EPARTERIAL BRONCHIAL SYSTEM. 135 



almost universal law, extension of the bronchial system by mi- 

 gration cephalad of some of the secondary branches brings 

 about asymmetry of the tree and a changed relation of the 

 cephalic primary bronchus to the pulmonary artery. 



The only forms in which the bilateral hyparterial type is 

 known to exist are : 



Hystrix cristata (Aeby), 



Balcena mystic ctiis and ajitipodiim (M. Weber), 



Taxidea anicricana (Huntington). 



Turning now to the conditions presented by the remaining 

 mammalia, I have selected the following series of typical modi- 

 fications, and will present them in the order in which the sub- 

 sequent general phylogenetic comparison will be made. 



III. Canis familiaris — Dog, 9 . 



Corrosion preparation of bronchial system and pulmonary 

 artery. Columbia University Museum, No. 1256. PI. XVIII. 



The type presented is the one followed by the vast majority 

 of mammalia, and is defined by Aeby as ''bronchial tree with 

 eparterial bronchus only on the right side, bronchial in deriva- 

 tion." There is a well-developed cardiac bronchus (C) supply- 

 ing the Azygos lobe. 



Even a cursory examination of this preparation reveals the 

 fact that, with the exception of the cardiac bronchus, a strict 

 equivalence of bronchial elements exists on the right and left 

 sides, but that their relation to the primary bronchus and the 

 main trunk of the pulmonary artery differs on the two sides. 



a. Left Side. 



The first bronchus is a short, thick stem, hyparterial in posi- 

 tion (A), which divides into an apical and a lateral branch (^', 

 A"^. Compared with Hystrix and Taxidea, it is not difficult to 

 recognize in the former the cephalic trunk {A) and in the latter 

 the two secondary branches {A' and A"^. The caudal portion 

 of the bronchial tree below the origin of A appears as the 



