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-sae. 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 1s easy 
