ALIMENTARY TRACT AND ITS \PPENDAGES 331 



than the right. The expansion goes on lapidly and by the 

 thirteenth to the fifteenth day they have reached the hinder end 

 of the body cavity, and have ah^eady expanded into it so far as 

 to form fusions with the mesentery. Recurrent bronchi begin 

 to develop from their base about the ninth day. 



The cervical sacs appear early from an anterior branch of the first 

 entobronchus (Fig. 191). They form no recurrent bronchi (Locy). 



The interclavicular sac, which is single in the adult, arises 

 from two sacs on each side, a lateral moiety from the first ento- 

 bronchus, and a mesial moiety from the third. These four parts 

 fuse to form the single sac of the adult (Locy). These sacs form 

 recurrent entobranchi. 



The anterior thoracic sac forms about the seventh day as a 

 dilatation of the ventral wall of the third entobronchus pro- 

 jecting into the pleuroperitoneal membrane near its median 

 edge; it thus lies just lateral to the pneumato-enteric recesses. 

 From this position it expands laterally and posteriorly in the 

 pleuroperitoneal membrane and thus gradually sphts it in two 

 layers (Fig. 190, 11 days). 



The posterior thoracic air-sac arises from the third latero- 

 bronchus somewhat later than the others, and grows at first 

 through the hinder portion of the pleuroperitoneal membrane to 

 enter the lateral body wall. In its subsequent expansion, it sphts 

 the posterior portion of the pleuroperitoneal membrane, as the 

 anterior thoracic air-sac does the anterior portion of the same 

 membrane. Anterior and posterior thoracic air-sacs then come 

 into contact, forming a septum. Both form recurrent bronchi. 



The lower layer of the pleuroperitoneal membrane, spht off 

 from the upper layer by expansion of anterior and posterior 

 thoracic air-sacs, constitutes the oblique septum. The most 

 posterior portion of the oblique septum, however, is derived from 

 the peritoneum of the lateral bo^dy wall by expansion of the pos- 

 terior thoracic air-sacs behind the pleuroperitoneal membrane. 



Like the abdominal air-sacs, "the remainder expand rapidly, 

 particularly from the fourteenth day on, among the thoracic 

 viscera, and fuse intimately with these and the walls of the body 

 cavity in a few days, the coelomatic fluid being in the meantime 

 absorbed. The interclavicular air-sac grows out to form the 

 subscapular air-sac and at the time of hatching has approached 

 close to the humerus." (Selenka.) 



