134 HUNTING TON. 



supply of the medial secondary branch {B") derived from the 

 right caudal trunk (^B) presents the same typographical peculiar- 

 ity found in the younger specimen. 



General Consideration of the '' Bilateral Hyparterial 

 Type," as Shown in the Preceding Preparations. 



1. Taxidea aincricana is a new form, presenting the bilateral 

 hyparterial type, now described in detail for the first time, al- 

 though I called attention to the peculiarities of the pulmonary 

 structure of this animal in the " Cartwright Lectures," delivered 

 in April 1896. 



2. Comparison with the remaining mammalian forms leads 

 me to regard the bilateral hyparterial type as the primitive con- 

 dition of the mammalian lung, whereas Aeby (i)and Wieder- 

 sheim (Vergl. Anat. Lehrb., p. 262-266) consider it a complete 

 reduction form, resulting from the bilateral suppression of the 

 " eparterial" bronchus. The reasons for the opinion expressed 

 are as follows : 



a. The tracheal lacuna or bulla corresponds to the condition 

 presented by the tracheal bud during the early stages of pul- 

 monary development in mammalian embryos.^ 



b. During the early developmental stages the pulmonary 

 artery passes caudad on each side of the tracheal stalk to the 

 point of division. The subsequent descent of the heart turns 

 the pulmonary trunk ventrad and caudad into the position which 

 it later occupies in relation to the tracheal bifurcation. Hence 

 the original position of the tracheal buds is "hyparterial. 



The appearance, therefore, both of the bronchial system and 

 of the pulmonary artery in Hystrix and Taxidea represents a 

 persistent embryonal type. 



3. We may add that this type appears as an exceedingly ex- 

 ceptional one in the mammalian series. In obedience to an 



^Robinson, Arthur, '' OVjservations on the earlier stages in the development of 

 'the Lungs of Rats and Mice," Jour. Anat. and Phys., Vol. xxiii, Ft. ii, January 

 1889, p. 224. 



