16 WILLIAM A, LOCY AND OLOF LARSELL 



The same general course is followed by the anterior intermedi- 

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



The cervical and interclavicular sacs attain their most rapid 

 growth after the twelfth day of development. The cervical 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. They give rise to several subdi- 

 visions in the cervical and axillary regions. 



The later stages of the interclavicular sacs require a more ex- 

 tended description than the others because of marked differences 

 in their formation. 



Returning to figure 47, the representation of the condition in 

 the ten and one-half day embryo, we note again that the mesial 

 moiety is bifurcated at its distal extremity. The more mesial 

 lobe thus produced expands in such a manner that its walls come 

 into contact with the walls of the corresponding lobe of the in- 

 terclavicular sac of the opposite lung. This phase is reached on 

 the fifteenth day of incubation (fig. 51). 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. This condition was demonstrated both by dissections 

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



On the sixteenth day of development portions of the mesial 

 moiety of both sides grow ventrally over the bronchus, and come 

 into contact with the lateral moiety of the interclavicular sac. 

 The membranous walls subsequently begin to fuse, and on the 

 eighteenth day union is approximately completed. 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 coalesce to form one sac (fig. 53). 



Thus the single large interclavicular sac of the adult is the 

 result of the union of two moieties on each side which arise from 

 different entobronchi. As diagrammatically illustrated in figure 

 53, the union of the sacs and the disappearance of the septum is 

 completed by the close of the first day after hatching. 



