488 



In an embryo of 9^/2 days incubation (Fig. 4 and 5, ^)?\) the 

 prebronchial sac shows only an increase in size, approximately propor- 

 tional to the general growth of the lung. 



The lateral jnoiety of the sub-bronchial sac (Fig. 5, s.2.) now 

 shows a well defined primordium outside the lung wall. The subsequent 

 history of this sac indicates that some of the intra-pulmonary part of 

 the first entobronchial tip, from which the sac arises must be considered 

 as a part of the air-sac primordium. The line of separation between 

 the entobronchus and the air-sac is where the more dorsal buds arise 

 "which are labeled in Fig. 5, r.br. Two groups of recurrent bronchi 

 belonging to this sac have also appeared. The bud which represents 

 the more ventral group of these bronchi extends posteriorly and 

 ventrally and is as yet undivided. The more dorsal group is indicated 

 by an already bifurcated bud which projects posteriorly and slightly 

 outward. These represent the only evidence of recurrent bronchi of 

 the sub-bronchial air-sac. The mesial moiety of this stage (s.l) a part 

 of which is shown in Fig. 5, is scarcely changed from the condition 

 described in the eighth day embryo, except that it has increased in size. 



The anterior intermediate air-sac (a.i.) at the close of the ninth 

 day of incubation has enlarged considerably, as compared with the 

 preceding stage. The recurrent bronchi (r.br.) have not greatly changed, 

 but the stem has elongated to some extent. The posterior intermediate 

 sac (p.i.) now projects well out from the lung, and its distal end forms 

 a flask-like swelling. The proximal part remains still constricted and 

 lies within the lung. From this part tw^o buds (r.br.) are seen pro- 

 jecting tow-ard the dorsal and anterior parts of the lung. The more 

 dorsal of these buds is divided at its tip. These indicate the beginnings 

 of the recurrent bronchi of the posterior intermediate air-sac. 



The abdominal sac (Fig. 4 and 5, ab.) has expanded greatly since 

 the eighth day stage, and now lies almost entirely outside the lung. 

 Its recurrent bronchi have also made an obvious growth. The bud 

 (Fig. 4, r.br.l) previously described has divided into two main branches 

 each of which has in turn bifurcated, as represented in the figure. 



A second group of recurrent bronchi, also belonging to the ab- 

 dominal sac, begins early in the ninth day of incubation. The bud for 

 this group starts from the air-sac primordium at a point just dorsal 

 to the base of the first-formed group. At the close of the ninth day 

 this outgrowth (Fig. 4, r.br. 2) has bifurcated so that there is an an- 

 terior and a lateral limb. The anterior limb curves gently upward. 

 The lateral branch makes a more sudden turn posteriorly. 



