THE EMBRYOLOGY OF THE BIRD's LUNG 449 



In early stages of development the outgrowths of the bron- 

 chial tree end blindly and this condition is maintained in the 

 adult mammalian lung, but in the bird lung, the terminals come 

 into contact and anastomose during embryonic development so 

 that in the adult lung there are no culs-de-sac. This condition 

 of anastomosis affects also the air capillaries that are radially 

 arranged around the parabronchi. Thus in following the devel- 

 opment of the air passages of the bird's lung we pass from the 

 primary condition of a bronchial tree to the modified condition 

 of uninterrupted bronchial circuits. 



Some of the points that require elucidation for understanding 

 the morphology and the physio' ogical action of the avian lung 

 may be enumerated: 



As a background, a knowledge of the phenomena of extra pul- 

 monary development, or the general course of its embryology. 



The intra-pulmonary development of the bronchial tree, its 

 ramifications and the establishment by anastomoses of unbroken 

 communications between the parabronchi and the air capillaries. 



To determine the method of formation of the air-sacs and of 

 their outgrowths, the recurrent bronchi. 



To observe the formation of the air capillaries and the estab- 

 lishment of anastomoses among them. 



To observe the origin and mode of development of the pul- 

 monary artery and of the general course of circulation within the 

 lung. 



In addition to the above there should be observations on the 

 diaphragmatic membranes and the muscular means of producing 

 respiratory movements accompanied by physiological experi- 

 ments, but observations of this nature have not been included 

 in our studies. 



Our observations are confined to the embryology .and mor- 

 phology of the lung and air-sacs, and in this study of limited 

 range we do not presume to have found answers to the ultimate 

 questions of morphology of the bird lung. We have assembled 

 our results merely as an objective account of what we have been 

 able to observe in the time and with the material at om* disposal. 



