ALIMENTARY TRACT AND ITS APPENDAGES 327 
At six days the mesobronchus within the lung describes a 
course nearly parallel to the cesophagus as far as the middle of 
the lung; in this part of its course it les near the median sur- 
face and ascends very slightly. About the middle of the lung 
it makes a sharp bend, almost at right angles, and passes 
towards the lateral and dorsal surface of the lung; here it enters 
a considerable thin-walled dilatation from which it is con- 
Fig. 189. — Photograph of transverse section through the lungs of an 8-day 
chick embryo. 
A. A. d., Right aortie (systemic) arch. D. art. d.,s., Right and left ductus 
arteriosi. Ent’b.1., Branches of first entobronchus. M. pl. pe., Pleurope- 
ricardial membrane. Mes’b. d., s., Right and left mesobronchia. (Es., 
(Esophagus. Pc., Pericardial cavity. pl. Cav., Pleural cavity. Rec. p. e.s., 
Left pneumato-enteric recess. V.c. a., Anterior ven cave. 
tinued straight backwards by means of a second curve, and 
ends in the same slight thick-walled dilatation that we noted 
on the fourth day. There are thus three very distinct divisions 
of the mesobronchus which we may name the anterior (or ven- 
tral) the middle (or ascending) and the posterior (or dorsal). 
Three evaginations arise from the dorsal wall of the anterior 
