ALIMENTARY TRACT AND ITS APPENDAGES 331 



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 splits it in two 

 layers (Fig. 190, 11 days); the original connection with the pul- 

 monary tube does not expand much, and thus gradually forms 

 a constricted neck. 



The posterior thoracic air-sac arises from the mesobronchus 

 near its hind end 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 splits 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. 



The lower layer of the pleuroperitoneal membrane, split 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 body wall by expansion of the 

 posterior thoracic air-sacs behind the pleuroperitoneal mem- 

 brane. 



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 ccelomatic 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. But more time is required to enable it to 

 enter the humerus through the foramen pneumaticum; certainly 

 more than twenty-two days, that is after the bone is nearly full- 

 grown. The marrow, rich in blood-vessels, makes room for the 

 air, as soon as the bone is fully formed." (Selenka.) 



The Laryngotracheal Groove. The embryonic primordium 



of the larynx and trachea communicates at first along its entire 



length with the postbranchial division of the pharynx (72 hours). 



At 96 hours the hinder portion of the groove is already converted 



^ A small projection also grows forward. 



