326 THE DEVELOPMENT OF THE CHICK 
the trachea to the hilum of the lung (extra-pulmonary bronchus) 
and its continuation within the lung, extending its entire length 
(mesobronchus). The secondary air passages and chambers 
of the lung (ecto- and entobronchi, parabronchi, canaliculi and 
terminal alveoli) and the air-sacs arise from the mesobronchi 
by a process of budding and branching and enlargement of ter- 
minal twigs. The mesobronchi are surrounded from the first 
by a thick mass of mesenchyme, covered of course towards the 
body cavity by a layer of mesothelium. In the early develop- 
ment the mesenchyme of the lung-primordia expands so rapidly 
as to provide adequate space for the branching of the mesobronchi 
entirely within the mesenchymal tissue. 
Although the development of the lungs of the chick has been 
studied by several investigators (Rathke, Goette, Selenka, Ber- 
telli) there has been no complete study made with the resources 
of modern technique, and our knowledge is therefore defective 
in many important respects. 
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 cesophagus. A very 
slight dilatation at the extreme end of the mesobronchus is usually 
interpreted as the beginning of the abdominal air-sac. 
