326 THE DEVELOPMENT OF THE CHICK 



the trachea to the hilum of the lung (extra-puhnonary bronchus), 

 and its continuation within the huig, extending its entire length 

 (mesobronchus) . All the air passages of the lung, and the air- 

 sacs, arise from the mesobronchi by processes of budding and 

 branching, enlargement of buds to form air-sacs, and by various 

 secondary anastomoses of branches. The mesobronchi are sur- 

 rounded from the first by a thick mass of mesenchyme, covered 

 of course towards the body cavity by a layer of mesothelium. 

 In the early development the mesenchyme of the lung-primordia 

 grows so rapidly as to provide adequate space for the branch- 

 ing of the mesobronchi entirely within the mesenchymal 

 tissue. 



Although the development of the lungs of the chick was 

 studied by several earlier investigators, our principal reliance in 

 this subject rests on the beautiful and complete study by Locy 

 and his students. 



We may note the general topographical development as 

 follows: The expansion of the lungs takes place into the pleural 

 cavities; they therefore raise themselves from their surfaces of 

 attachment, oesophagus and pleuroperitoneal membrane, and 

 project in all directions, but especially dorsally and anteriorly 

 (Fig. 189). We may thus distinguish free and attached surfaces; 

 the latter is nearly a plane surface and on the whole ventral in 

 position, and the free arched surfaces are dorsal. However, it 

 should be remembered that the pleuroperitoneal membrane 

 which forms the attached surface, lies at first in a sagittal plane, 

 and only secondarily becomes frontal. In successive stages, the 

 attached surface of the lung (pleuroperitoneal membrane) 

 rotates from a sagittal to an approximately frontal plane (Chap. 

 XI). An anterior lung lobe grows out in front and dorsal to the 

 mesobronchus, beginning at six days, and the extra-pulmonary 

 bronchus thus acquires a ventral insertion into the lung. 



Stages in the development may be described as follows: 

 At 96 hours, the bronchi arise from the end of the trachea, ven- 

 tral to the cesophagus and pass back on either side of the latter, 

 describing near their centers a rather sharp curve that brings 

 the dorsal ends to a higher level than the oesophagus. A very 

 slight dilatation at the extreme end of the mesobronchus is usually 

 interpreted as the beginning of the abdominal air-sac. 



