ALLMEXTARY TRACT AND ITS APPEXDAGES 



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to imagine the manner in which they must form, from the anatomy 

 of the aduh lung; small branches must arise radially around the 

 central parabronchi and penetrate to the margins of each hexagonal 

 area, branching on the way and terminating in the final alveoli. 



/f-f 



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 



