484 



The youngest embryos in which any of the air-sacs appear as 

 projections from the lung proper are in the early part of the seventh 

 day of development. In, an embryo of six days, six hours, incubation 

 the abdominal air-sac (Fig. 1) projects as an extension from the lung 

 proper. The primordium of this sac is well shown in the illustration 

 as the distal portion lying beyond the bend of the central lung tube. 

 In the same embryo may be seen the first indication of the 

 prebronchial air-sac Qjr.). The bud from which it is formed projects 

 forward from the distal extremity of the first entobronchus {en.l). 



The mesial moiety of the sub-bronchial sac and the anterior 

 intermediate sac are also foreshadowed in this specimen as a bud (r. 1) 



of the third entobronchus. 

 It will be seen that the 

 third entobronchus (Fig. 1, 

 en. 3) shows an unequal 

 bifurcation. The more 



Fig. 1. Dorsal aspect of 

 the bronchial tree of both 

 lungs of chick embryo of six 

 days six hours incubation. 

 The bronchial tree injected 

 with air as explained in the 

 text, showing the beginning 

 of the abdominal air-sac (ab.) 

 and the bud of the prebron- 

 chial air-sac (pr.) ab., abdomi- 

 nal air- sac; br., bronchus; d.r., 

 dorsal ramus of second ento- 

 bronchus; ec.1.2 etc., ecto- 

 bronchi; en. 1.2, etc., ento- 



bronchi; lat.l, first laterobronchus ; we«., mesobronchus; pr., prebronchial air-sac; 



r. i, bud which gives origin lo mesial moiety of subbronchial sac and to the anterior 



intermediate air-sac. Camera sketch. X 73. 



posterior and larger branch develops into the entobronchus proper, 

 but the forward projecting bud {r.l) becomes eventually differentiated 

 into the anterior intermediate air-sac, and into the mesial moiety of 

 the sub-bronchial air-sac. Since the three air-sacs mentioned do not 

 appear at the surface of the lung until a considerably later stage, it 

 is premature to designate the buds as primordia. 



The Ninth Day Stage. 

 In the interval between the seventh and ninth day the entire 

 bronchial tree grows rapidly and the air-sacs enlarge. 



