PULMONARY EVOLUTION IN MAMMALIA 167 
district may derive its bronchial supply from the following 
sources: 
1. Typical ventro-medial accessory bronchus. 
2. Accessory bronchus arising further ventrad or dorsad. 
3. Ventral accessory bronchus. 
4. Accessory bronchus arising from the angle between the 
stembronchus and one of its primary ventro-lateral branches. 
5. From the proximal portion of the latter itself. 
This does not imply that the accessory bronchus has ‘migrated’ 
or ‘wandered’ from the point of its inception, the ventro-lateral 
primary bronchus, first to the angle between it and the stem- 
- bronchus and then to the stembronchus itself, but that the ven- 
tro-medial pulmonary area in question can be supplied by an 
accessory bronchus derived from any one of several adjacent 
points on either the stembronchus or on its main ventro-lateral 
branch. This is the very simple explanation of all the observed 
facts upon which the entire theory of migration of bronchial 
components has been erected. 
If three corrosion preparations of the adult lung illustrating 
three types of bronchial supply for a given ventro-medial pul- 
monary district (e.g., the cardiac lobe) are compared (fig. 
10', ?, 3), they might suggest that the ventro-medial accessory 
bronchus supplying the identical peripheral area (X) in all three 
lungs had ‘migrated’ from the initial position it occupied in 
figure 10! on the ventral bronchus to the angle between it and 
the stembronchus (fig. 102), and had reached by continued ‘shift- 
ing’ or ‘wandering’ the ventro-medial surface of the stembron- 
chus itself (fig. 108). 
Conditions identical in significance with those above outlined 
lie at the base of all other cases of supposed ‘migration’ of what- 
ever kind or extent. In our present instance the sound inter- 
pretation recognizes, that the three bronchi, A, B, and C, sup- 
plying the same peripheral area, X, in their terminal distribu- 
tion, are functional equivalents. 
In respect to derivation they are not, however, the identical 
morphological element shifted by migration from 1 to 2 or 3, 
but the result of epithelial proliferation from three separate and 
