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-saes, ‘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 ccelomatie 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 
1A small projection also grows forward. 
