PULMONARY EVOLUTION IN MAMMALIA 161 
mydas) illustrate the initial stage, those of Thalassochelys and 
Dermochelys the completion of the evolutionary process in the 
invasion of the lung by the stembronchus. 
A lung which has attained this stage of advancement repre- 
sents in its entirety that portion of the more highly developed 
mammalian organ which we can regard as the archeal fundament 
of the whole, the ‘lung-stem’ of Aeby, commonly defined as the 
‘lower lobe,’ supplied through the primitive stembronchus with 
main ventral and dorsal derivatives. All further additions to 
the primitive lung-stem are acquired secondarily, as the struc- 
tural expressions of heightened respiratory demands in the more 
complex mammalian types. 
Before taking up this second chapter in mammalian pulmonary 
differentiation and extension it is desirable to clear the ground 
for the same by considering some of the details of the intrinsic 
structure of the lung-stem and their bearing on further evolu- 
tionary progress. For the purposes of the present inquiry we 
need only dwell on certain main ontogenetic and phylogenetic 
facts. 
1. The distinction between the conducting and the respiratory 
components of the lung, marked as this becomes in the proximal 
segments of the higher lung, is not definite and absolute in the 
peripheral portions. Their meeting point in the adult lung lies 
within the intermediate area of the respiratory bronchus, effect- 
ing a gradual transition between the conducting tube and the 
alveolus. Ontogenetically the identical derivation of both struc- 
tures from a common primitive anlage enables them to become 
mutually interchangeable up to a certain stage of differentiation, 
as Shown by the genetic intrapulmonary cycle of the Monotremes 
and Marsupials (cf. p. 192). Phylogenetically the same initial 
indefinite demarcation of the two components is encountered, 
as in the tracheal lung of the colubrine Amblycephalidae, in the 
eparterial bronchi of the cetacean and artiodactyl lung derived 
from the trachea, and in many bronchial variants. 
2. This interchangeability of conducting and respiratory com- 
ponents is based upon the primary fundamental inherent capacity 
of the entire entodermal respiratory tract to inaugurate bronchial 
THE AMBRICAN JOURNAL OF ANATOMY, VOL. 27, No. 2 
