482 



their development that the two structures should be considered together. 

 In the course of development they unite with twigs of the bronchial 

 tree and thus establish a complete circuit with the air passages within 

 the lungs. In the adult lung the air passes from the air-sacs through 

 these recurrent bronchi, entering the lung by a returning current, and, 

 in this sense, the air circuit through these bronchi is a recurrent one. 



The credit for the recognition of the morphological arrangement 

 as well as for the part which the recurrent bronchi play in the respi- 

 ration of birds, belongs chiefly to Juillet (1911). Although figured 

 by earlier observers (Campana '75, Fischer '05), Juillet was the first 

 to observe their development, and to indicate with clearness their 

 relation to other bronchi. 



This paper is part of a more extended study of the development 

 of the lungs of birds which will appear elsewhere under the joint 

 authorship of Professor Logy and myself. 



The observations were carried on on during 1913-^1914 in tho 

 Zoological Laboratory of Northwestern University under the direction 

 of Professor William A. Locy. It is a pleasure to express my sense 

 of obligation to him for encouragement, for supervision and for criticism 

 of the manuscript. 



Review of the extensive literature on the air-sacs may be omitted 

 except to say that little has been added to the embryology of the 

 air-sacs since Selenka's paper of 1866 in which he described and 

 figured their development in the chick. 



The recurrent bronchi, however, have come into notice more 

 recently and a brief account of the published observations on these 

 structures should be given. 



Campana ('75) in an extensive memoir dealing with the respiratory 

 apparatus of birds and cofined chiefly to a description of the adult 

 structures, described the air-sacs and bronchial tree. He also made 

 a careful analysis of the orifices connecting lung and air-sacs. Exa- 

 mination of his figures shows that some of the recurrent bronchi also 

 stand out quite distinctly, but Campana considered them as the result 

 of a reconstitution into a single trunk of several tertiary bronchi, without 

 recognizing their true nature. (He apparently considered them as 

 merely a part of the network of air passages with no special significance 

 attached to them.) On the other hand Campana used the term "Broncho 

 recurrente" in an entirely different connection, applying it to the first 

 entobronchus. 



