a 
34 The Development of the Lungs 
The branches of the primitive bronchial tree in a pig 8.5 mm. long, 
then, may be tabulated as follows: 
TRACHEA. 
Lateral 1. 
Right stem bronchus. Left stem bronchus. 
Lateral 2. Lateral 2. 
At 10 mm., the trachea (PI. I, Figs. 11, 12 7) has increased consider- 
ably in size and Lateral 1, which appeared as a simple swelling in the 
earler stages, has now grown to a button-like enlargement (PI. I, Figs. 
11, 12, L. 1) sharply constricted from the wall of the trachea. It points 
lateralwards and also shghtly ventralwards. The division of the trachea 
into right and left bronchi shows still the asymmetry of the preceding 
stages as the plane of the left stem is still more transverse than the right. 
At the same time the general direction of the right bronchus does not ex- 
tend so far dorsalwards, as the growth of the right bronchus has appar- 
ently been directed more towards the tail end of the embryo. Just at 
the point of bifurcation, the second lateral bronchi on either side are 
seen; the one on the right is somewhat larger than the corresponding 
branch on the left. Both, however, are now fairly symmetrically placed, 
although right Lateral 2 is shghtly more ventral and the left more apical 
in its direction. Beneath right L. 2, a slight bulging is visible on the 
axial bronchus directed ventralwards. This is the anlage of Ventral 2, 
the infracardiac bronchus (Pl. I, Fig. 11, V. 2) which arises directly 
from the stem bronchus and not from L. 2. At the same time, directly 
opposite the anlage of Ventral 2, there is also a slight dorsal evagination 
of the stem, indicating the first traces of Dorsal 2 (Pl. I, Fig. 12, D. 2) 
on the right side. The appearance of Ventral 2 (Bronchus infra- 
cardiacus) and Dorsal 2 is accompanied by an apparent lateral flatten- 
ing of the stem bronchus due to the extension of the buds dorsalwards 
and ventralwards from the axis of the mother branch giving it, in cross- 
section, a marked oval shape, while above and below, it resumes its 
cylindrical form. This may be nicely seen in Figs. 11, 12, and 13, where 
11 shows a transsection of the stem bronchus above, 13 below, and 12 
at the level of the primitive dorsal and ventral branches (Fig. 12, V. 2, 
D. 2). In the inner row of epithelium in these projections, karyokinetic 
figures are much more numerous than in other parts of the stem bronchus 
save in the neighborhood of the terminal bud. At the same time there 
is a packing of the nuclei at the base nearer the basement membrane 
which is now less distinct and gives the epithelium the appearance of 
having an extra row of cells at this point. 
The left bronchus is considerably shorter than the right and projects 
