ALIMENTARY TRACT AND ITS APPENDAGES 327 



At six days the mesobronchus within the hing describes a 

 course nearly parallel to th(^ cesopha^us 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, and passes toward the lateral and dorsal 

 surface of the lung; here it enters a considerable thin-walled 

 dilatation from which it is continued straight backwards by means 





X 



rye. C(juir. 





J).o/yc.d. i 





„Jj.iZ?-C.^. 



■Me5'h.d%,. 



■ Oes. 

 iV.c.a. 



^ft' ^ 



\ ■: 



/ ^> 



Fig. 189. — Photograph of transverse section through the lungs of an 8-day 

 chick embryo. 



A. A. d., Risrht aortic (systemic) arch. D. art. d., s., Right and left ductus 

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

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

 OEsophagus. Pc, Pericardial cavity, pi. Cav., Pleural cavity. Rec. p. e. s., 

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



of a second curve, and ends in the same slight thick- walled dila- 

 tation that we noted on the fourth day. There are thus three very 

 distinct divisions of the mesobronchus which we may name the 

 anterior, the middle, and the posterior. 



Four evaginations arise on the sixth day from the mesial 



