THE EMBRYOLOGY OF THE BIRD's LUNG 475 



layer of mesothelium. Accordingly, the external boundaries of 

 the lung are formed by a wall of mesoderm which gives no indi- 

 cation of the internal configuration of the endodermal lining. 

 The formation of the branches of the tree must be traced by re- 

 constructions or by air injections, since the study of the lungs as 

 transparencies, without such injections, is very limited in its ap- 

 plication and does not show the outgrow^ths with any degree of 

 completeness as to detail. 



At the 96-hour stage the simple cylindrical tube of endoderm 

 extends into the mesenchyme of the lung primordium and is 

 slightly expanded at its distal extremity (fig. 24, A and B). 

 The trachea has already begun to differentiate from the pharyngo- 

 tracheal groove, and in a few hours' time is definitely formed. 

 The stomach enlargement (ventriculus) pushes on the left lung 

 so as to throw it out of alignment and gi\'e an appearance of 

 asymmetry to the lungs. 



At the proximal end, a short portion of the lung tube lies out- 

 side the boundary of the mesenchymal swelling of the lung pouch 

 and forms the primordium of the extra-pulmonary bronchus 

 (fig. 24). 



By the 100-hour stage the trachea is distinctly formed (fig. 

 25) ; it is of larger calibre than the bronchi that connect with it. 

 The oesophagus curves abruptly from it in the dorsal plane, and 

 then, with a more gentle curvature, it passes caudad for some 

 distance and finally curves ventrally and passes into the stom- 

 ach enlargement between the lungs. 



On the second half of the fifth da}' of development a spindle- 

 shaped expansion arises within the lumen of the lung tube (fig. 

 26). This is a convenient anatomical landmark of the central 

 lung tube of the embryo and may be designated as the embry- 

 onic vestibulum. It does not correspond however to the vestib- 

 ulum of the adult, into which the entobronchi open, neither is it 

 the genetic forerunner of the adult vestibulum. It is a relatively 

 thin-walled dilation of the central lung tube located further cau- 

 dad than the adult structure of that name. Figure 26 shows its 

 dimensions at four days and twenty hours' incubation. Its po- 

 sition varies from about two-fifths to two-thirds the space from 



