THE EMBRYOLOGY OF THE BIRD's LUNG 489 



chi (fig. 39, Par.). The original bifurcation can be readily seen 

 in the later stages (fig. 40). 



The second ectobronchus (figs. 31 and 33, Ect. 2) arises early 

 on the seventh day of development as a bud from the dorso-mesial 

 surface of the vestibulum (embryonic). Its point of attach- 

 ment is slightly more mesial than that of the first ectobronchus 

 and at first its course is nearly parallel with the first ectobronchus, 

 but in late stages it becomes transverse. A bud arises on its dis- 

 tal extremity towards the close of the seventh day but there is 

 no definite branching until early on the ninth day at which time 

 the distal end is divided into three lobes (figs. 35 and 36). These 

 lobules are the fu'st indications of the subdivisions whose branches 

 become parabronchi as shown in figure 39. In the adult the 

 parabronchi of the second ectobronchus supply the dorso-medial 

 face between the third and fourth ribs. 



The embryonic history of the remaining four ectobronchi is 

 so similar that only brief descriptions are required. It is suf- 

 ficient to note that each has its origin slightly more mesially than 

 the preceding on the dilatation of the lung tube and, after 

 branching has begun, they incline more towards the caudad 

 border of the lung. Beginning with ectobronchus number three 

 this caudal bending becomes more marked. In the adult their 

 parabronchi supply the mesial region of the posterior half of the 

 lung. The parabronchi that originate from the sixth ecto- 

 bronchus spread out in such a manner as to be distributed to 

 the dorso-caudal part of the lung. 



None of the ectobronchi have any direct connection with air 

 sacs. 



A seventh ectobronchus is sometimes present in the chick and 

 becomes connected with recurrent bronchi from the abdominal 

 air-sac. 



As to their number in birds in general, Schulze has pointed out 

 that they vary from six to ten and that seven is the usual num- 

 ber. 



The later obronchi. The laterobronchi correspond to those 

 designated 'bronches secondaires externes' by C'ampana. Six 



