330 



THE DEVELOPMENT OF THE CHICK 



lungs of the adult ''have no peritoneal covering," although this 

 is denied by other authors. 



The air-sacs are terminal expansions of entobronchi or of the 

 mesobronchus (Fig. 191). From all of them with the exception 

 of the cervical sac there grow bronchial tubes which connect with 

 parabronchi secondarily within the lung proper. Owing to their 

 method of origin, and also to the fact that the current of air through 

 them in the functional lung is from the air-sacs, these tubes are 

 known as recurrent bronchi. The lungs of birds thus differ from 



Cerir.S. 

 -Br. 



.I-CLt.TnoC- 



-Mes.moi 

 ^Bixt. l- 

 ~Ent.2- 



-EntA. 



BcU4--^ 



Dors. 



---i.at.3--' 

 — HccBr.-- 



-Abd,S. 



Fig. 191. — The air passages of the limg of the chick early on the ninth day 

 of incubation. A Lateral view; B. Mesial view. (After Locy and Larsell.) 

 Abd. S., Abdominal Air-sac. Ant. Th. S., Anterior thoracic air-sac. Br., 

 Bronchi. Cerv. S., Cervical air-sac. Dors., Dorsibronchi. Ect. 1, Ect. 2, 

 etc., First to fourth Ectobronchi. Ent. 1, Ent. 2, etc., First to fourth Ento- 

 bronchi. Lat. 3, Third laterobronchus. Lat. moi.; Mes. moi.. Lateral and 

 mesial moieties of the interclavicular air-sac. Rec. Br., Recurrent bronchi. 



those of other vertebrates in having no terminal alveoli, contain- 

 ing residual air; there is instead a system of communicating tubes 

 through which the air flows. 



The abdominal air-sacs do not undergo any considerable 

 expansion until after the eighth day (cf. Fig. 191). Then they 

 push through the hinder end of the pleuroperitoneal membrane, 

 now fused with the lateral body- wall, and penetrate the latter 

 just beneath the peritoneum. About the tenth day they begin 

 to expand into the abdominal cavity just behind the liver, thus 

 evaginating the peritoneum. The left sac is somewhat larger 



