THE EMBRYOLOGY OF THE BIRD's LUNG 497 



of the adult lung, along the dorsal margin of which the}' foi'in a 

 distinct line. This line also marks the region of anastomosis, 

 on the dorso-lateral border between the first ento- and tlu^ first 

 ectobronchus. 



During the eighteenth day of development, and subsequently, 

 similar connections are established between parabronchi, both ex- 

 ternally and internally, in other parts of the lung. There is a 

 well defined area along the ventro-mesial margin of the adult 

 lung, but this is not the result of terminal anastomoses, as in the 

 case just described, but the union effected between adjacent 

 parabronchi by the sending out of lateral buds. 



Figures 42 and 43 are surface studies of air injected specimens 

 to show the nature of the peripheral anastomoses. Figure 42 

 exhibits the condition on the fifteenth day of development, with 

 the slender parabronchial tips coming into contact. A camera 

 tracing of the eighteenth day stage (fig. 43) shows the parabron- 

 chi of the same region having attained their full size. As before 

 indicated they are of substantially the same diameter throughout 

 their length and they are united by frequent transverse branches. 

 The diameters of parabronchi vary in different birds; they are 

 relatively large in the domestic fowl where they attain a diameter 

 of about 2 mm. on the surface and about 1 mm. in intrapulmonarj^ 

 situations. These figures show the formation and union of para- 

 bronchi at the surface, and it is to be understood that similar anas- 

 tomoses occur at different le\Tls within the interior of the lung. 



The branches of latero- and dorsobronchi give rise to parabron- 

 chi which are relatively' short, and which anastomose, at differ- 

 ent levels within the lung, with those of adjacent latero- and 

 dorsobronchi, and to some extent with recurrent bronchi. By 

 these anastomoses an elaborate network of air-tubes is formed in 

 the dorso-lateral and lateral portions of the lung and, also, in the 

 caudal region of the ventral face. In the latter region, the sec- 

 ond laterobronchus and the first ramus of the fourth entobron- 

 chus play the most important part. 



The recurrent bronchi from the two posterior air-sacs, also, as 

 more fully descidbed below, help materially in forming the net- 

 work of this region. 



