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ito) 
Joseph Marshall Flint 
group of elongated fusiform cells having a distinctly circular arrange- 
ment may be noted, representing the earlier stages of the production of 
the muscular coat. 
The epithelium in all the large and in the majority of small bronchi 
still consists of two layers of cells, the inner columnar, the outer poly- 
gonal in form. But in the youngest branches of the oldest bronchi, 
namely Lateral 1 or 2, there is now a reduction to a single layer of 
columnar cells (Fig. 220). Cilia are as yet invisible in these speci- 
mens, but the cuticula at the inner margin of the cells is already differ- 
entiated. At the root of the lung, a few dilated lymphatics may be noted 
near the bronchi and pulmonary vessels; they have not, however, grown 
beyond this point into the substance of the lung wings. 
Embryo 5 mm. long (Fig. 23). The general framework (Fig. 23 a) 
of the lung at this period has undergone a further differentiation over 
the preceding stages, consisting in an increasing density and complexity 
of the young fibrils, which now possess a more distinctly fibrillar appear- 
ance, while the quantity of endoplasm about the connective-tissue cells 
has slightly diminished, except in the immediate neighborhood of the 
larger bronchi. The pleural epithelium (Fig. 23 p) is much more flat- 
tened and the nuclei of the individual cells consequently further apart. 
As shown by points where the plane of section falls tangential to its 
surface, the basement membrane beneath this epithelium is distinctly 
reticulated. About the larger bronchi, there is a distinct circumferential 
arrangement of the exoplasmiec fibrils in which are imbedded a great 
many cells. The basement membrane is slightly thickened and just 
beneath the latter there is now a well-marked layer of fusiform cells 
with elongated nuclei running circularly about the bronchial tube. Ex- 
ternal to this stratum, is a looser circular arrangement of the exoplasmic 
fibrils as well as the cells embedded in it. When the bronchi are cut 
longitudinally, these circumferential lamelle of cells and exoplasm run 
parallel to the long axis of the tube. The epithelium, as in the preced- 
ing stages, shows a distinct division into two or three layers, with the 
nuclei situated approximately in the middle of the cell. The thickening 
‘on the edge of the cell lining the lumen is apparent, although cilia are as 
yet unformed. As the branches of the tree are followed towards the 
periphery, the layers of circularly directed syncytial cells disappear and 
we have simply the primitive basement membrane with the connective- 
tissue cells immediately about it. In the most terminal parts of the 
air passages, the double layer of epithelium has been replaced by a single 
layer of lower columnar epithelium (Fig. 230). All of the bronchi 
