490 



The lateral moiety {s.2) of the sub-bronchial sac, as may be 

 seen by reference to the figure, now projects well out from the lung 

 surface. The beginnings of the recurrent bronchi of this sac (first 

 seen in the ninth day stage) have now elongated considerably and 

 also divided so that they are distinctly regognizable as recurrent 

 bronchi (Fig. 6, r.br.). 



The anterior intermediate air-sac (Fig. 6, a.i.) except for increase 

 in size remains practically unaltered. As will be seen, however, its 

 recurrent bronchi are now much farther developed. 



Fig. 6 also illustrates the relation between the mesial moiety of 

 the sub-bronchial sac and the anterior intermediate sac, both arising 

 from the same branch of the third entobronchus (entobronchus not shown). 



The posterior intermediate and abdominal air-sacs (p.i. and ab.) 

 have greatly increased in size, especially the latter. Only the proximal 

 ends of the recurrent bronchi of these sacs have been figured at this 

 stage but it will be seen the relationship is the same as in the ninth 

 day stage. 



The Fifteenth Day Stage. 



Fig. 7, which represents a partly diagrammatic dorso-lateral view 

 of the lung of a chick near the close of the fifteenth day of incu- 

 bation, shows the condition of the recurrent bronchi of the two posterior 

 sacs at this stage of development. As will be noted, these bronchi 

 have elongated toward the anterior end of the lung The distal tips 

 of the longest recurrent bronchi of the abdominal air-sac have anast- 

 omoed with the latero-ventral parabronchi of the first entobronchus 

 {par.). The more dorsal branches anastomose with the laterobronchi. 

 It is worthy of attention that the bases of the first group (r.br.l) and 

 of the second (r.br. 2) of recurrent bronchi of this sac have united 

 so that the second group at this stage appears to be a branch of the 

 first. There is thus a single orifice (d.o.ab.) opening from the sac 

 into the recurrent bronchi. Strictly speaking this orifice should be 

 considered as the constricted anterior portion of the air-sac, which 

 later expands as explained below. 



The recurrent bronchi (r.br.) of the posterior intermediate sac do 

 not extend so far forward as do the branches of the preceding group. 

 They occupy the extreme ventral part of the lung. Their anastomoses 

 (not shown in the figure) are principally with the first and second 

 laterobronchi (lat. 1, 2). 



