THE EMBRYOLOGY OF THE BIRD's LUNG 477 



the anterior to the posterior end of the king tube. The ectobron- 

 chi arise from this expanded region. Later the more cephahid 

 region of the hmg tube increases in cUameter and, owing to un- 

 equal growth, the embryonic dihition merges into the rest of the 

 hmg tube and is no longer conspicuous. The region of the adult 

 vestibulum is, in reality, occluded at this stage of development. 

 As shown in figiu'e 26, the greater part of the future lumen be- 

 tween the trachea and the anterior end of the lung becomes oc- 

 cluded early on the fifth day and this condition persists through- 

 out that day of incubation. This occluded portion embraces 

 the region of the adult \-estibulum. 



The embryonic dilation referred to was first figured by Selenka 

 in 18G6. Its rise served to divide the central lung tube into 

 three regions — an interior, a middle and a posterior. The lung 

 tube as a whole is in the shape of an elongated S and the dilata- 

 tion of the middle region arches so as to come close to the dorsal 

 surface of the lung. 



Shortly after the appearance of the embryonic \-estibulum, the 

 primary lung tube, earh' on the sixth day, begins to give off buds 

 which form the beginnings of the secondary branches of the bron- 

 chial tree. The first bud to be formed (fig. 27, cut. 1, 5 days, 9 

 hours) is from the internal (mesial in the adult) wall of the intra- 

 pulmonary bronchus This is the primordium of the first ento- 

 bronchus. Behind this (in the same figure) is the smaller bud 

 of the second entobronchus. It will be noted that both are in 

 front of the embryonic vestibulum. 



Two similar hernia-like enlargements follow in quick succes- 

 sion and form the primordia of the third and fourth entobronchi. 

 It results that, at the stage of five days twenty hours' incubation 

 there are present four bladder-hke outgrowths (fig. 28, eyii. 1, 

 2, 3, 4) which are connected with the bronchus by slender stalks. 

 The third and fourth have their attachment to the bronchus 

 somewhat more mesially than do the first and second. These 

 four entobronchi, although arising on the internal wall, curve 

 around the bronchus as the>' grow so as to occupy the ventral 

 face of the lung. 



