114 GEO. S. HUNTINGTON 
dent relation rests upon the common morphogenetic and func- 
tional factors which link the reptilian respiratory tract to the 
early phases of its adaptation to mammalian requirements. 
This does not involve a hypothetical reptilian eparterial bronchial 
system which does not exist. When it appears in the mamma- 
lian line it does so as a new acquisition in response to very 
definite environmental and functional factors 
With d’Hardiviller’s and Bremer’s position in the matter 
defined, it is possible to return to the critical consideration of 
Aeby’s Reduction Theory on its own merits 
This theory involves the acceptance of the following dogmata: 
1. The primitive ancestral mammalian lung was, in its bron- 
chial architecture, a bilaterally symmetrical organ, with the 
eparterial bronchus developed in both lungs and arising from 
the stem (Aeby’s type I’). 
2. The reduction has chiefly affected the cranial portion of 
the left lung, entailing the phylogenetic suppression of the left 
eparterial bronchus, and resulting in the establishment of the 
dominant mammalian type with the eparterial bronchus lim- 
ited to the right lung (Aeby’s type II*). 
3. Only relatively few forms have retained the original bilat- 
eral eparterial organization (Aeby’s type I‘). 
4. A still more limited number of living mammalia have 
undergone reduction also of the right lung, thus establishing the 
bilateral hyparterial distribution of Aeby’s type III. 
An analysis of these theses provokes the following critical 
considerations: 
1. The assumption of a bilaterally symmetrical bronchial tree 
in the most primitive mammalia or promammalia rests on pure 
hypothesis and is incapable of direct proof. With the elimina- 
tion of the evidence offered by d’Hardiviller and Bremer, the 
ontogeny of the mammalian lung does not throw the light on 
the problem which was confidently anticipated by the author of 
the theory. The type of bronchial architecture characteristic of 
any given mammalian species manifests itself unmistakably in 
the earliest pulmonary anlage of its embryos as soon as the dif- 
ferentiation of individual bronchial components appears. 
