ALIMENTARY TRACT AND ITS APPENDAGES 329 
to imagine the manner in which they must form, from the anatomy 
of the adult 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. 
Fic. 190. — Transverse section through the lungs of a chick embryo of 11 days. 
a. th. A. 8., 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. Pl. C., Pleural cavity. ~Pts Ce 
Peritoneal cavity. R., Rib. Se., 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 
