bronchus dorsally (fig. 32) and at its tip bends slightly ventro- 

 lateralward. This is the dorsal ramus of the adult second ento- 

 bronchus. It is first noticeable in the latter part of the sixth 

 day. 



Aside from increase in dimensions, but little change occurs in 

 the second entobronchus or its dorsal ramus until the ninth day 

 of development. At this time, as illustrated in figures 35 and 37, 

 the extremity of the entobronchus has divided into two unequal 

 lobe-like branches, the more posterior of which is bifiu'cated (fig. 

 37). The dorsal ramus has elongated and taken on a more ven- 

 tral curve (fig. 37) and is a distinct anatomical feature. 



The second entobronchus is smaller and of less wide-spread 

 distribution than the first. The three lobes, formed on the eighth 

 day, subdivided into branches, which on the ninth day, begin to 

 differentiate into parabronchi that ultimately supply the an- 

 terior mesial region of the adult lung. The parabronchi of the 

 dorsal ramus supply the interior of the cranial part of the lung 

 between the stem of the first entobronchus and its transverse 

 branch. 



Exceptionally the second entol^ronchus gives off near its base 

 a mesial branch (fig. 38) from which develops the mesial moiety 

 of the interclavicular sac. The mesial moiety, however, as de- 

 scribed below, usually has its origin from the third entobronchus. 

 This exceptional point of origin may account for the fact that 

 some observers have ascribed the origin of the interclavicular 

 sac to the second entobronchus while others have claimed that 

 it arises from the third entobronchus. 



The third entobronchus arises (fig. 28) somewhat more mesially 

 on the intra-pulmonary bronchus than the first two. On the 

 second half of the sixth day it extends dorsally a short distance, 



