496 WILLIAM A. LOCY AND OLOF LARSELL 



Their embryonic development may be briefly summarized. 

 The formation of lobular branches of ento- and ectobronchi, at 

 the close of the ninth, and during the tenth day, has already been 

 described. In spreading out on the ventral (entobronchi) and 

 dorsal (ectobronchi) facets of the lung, these branches elongate 

 towards the mesial border and undergo further subdivision. 

 Numerous outgrowths of ento- and ectobronchi also occur within 

 the interior of the lung. 



The digitations of the ento- and ectobronchi gradually approach 

 each other, and, in order to get from the ventral to the dorsal 

 face, the branches of the entobronchi bend a'most at right angles 

 around the mesial face of the lung. On that facet they run 

 nearly parallel to one another. These are parabronchi and are 

 diagrammatically represented in figure 56. 



The branches of the ectobronchi, on the dorsal face, grow to- 

 wards the approaching entobronchi, and by the eleventh day, 

 there is only a narrow lane-like area of mesenchymal tissue be- 

 tween the two groups that is unoccupied by bronchial tubes. 

 This area extends along the dorso-mesial face from the caudal 

 to near the cranial border of the lung, where it is turned to one 

 side owing to the fact that the branches of the first entobronchus 

 are in the path. These branches extend posteriorly and gradu- 

 ally approach the similar branches of the first ectobronchus and 

 of the first laterobronchus, and, shortly these opposite systems 

 of tubes become connected by parabronchi. Parabronchi, un- 

 like other bronchial tubes, are substantially of uniform calibre. 

 Those of the periphery are of somewhat larger diameter than 

 those within the interior of the lung. 



The manner of the anastomosis of parabronchi is interesting. 

 On the twelfth day of development the tips of the aproaching 

 parabronchi are nearly in contact. They now bifurcate, forming 

 slender twigs which come into contact and anastomose with simi- 

 lar twigs coming from the opposite direction. Complete union 

 has occurred by the fifteenth day of incubation, but the anasto- 

 mosing parts are as yet very slender (fig. 42). These twigs at- 

 tain substantially the diameter of their parent branches by the 

 eighteenth day (fig. 43) and remain as the parabronchial network 



