486 



ting from this division becomes the mesial moiety of the sub-bronchial 

 air-sac. As shown in Fig. 2 (s.l) the mesial moiety, although very 

 slender on the eighth day, is nevertheless sufficiently elongated to 

 project beyond the lung wall. The larger and more posterior branch 

 of the ramus (r.l) becomes the anterior intermediate air sac. Usually 

 the sub-bronchial sac is formed by the union of two projections, one 

 (the laleral moiety) springing from the first entobronchus, and the other 

 (mesial moiety) arising from the third entobronchus, in connection 

 with the anterior intermediate sac. Sometimes, however, the mesial 

 moiety arises as a branch of the second entobronchus, in which case 

 the ramus of the third entobronchus gives origin only to the anterior 

 intermediate sac. This condition is illustrated in Fig. 3 which represents 

 a slightly earlier stage than the one just described. 



By unequal growth the anterior intermediate air-sac (Fig. 2, a.i.) 

 has increased relatively much faster than has the mesial moiety of 

 the sub-bronchial. It forms, at this stage, a pronounced cavity in the 

 ventro-mesial part of the lung, and remains until the eleventh day 

 of incubation the largest of the embryonic air-sacs. Extending forward 

 from its ventral anterior part may be seen three small papilla-like 

 buds (r.hr.) connected with the sac by a short stem. These buds are 

 the beginnings of the recurrent bronchi of the anterior intermediate 

 air-sac. They make their first appearance (not figured) as a single 

 bud during the latter part of the seventh day of incubation and by 

 division oft the distal end of this bud, the proximal end of which 

 remains as the stem, the three papillae are formed. 



The posterior intermediate air-sac (Fig. 2, p.i.) also makes its 

 first appearance as a projection through the lung wall at the eighth 

 day stage. It is the distal continuation of the third laterobronchus 

 ("Bronche secondaire externe" of Campana). At this stage it is but 

 slightly distended and shows no indication of recurrent bronchi. 



The abdominal air-sac (a&.), the primordium of which has already 

 been described, is at eight days of incubation, greatly elongated. 

 From its anterior end a bud (Fig. 2, r.hr.) represents the beginning 

 of the recurrent bronchi of this sac. The distal end of the sac is 

 but slightly more expanded than it was on the sixth day, but about 

 two-thirds of it now projects beyond the lung proper. 



