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RESPIRATORY AND BUOYANCY SYSTEMS 



rapidly into the surrounding mesenchyme and they soon project dorsally, as 

 indicated in figure 306D. The latter figure presents the developmental condi- 

 tion of the lung rudiments late on the fourth day of incubation. Two areas of 

 the lung rudiment are evident, namely, the tracheal and lung rudiments proper. 

 The external appearance of the developing lungs on the ninth day of incuba- 

 tion is shown in figure 306E, while that of the twelfth day with the forming 

 air sacs is shown in figure 306F. 



2) Formation of Air Sacs. The air sacs arise as extensions from the main 

 bronchi during the sixth to seventh day of incubation. During the ninth day, 

 they are present as well-developed structures (fig. 306E). The abdominal 

 air sac appears as a posterior continuation of the mesobronchus or primary 

 bronchus of the lung, while the cervical air sac arises from the anterior ento- 



Fio. 307. Lung development in the chick. (All figures, after Locy and Larsell: '16, 

 Am. J. Anat., vols. 19, 20.) (A) Diagram of dissection of lung of 9'/2-day embryo, 

 designed to show entobronchi and air-sac connections with bronchial tree. (B) Diagram 

 of mesial aspect of adult lung, showing parabronchial connections between entobronchi 

 and ectobronchi. Dorsal and lateral bronchi are not shown. (C) Simplified diagram 

 to show air capillaries in relation to infundibula and parabronchus. (Blood capillaries 

 added to one sector of figure represent a modification of the original figure.) (D) Dia- 

 gram of lateral surface of right lung of 15-day embryo, showing recurrent bronchi of 

 abdominal and posterior intermediate air sacs. Anastomoses of recurrent bronchi are 

 also shown. 



