328 THE DEVELOPMENT OF THE CHICK 



division of the mesobronchus, which is otherwise unbranched. 

 These represent the three anterior entobronchi; the first or 

 anterior one is the largest, and the third or posterior the smallest. 

 Their direction of growth is, on the whole, dorsal, with an inclina- 

 tion anteriorly and towards the middle line. It is evident that 

 the part of the mesobronchus from which they arise will form 

 the vestibulum of the adult lung. 



On the eighth day the fourth entobronchus is formed, 

 extending posteriorly from the hind end of the vestibulum in the 

 ventral wall of the lung; it gives off a couple of latero- ventral 

 small branches. The first entobronchus is now much subdivided 

 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 similarly branches dorsally and posteriorly, 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. 



Behind the vestibulum, the mesobronchus ascends towards 

 the dorsal surface of the lung, remaining unbranched, then turns 

 posteriorly and gives off five or six branches, presumably the 

 ectobronchi. It terminates posteriorly in the small abdominal 

 air-sac, which is still contained within the lung substance. Just 

 anterior to this is a slight diverticulum, possibly the primordium 

 of the posterior thoracic air-sac. The parabronchi or tertiary 

 bronchi are not yet formed. 



Between the eighth and eleventh days, numerous tertiary 

 bronchi (parabronchi) arise from the ento- and ectobronchi 

 (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 of the 

 lungs, and ending always in slight enlargements. They do not 

 appear to anastomose, though they are known to do so later. 

 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. The canaliculi and alveoli arise later, and it is easy 



