THE EMBRYOLOGY OF THE BIRD's LUNG 487 



moiety of the interclavicular sac, the branch of the third ento- 

 bronchus just described does not divide and produces only the 

 anterior intermediate air-sac. 



The other division of the third entobronchus elongates and 

 expands caudo-dorsally until, early on the ninth day, its dorsal 

 face becomes tri-lobed (fig. 37). At the ten-day stage these 

 lobes branch and later give rise to the parabronchi of the middle 

 portion of the mesial lung facet. 



In the adult, the third entobronchus extends obliquely toward 

 the caudal extremity of the lung on its ventral face (fig. 44). 

 From its medial border six or seven parabronchi are given off 

 and on its lateral border is a transverse branch connecting it 

 with the fourth entobronchus. 



The fourth entobronchus has an embryonic history very simi- 

 lar to that of the third, but it is not connected with any of the 

 air-sacs. Its origin is on that part of the mesobronchus where 

 the swelling of the embryonic vestibulum begins, or where the 

 anterior part of the mesobronchus merges with the middle part. 

 Its orifice is somewhat more mesial in position than that of the 

 third entobronchus. 



Shortly after the beginning of the seventh day the fourth en- 

 tobronchus is short and extends mesially with an expanded end 

 which is directed caudally (fig. 30). 



On the eighth day a branch is budded off very close to its base, 

 which early on the ninth day forms a sickle (figs. 34 and 37) 

 with its point directed laterally. In the adult this sickle-shaped 

 branch becomes the great transverse branch, extending obliquely 

 across the ventral face of the lung and forming a prominent ana- 

 tomical landmark (fig. 44). Campana ('75) enumerated this large 

 transverse branch as a fifth entobranchus, but since it has no 

 direct connection with the central lung tube, even in the embryo, 

 we have, as most previous observers, considered it a branch of 

 the fourth entobi'onehus. 



The principal trunk of the fourth entobronchus extends cau- 

 dally and mesially in a course parallel to the caudal branch of 

 the third entobronchus. The territory included between the 

 main trunk and the transverse branch is overrun with small 



THK AMKKICW JOrKNAI. OF ANATOMY, VOL. 19, NO. '.i 



