136 HUNTING TON. 



"stem-bronchus" of Aeby, from which the remaining sec- 

 ondary branches are derived. Compared with Hystrix and 

 Taxidea, we recognize the element B, between the origin of the 

 first hyparterial trunk A, and the origin of the lateral branch 

 B' , corresponding to the caudal trunk B of Hystrix and Tax- 

 idea. The lateral branch B' corresponds to the same element 

 in the bronchial system of Taxidea and Hystrix. 



The continuation caudad of the stem-bronchus occupies the 

 site of the secondary caudal branch B" in Hystrix and Taxidea, 

 and, like this branch, divides into two nearly equal segments, a 

 medial and a lateral, each of which gives off monopodic dorso- 

 medial and ventro-lateral twigs. 



The general comparison, therefore, of the left bronchial sys- 

 tem of Canis with the bilateral hyparterial type of Hystrix and 

 Taxidea results as follows : • 



Hystrix and Taxidea. Canis. 



A = A 



A^ = A^ 



A'' . = A" 



B =^ B 



B' = B^ 



B" = B'' 



Aeby's ** stem-bronchus" appears as the result of the follow- 

 ing rearrangement and further development : 



1. The proximal part, between the bifurcation and the origin 

 of the cephalic trunk A (" primary left bronchus") results from 

 the segmentation and division of the tracheal bulla. 



2. The second segment of the stem-bronchus is formed bv 

 the element B (caudal trunk) between the origin of A and the 

 derivation oi B' . 



3. The third segment is continued caudad as the representa- 

 tive oi B" (medial secondary caudal branch), while the lateral 

 branch (B'^ appears as its secondary derivative. Hence we may 

 regard the typical ** stem-bronchus" as it appears in the ma- 

 jority of mammalia in the following light : 



" Stem-bronchus" = segmented tracheal bulla -\- B -\- B" , 

 medial division. 



A and its two secondary divisions A' and A'\ B' , as well 



