THE EMBRYOLOGY OF THE BIRD \S LUNG 



508 



indrical conduits measuring from 0.10 to 0.14 mm. in diameter 

 and there are about twenty of them within the circumference of 

 a cross-section of a parabronchus. Soon the vestibules divide 

 at their tips by unequal dichotomy and the branches thus formed 

 may subdivide. As a rule, subdivision does not occur more than 

 twice, but, in the adult lung, there are numerous examples of a 

 third and even a fourth division. The first bifurcations occur 



Fig. 4() Simplified diagram to illustrate the anastomosis of 1 h 



e air canulane 



early in the fifteenth day, and by the close of that day branches 

 are well defined. By continuous growth these branches pene- 

 trate further into the tissue surrounding the parabronchus and 

 the air ca]:)illaries are soon formed on their distal extremities. At 

 first these terminate blindly, but between the nineteenth and the 

 twenty-first days of development they anastomose profusely 

 and thus establish a network of intercommunicating air pas- 

 sages. In the chick the anastomosis l)etween air-capillaries is 

 confined to the limits of the hexagonal prisms. In good flyers, 



THE AHiERICAX JOURNAL OF ANATOMY, VOL. II), NO. S 



