ALIMENTARY TRACT AND ITS APPENDAGES 



329 



another system, that of the recurrent bronchi, develops from the 

 air-sacs wliich we now go on to consider. 



M&MM- 



Fig. 190. — Transverse section through the lungs of a chick embryo of 11 

 days, 

 a. th. A. S., Anterior thoracic air-sac. Ao., Aorta. Aur. d., s., Right and 

 left auricles. B. d., s.. Right and left ducts of Botallus. F., Feather germs. 

 Li., Liver. P. C, Pericardial cavity. " p. p. M., Pleuroperitoneal membrane. 

 P. v., Pulmonary vein. Par'b., Parabronchi. PI. C, Pleural cavity. Pt. C., 

 Peritoneal cavity. R., Rib. Sc, Scapula. V. d., s., Right and left ventri- 

 cles. 



The expanding lungs nearly fill the pleural cavities on the 

 eleventh day. Subsequently, the pleural cavity is obliterated 

 by fusion of the free surfaces of the lungs with the wall of the 

 pleural cavities. Thus it happens that the dorsal surfaces of the 



