Joseph Marshall Flint 39 
and more developed than the left. At the same time, there has been a 
more symmetrical readjustment of the two second lateral bronchi, making 
them both with reference to their direction and the distance which 
separates them from the trachea practically mirror images of each other. 
L. 2 on the right side passes laterally and somewhat superior, undergoing 
like the tracheal bronchus a division into two practically dichotomous 
branches. Of these, one branch, which will continue as the main 
bronchus (PI. II, Figs. 15, 16 i) les ventralwards, while the other is 
directed dorsally and slightly inferior. The latter is the dorsal inferior 
branch (PI. II, Figs. 15, 16 di) of the right L. 2 in the adult, and its 
downward course is due, as we shall see later, to the presence of L. 1 
above, which prevents its growing upwards to the apex of the lung like 
the corresponding branch of the left side (Pl. II, Figs. 15, 16ap). In 
comparing the growth of the three first divisions of the bronchial tree 
until they have reached their present development, it is possible to note 
in the progress of L. 1 and L. 2 on each side their passage through 
practically the same stages simultaneously. If the apical branch of 
L. 2 on the left side is equivalent of L. 1 or the tracheal bronchus as 
Willach, Narath, and others suggest, it is difficult to explain the tardy 
appearance of the left element and to give a reason why the right should 
be so well developed. As a matter of fact, this apical branch of the left 
Lateral 2 is not the homologue of L. 1, but of the dorsoinferior branch of 
right Lateral 2, a branch, which, in the adult lung, is practically but not 
quite as well developed as the apical branch itself. The difference be- 
tween the two lies in the different nature of the environment in which 
they grow. Of equivalent age and value in the bronchial tree, the dorso- 
inferior branch on the right side, influenced by its space relationships 
and the presence of L. 1 above is forced to grow downwards and back- 
wards, while on the left side, the corresponding branch, unobstructed 
through the absence of L. 1, mounts upwards to the apex of the lung 
to supply the territory through which the tracheal bronchus runs on 
the opposite side. This power of substitution, which the bronchi pos- 
sess is not confined to this branch alone, but may take place in many 
other parts of the tree, as we shall see in the later stages. In my corro- 
sions, I have never found an instance of the suppression of L. 1 in the 
pig. Narath, or (Pl. VII, Fig. 5), however, shows a case in the human 
lung which indicates how, under these circumstances, this dorsoinferior 
branch of right L. 2 with an unobstructed environment may take a 
course almost exactly like the corresponding branch on the opposite side. 
Arising as in the preceding stages from the axial bronchus between 
L. 2 and L. 3, Ventral 2 on the right side has increased considerably in 
