THE EMBRYOLOGY OF THE BIKD's LUNG 493 



laterobronchi. These are the beginnings of the larger or prin- 

 cipal dorsobronchi and they arise on the eighth day of incubation. 

 The stage figured shows their degree of development early on the 

 ninth daj^ 



After this stage additional buds appear, in rather rapid suc- 

 cession, within the space on the wall of the niesobronchus between 

 the bases of the latero- and ectobronchi and, also, more caudad. 

 By the eleventh day so many have developed that, when they 

 are injected with air, they tend to obscure the other air passages, 

 of the lungs. Accordingly, all but the larger dorsobronchi have 

 been omitted in the sketches subsequent to the ninth day, in 

 order not to confuse the more important features of the sketches, 



In the adult lung they form a very important area on the 

 dorsal surface of the lung which will be described later. As il- 

 lustrated in figure 55, the bases of the dorsobronchi are arranged 

 roughly in three rows. The middle row is composed of the 

 larger ones, which are the first to develop, and arranged, in a 

 general way, alternating with those of the middle row, mesially 

 and lateral to it, are the more numerous smaller ones. In the 

 diagram (fig. 55) the bases of twenty-one dorsobronchi are shown 

 — five of the larger, opposite the bases of the laterobronchi, and 

 sixteen of the smaller ones. This diagram is made from the study 

 of a Wood's metal cast in which the dorsobronchi were clipped off 

 at their bases. There is individual variation as to the total num- 

 ber present as well as to the pattern of their arrangement on the 

 mesobronchial wall. C'ampana cites a number of \'ariations in 

 this arrangement. We have examined sjoecimens with twenty- 

 five dorsobronchi — five larger and twenty smaller ones — but to 

 what limit the number of smaller ones may go in the adult we 

 have not determined. 



These dorsobronchi occupy a position along the dorsal wall 

 of the central lung tube from the stem of the first ectobronchus to 

 the posterior end of the niesobronchus, 



The branches of the dorsobronchi project towards the dorsal 

 surface of the lung, and arriving there, their parabronchi form a 

 well-marked, nearly circular area, or network, in the center of 

 the dorsal face. After the tenth day this area can readily be 



