328 THE DEVELOPMENT OF THE CHICK 



wall of the anterior division of the mesobronchus, which is other- 

 wise unbranched. These represent the entobronchi; they arise 

 in antero-posterior order, and the first is therefore the largest. 

 The part of the mesobronchus from which they arise will form 

 the vestibulum of the adult lung. 



Later on the same day the ectobronchi, six in number, begin 

 to arise from the dorsal surface of the dilated portion of the 

 middle division of the mesobronchus. Other independent out- 

 growths of the same division of the mesobronchus are the 

 so-called laterobronchi and dorsobronchi (Locy). These four 

 groups of out-growths may be classed as secondary bronchi (Fig. 

 191). 



On the ninth day (Fig. 191) the first entobronchus has formed 

 a number of branches in the anterior lobe of the lung, and two of 

 its terminal twigs, one in the antero-dorsal, the other in the antero- 

 ventral tip of the lung, are slightly dilated and project as primordia 

 of the cervical and interclavicular air-sacs respectively. The 

 second entobronchus is also subdivided several times; its terminal 

 branches extending to the dorsal surface of the lung. The third 

 entobronchus bends ventrally, and from its base a narrow canal 

 extends into the pleuroperitoneal membrane, where it expands 

 into the anterior thoracic air-sac, which is much the largest of 

 the air-sacs at this time. 



Between the eighth and eleventh days, numerous tertiary 

 bronchi (parabronchi) arise from the secondary bronchi (Fig. 

 190). These are considerably smaller than the tubes from which 

 they arise, and are extremely numerous, radiating from all parts 

 of the secondary bronchi towards the free surfaces and interior 

 of the lungs. They are embedded in the mesenchyme of the lung, 

 which is already marked out into areas hexagonal in cross-section, 

 with the parabronchi in the centers, by the developing pulmonary 

 blood-vessels. 



From the twelfth to the eighteenth days parabronchi of dif- 

 ferent origin meet and fuse in a most extensive fashion, thus form- 

 ing an intercommunicating net-work of tubes throughout the 

 lung. Air-capillaries finally arise from the parabronchi in the 

 centers of the hexagonal areas and form an anastomotic net-work 

 arising from and surrounding the parabronchi. This completes 

 the system of tubes arising from the secondary bronchi; but 



