Joseph Marshall Flint 81 
V. 3 and V. 4. , The latter, however, are very faint and are separated 
from the rest of the lobes by very shallow grooves. On the left side, 
the Lobus superior (Fig. 19, L. 2) is separated from the lower lobe by 
a deep cleft, while the development of the apical branch of L. 2 has 
pushed up with it a segment of this lobe which also grows backward 
until it lies above the series of dorsal swellings (Fig. 19 B) and bears a 
marked resemblance to the Lobus superior on the opposite side. Ex- 
cepting for the Lobus infracardiacus, the lower lobe has characteristics 
practically homologous to the corresponding lobe of the right side. The 
dorsal flexion of both lower lobes still persists and the lateral tips or 
margins of the median lobes now begin to show, at their lateral extremi- 
ties, a shght bending ventralwards as they begin to fold around the 
heart. 
As the lung continues to grow, with the successive appearance of new 
branches, new elevations are formed on the surface of the primitive lobes 
until finally, as Narath describes, they have an appearance like the sur- 
face of a mulberry. The primitive lobes, however, keep their independent 
character: and alter in form by two chief factors, namely, the intrinsic 
growth of the lung itself, and the change in its environment formed by 
the chest wall, heart, liver, and diaphragm. Narath has given as the 
cause of the lobe formation, the extremely rapid growth of the first 
branches of the tree, while the later branches of slower growth fail to 
form furrows in the mesoderm deep enough to subdivide the lung further. 
With this view, I am in complete accord, but it ought, it appears to me, 
to be extended to include the character of the mesoderm. In the early 
stages, this is in extremely plastic form, which easily moulds itself to the 
pressure of the growing bronchi beneath. Up to 10 mm. there is scarcely 
any differentiation in the mesoderm into distinctly fibrillar and cellular 
portions, while at 12 mm. this change is inaugurated and fibrils appear 
particularly in the region of the root of the primitive lung. At 20 mm. 
the whole mesodermic portion is composed of young connective tissue 
with well-marked fibrils. As the mesoderm differentiates, therefore, it 
becomes firmer and is less easily influenced by the growth of the young 
bronchi. 
Fig. 20 is an outline drawing of the lateral and diaphragmatic aspects 
of the lings of an embryo 19 em. long. At this time, all of the important 
adult topographical features of the lungs are present. A., shows well how 
the right Lobus superior has grown down and back into the dorsal area, 
moulding itself even more than in an embryo 18.5 mm. long (Fig. 19) 
to that portion of the thoracic cavity and extending now up over the 
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