PULMONARY EVOLUTION IN MAMMALIA 145 
2. In support of the second hypothesis of the Migration Theory, 
that the eparterial bronchus is really the first dorsal bronchus 
displaced craniad upon the stembronchus, Narath cites an in- 
complete corrosion preparation of the right lung of Echidna hys- 
trix in which the eparterial trunk arranges itself with the follow- 
ing more caudally placed dorsal branches in a ‘handsome row’ 
and resembles them in shape, size and relation to the pulmonary 
artery, the vessel descending on the lateral surface of the stem- 
bronchus between the dorsal and ventral series of bronchi, thus 
bringing the eparterial or apical bronchus, as D', into the same 
relation as D?, D?, D4. In evaluating this argument it must be 
remembered in the first place that the eparterial district of the 
Monotremes is exceedingly restricted, almost rudimentary. The 
right upper lobe is hardly more than a short projection craniad 
from the dorsal pulmonary border, and hence a resemblance 
between the eparterial and the succeeding dorsal bronchus is of 
itself suggested by their nearly uniform size and by the restricted 
area supplied by each. Beyond this resemblance there is abso- 
lutely nothing to prove that the element described as the epar- 
terial or apical bronchus is really the transplanted first dorsal. 
Once granted that pulmonary evolution is not limited to a fixed 
and immutable number of bronchial units, rearranged in the 
special types by ‘migration,’ the bronchus in question is far 
more likely to be a neomorph, developing in accordance with the 
beginning cranial extension of the Monotreme lung, and thus 
acceding to the dorsal series at this stage of its evolutionary un- 
folding. There is not an atom of morphological evidence in 
support of the assumed ‘migration’ of a preexisting element (D') 
to the site now occupied by the new eparterial bronchus. D! 
remains as before, and Ep. develops in advance of it. 
Kchidna is not a good form for the the comparison of the typical 
mammalian eparterial bronchus with the other components of 
the bronchial tree, because of the limitations of the cranial pul- 
monary districts. It can serve as showing the rudimentary 
character of cranial extension of the lung in the most primitive 
mammal, but it cannot be used to demonstrate the origin of the 
bronchus supplying this extension. 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 27, NO. 2 
