4 WILLIAM A. LOCY AND OLOF LARSELL 



''sufficiently conspicuous in the ostrich to admit the points of 

 my fingers." 



It will be advantageous to describe the development of air- 

 sacs and recurrent bronchi by stages beginning with the seventh 

 day. 



The seventh day stage. There are five air-sacs in the lung of 

 the adult fowl, and as will be shown later, one of these (the in- 

 terclavicular) is the result of the fusion of four moieties, two from 

 each lung, that arise independently. The names emploj^ed in 

 the following descriptions are: cervical, interclavicular, anterior 

 intermediate, posterior intermediate, and abdominal air-sacs. 

 All the air-sacs, except the interclavicular of the adult, are paired. 

 The cervical and interclavicular arise anteriorly, the other three 

 upon the ventral and caudal surface of the lung. 



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

 projections beyond the lung wall are of about six days six hours 

 incubation. As shown in figure 30, the abdominal air-sac of this 

 stage projects as an extension from the lung proper. The pri- 

 mordium of this sac is the slightly expanded distal portion of the 

 mesobronchus lying beyond the bend of the central lung tube. 



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

 cervical air-sac in the form of a bud projecting from the distal 

 extremity of the first entobronchus. In its later develoi)ment the 

 entobronchus becomes much branched, and the orifice of the 

 air-sac is not terminal, as in the embryo . but on the body of the 

 cranial branch of the entobronchus. 



The anterior intermediate air-sac, with the mesial moiety of the 

 interclavicular united to it, is also foreshadowed in this specimen 

 as a bud of the third entobronchus. The third entobronchus 

 shows at this stage. The beginning of an unequal bifurcation 

 which shortly (figs. 34 and 37) becomes well differentiated. The 

 more caudad, and longer, branch of the bifurcation develops into 

 the foliate division of the entobronchus, and the forward project- 

 ing bud becomes eventually differentiated into the anterior in- 

 termediate air-sac and the mesial moiety of the interclavicular 

 sac. To avoid confusion, one should constantly keep in mind 

 that the intercla^-icular air-sac arises from two moieties on each 



