492 



The abdominal sacs expand so as to fill the abdominal cavity, 

 and partly surround the viscera therein contained. About the four- 

 teenth day the walls of the sacs begin to fuse with the peritoneum 

 and this fusion is apparently completed sometime before the eighteenth 

 day of development. The left abdominal sac is somewhat larger than 

 is the right. 



The history of the posterior intermediate sacs after the tenth day 

 is closely parallel to that of the abdominals and does not require 

 detailed description. 



The same general course is followed by the anterior intermediate 

 sacs. Their walls fuse with the lining of the thoracic cavity. The 

 prebronchial and sub-bronchial sacs attain their most rapid growth 

 after the twelfth day of development. The prebronchial sacs grow 

 forward toward the neck of the chick and between the fifteenth and 

 nineteenth days of incubation their walls fuse to some degree with 

 the pleura. 



The later stages of the sub-bronchial sacs require a more extended 

 description than the others because of marked differences Avhich appear. 



In the description of the eleventh day stage it was pointed out 

 that the mesial moiety (Fig. 6, s.l) had bifurcated at its distal extre- 

 mity. The more mesial lobe thus formed expands in such a manner 

 that its walls come into contact with the walls of the corresponding 

 lobe of the sub-bronchial sac of opposite lung. This phase is reached 

 on the fifteenth day of incubation. By the nineteenth day fusion 

 of the walls has taken place, but there appears to be no breaking down 

 of the septum thus formed. This appears also to be the case with 

 the fused walls of the more anterior portion of the sac, as is describ- 

 ed below. This condition was demonstrated both by dissections and 

 by Wood's metal casts of the adult lungs and air-sacs. 



On the fifteenth day of development portions of the mesial moiety 

 engirdle the bronchus, ventrally, and come into contact with the lateral 

 moiety of the sub-bronchial sac. The membranous walls subsequently 

 begin to fuse and on the nineteenth day union is approximately com- 

 pleted. The single septum thus formed disappears sometime between 

 the nineteenth day and the end of the first day after hatching, so 

 that the two hitherto independent moieties coalese to form one sac 

 Fig. 8, s.l, S.2). 



The single large sub-bronchial sac of the adult is the result of 

 the union of two moieties which arise from different entobronchi. As 



