ALIMENTARY TRACT AND ITS APPENDAGES 327 



At six days the mesobronchus within the lung describes a 

 course nearly parallel to the oesophagus as far as the middle of 

 the lung; in this part of its course it lies 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 aortic (systemic) arch. D. art. d., s., Right and left ductus 

 arteriosi. Ent'b.l., Branches of first entobronchus. M. pi. pc., Pleurope- 

 ricardial membrane. Mes'b. d., s., Right and left mesobronchia. (Es., 

 (Esophagus. PC., Pericardial cavity, pi. Cav., Pleural cavity. Rec. p. e. s., 

 Left pneumato-enteric recess. V. c. a., Anterior venae cavse. 



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 w r e 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 



