178 GEO. S. HUNTINGTON 
proximal portion of the stembronchus cranial to the angle of the 
tracheal bifurcation, or even in part from the right lateral wall 
of the trachea. Every position between these two extremes is 
encountered in the mammalian series. These differences depend 
upon the point selected for the development of the eparterial 
bud in the different mammalian forms possessing the prevalent 
type of bronchial organization. Individual differences in the 
level of origin of the eparterial bronchus are also found within 
a single species. 
However placed in this respect, the right eparterial bronchus 
and its functional equivalent of the left lung, the ascending 
branch (A) of the first left ventral hyparterial bronchus, both 
possess fundamental morphological characters of the highest im- 
portance. Each serves for the neomorph cranial pulmonary dis- 
trict in the same capacity as does the main stembronchus for 
the lung-stem, constituting its excentric axial structure. Onto- 
genetically this is the result of the identical processes which we 
noted above as responsible for the intrapulmonary organization 
of the primitive lung-sac (cf. p. 160). 
The eparterial bud of the later phyletic stages of evolution is 
the exact homologue of a secondary lung-sac of the early embry- 
onal period, and develops its conductory system on exactly the 
same morphogenetic lines. We speak, for the sake of brevity, 
of the anlage of the ‘eparterial bronchus.’ In the strict sense it 
is not the ‘bronchus’ which buds, but the respiratory entodermal 
outgrowth which will later be represented in the bronchial tree 
by the eparterial bronchus (figs. 13 and 14, Ep). This secondary 
cranial stembronchus for the upper lobe develops along the same 
path as its ancestral prototype, the primary stembronchus of 
the lung-stem, and like it acquires ventral and dorsal branches, 
and, sparingly, accessory bronchi. Its terminal distribution sup- 
plies the apex of the lung, just as the main stembronchus breaks 
up into a leash of small branches for the caudo-dorsal lung-pole. 
In this evaluation of Hp and A, as the stembronchi for the cra- 
nial extension from the lung-stem of their respective sides, their 
dorsal and ventral derivatives are seen to fall into serial line with 
the corresponding branches of the main stembronchus in the 
