6 WILLIAM A. LOCY AND OLOF LARSELL 



By unequal growth the anterior intermediate air-sac (fig. 37) 

 has increased relatively much faster than the mesial moiety of 

 the interclavicular and forms, at this stage, a prominent landmark 

 on the ventro-mesial part of the lung. It 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 {Bec.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 of the 

 distal end of this bud the three papillae are formed. The proxi- 

 mal end remains as the stem and probably forms the basal pocket 

 of Schulze. 



The posterior intermediate air-sac (fig. 36, P.int.sc.) also makes 

 its first appearance as a projection beyong the lung wall on the 

 ninth day. It is the distal continuation of the third laterobron- 

 chus, and at this stage is but slightly distended and shows no 

 indication of recurrent bronchi. 



The abdominal air-sac, is on the ninth day of incubation greatly 

 elongated. From its anterior end and point of union with the 

 mesobronchus a pouch is developed which represents the begin- 

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

 is but slightly more inflated than it was on the seventh day, but 

 about two-thirds of it now project beyond the lung proper. 



From the position of the pouch of the recurrent bronchi one 

 would infer that the anterior limit of the abdominal air-sac of the 

 embryo is more cephalad than is usually recognized. This also 

 changes our idea of the position of the morphological tip of the 

 mesobronchus. 



The tenth day stage. From the beginning of the ninth day of 

 incubation to the close of the tenth there is a steady growth of 

 the various air-sacs and of their recurrent bronchi. It is not 

 necessary to follow in detail the various steps between the two 

 stages, since a description of the conditions found in the later 

 stage will sufficiently indicate the changes through which the 

 various structures have passed. 



