THE EMBRYOLOGY OF THE BIKD's LUNG 455 



1. THE EXTERNAL ASPECTS OF LT^NG DEVELOPMENT 



Under this heading the external features of hmg formation 

 will be described while the intra-pulnionary changes will receive 

 separate consideration in the following section. 



The time, the place and the method of formation of the primi- 

 tive lung of the chick has been well described by various ob- 

 servers. In reference to the time, it should be remembered that 

 in all embrj'onic development there is individual variation as 

 well as variable methods of estimating stages. It is not, how- 

 ever, so important to establish an exact correspondence in chro- 

 nology of different observers as to determine the method of lung 

 formation and the normal sequence of changes. 



The first external appearance of the lung of the embryo chick 

 comes in the early part of the third day. Many specimens of 

 30-31 somites show a slight ridge-like enlargement on each side 

 of the latero-ventral surface of the pharynx just behind the 

 fourth gill-pouch. This is in the narrowed respiratory divi- 

 sion of the pharynx, as distinguished from the broadly expanded 

 branchial division. 



Cross sections show that the ridge-like formation is owing to 

 an evagination of endoderm into the surrounding mesenchyma. 

 Figure 1, from a specimen of 30 somites, estimated as 50 hours' 

 develo])ment, is cut through the more prominent part of this out- 

 growth. Figure 2 is from a slightly older specimen, estimated 

 as the 52-hour stage. Both sketches are from camera lucida 

 tracings, so that the outlines are correctly represented, but in 

 finishing, the details, especially the nuclei of cells, have been 

 made diagrammatic. The shallow pockets on the ventral bor- 

 der of the pharynx are the beginnings of lung pouches ; they push 

 out into the mesenchyma which is bordered by a very pronounced 

 mesothelium. At their beginning, therefore, the primitive lungs 

 are paired, and consist of two shallow pouches that open widely 

 into the floor of the pharynx. The surrounding mesoderm is 

 also a i^art of the lung anlage and increases pari passu with the 

 growth of the endodermal part. The endoderm by budding gives 

 rise to the lining membrane of the bronchial tree, the mesoderm 



THE AMERK AN JOUKNAL OF ANATOMY, VOI,. lii, NO. 3 



