THE EMBRYOLOGY OF THE BIRD's LUNG 48.'-5 



previous!}^ ventro-lateral border laterad, and the previously dor- 

 sal border towards the median plane. 



During the last half of the sixth day (fig. 28, 5 days, 20 hours), 

 the slender stalk of the first entobronchus extends dorsally for 

 a short distance, then curves mesially and expands, the trend is 

 again dorsad, and finally, laterad. The enlarged bladder-like 

 extremity lies directly above the bronchus (fig. 29) and some- 

 what anterior to the point where the stalk joins the bronchus. 



Soon the distal enlargement of the entobronchus begins to 

 divide as shown in figures 30 and 32. One bud-like outgrowth 

 (Cerv.sc.) extends cephalad and constitutes the primordium of 

 the cervical air-sac. The other bud, visible from this position, 

 extends laterad and ventrad. 



It is not, however, until a later period that the divisions of the 

 first entobronchus can be clearly seen. On the ninth day of 

 development (figs. 36 and 37), the first entobronchus exhibits 

 three principal divisions, or branches, each of which is subdivided 

 by lobular branches. The cranial branch {Cr.hr.) extends for- 

 ward and bears at its tip the cervical sac {Cerv.sc.) which at this 

 period extends beyond the lung wall. The cervical sac is not 

 terminal in the adult as in the embryo, on the contrary, its orifice 

 opens upon the surface of the main part of the cranial branch. 

 The transverse, or lateral branch (figs. 36 and 37, Lat.br.) extends 



Fig. 32 Dorsal view of tlie lung of an embryo incubated 6 days 20 hours. Il- 

 lustrates ento- and ectobronchi and the early condition of laterobronchi. 



Fig. 33 Mesial view of the same specimen. 



Fig. 34 Ventral view. Abd.sc, abdominal air-sac; A.ititr.sc, anterior in- 

 termediate air-sac; Bd., bud of recurrent bronchi; Ceri'.sc, cervical air-sac; Dor., 

 dorsobronchi; Di-., dorsal ramus of second entobronchus; Ect. 1, 2, 3, etc., the 

 corresponding ectobronchi; £■««., 1, 2, 3, 4, the corresponding entobronchi; Lat., 

 1, 2, etc., the corresponding laterobronchi; Lat.moi., lateral moiety of the inter- 

 clavicular air-sac; Mes.7noi., mesial moiety of the interclavicular air-sac; P.intr.sc. 

 posterior air-sac. 



Note: Figures 34, 35, 3G and 37 represent the air injected right lung of an em- 

 bryo of the early ninth day of incubation viewed from different aspects. At this 

 period the principal divisions of the bronchial tree are established, although still 

 in a relatively simple stage of development, and the air-sacs with the exception 

 of the posterior intermediate are well outlined. The reference letters are the 

 same for all figures. 



Fig. 35 Dorsal view. (Reference letters as above.) 



