Joseph Marshall Flint 123 
This difference can usually be explained by the point of origin of the 
two stem bronchi with reference to the pair designated as Lateral 2. 
If the stems originate low down, then Lateral 1 is thrown on to the 
trachea, while if their origin is higher up, the first lateral arises from 
the stem bronchus. Apparently Lateral 1 is characteristic of mammals 
and, according to Aeby, of birds. A bronchus corresponding to it has 
not been found either in reptilia or amphibia. In almost all mammals 
it is an unpaired element. No satisfactory proof has even been brought 
to show a bilateral development of Lateral 1 with a subsequent degenera- 
tion of the left bronchus, notwithstanding the fact that this process has 
been described in two species. At no time in the life history of the pig 
is there a Lateral 1 formed on the left side. There is furthermore no 
embryological evidence to show a relationship between Lateral 1 and 
the dorsal series of bronchi. These characteristics are secondary and 
result from the antagonistic effects of the growth of Lateral 1 and 
Lateral 2. The latter is foreed somewhat ventralwards, while the former 
is pressed dorsalwards, until its lower branches le above the dorsal series 
of bronchi. 
3. The remainder of the lateral series originate in succession from 
the lateral side of the stem bronchus as lateral outgrowths or hernia- 
like expansions of the wall of the stem bronchus near the terminal bud. 
These elements in their growth outwards finally reach the chest wall. 
Here they are compelled to grow in the space between the ribs and the 
liver and consequently follow the curvature of the chest wall which 
ultimately gives them, more or less, the appearance of ventral bronchi, 
a fact which led Aeby, who studied only the finished tree, to call them 
the ventral series. 
4. The dorsal series of bronchi, originating like the lateral group as 
outgrowths from the stem bronchus, are usually paired. They alternate 
with the paired lateral bronchi and are independent productions of the 
stem. They do not either ontogenetically or phylogenetically originate 
from the lateral bronchi. For convenience, the first pair are called 
Dorsal 2, to keep the designation harmonious with the larger series of 
lateral bronchi. . 
5. The ventral bronchi originate as outgrowths from the ventral sur- 
face of the stem. They, like the other series, are independent produc- 
tions of the main bronchus. They are not originally formed on the 
lateral bronchi and subsequently transferred to the stem bronchus. Con- 
sequently, they are chief bronchi and not accessory in the sense of Aeby. 
In the pig and in the great majority of mammals, left Ventral 2 is 
