488 WILLIAM A. LOCY AND OLOF LARSELL 



branches springing from both the main division and the trans- 

 verse branch. 



It is evident that the entobronchi begin very early to play a 

 leading part in the formation of the bronchial tree and although 

 limited in number they make a large showing on the metal casts 

 of the adult lung. Generally speaking, the entobronchi of the 

 adult are distributed on the ventral surface of the lung as shown 

 in figure 44. The parabronchi that spring from their branches 

 bend around the mesial border of the lung and curve on to the 

 dorsal surface where they meet parabronchi coming from the 

 ectobronchi and thus a connection is established between these 

 opposed bronchi and the characteristic bronchial circuits. 



The ectobronchi. The ectobronchi are outgrowths from the 

 central lung tube arising somewhat later than the entobronchi 

 and more mesial in position upon the wall of the lung tube. 

 These are the bronches costales of Sappey and the secondaires 

 internes of Campana. While the ento- come from the anterior 

 division of the mesobronchus, the ectobronchi spring from the 

 wall of the embryonic vestibulum in the middle division of the 

 central lung tube. 



The first (fig. 30, Ect. 1) arises as a forward and upward pro- 

 jecting bud from the anterior part of the expanded region of the 

 tube at the beginning of the seventh day. By rapid growth it 

 becomes elongated, and, at six days, twenty hours, may be seen 

 the first indication of branching in the form of a ventral bud 

 springing from the distal end of the ectobronchus (fig. 32). The 

 two lobes produced in this manner rapidly increase in size and 

 give rise to other lobe-like branches. As illustrated in figure 36 

 there are five such lobules at the beginning of the ninth day. 

 The two most dorsal are the result of the division of the original 

 dorsal lobe, and the three ventrally placed lobules come from 

 the other division of the first bifurcation. 



These lobular outgrowths foreshadow the parabronchi of the 

 anterior lateral and of the dorsal region of the lung. By the 

 close of the ninth day the lobules have greatly increased in num- 

 ber and, on the tenth day, show the beginnings of some parabron- 



