Joseph Marshall Flint 91 
siderably denser. The circularly arranged fusiform cells noted in the 
earlier stages about the main bronchi are collected into bundles to form 
the muscular layer outside of the mucosa, while still external are stages 
in which the chondrification of the syncytium is progressing as the latter 
passes over into the precartilage stage at the periphery, and into young 
cartilage in the center to form the simple chondral rings of the trachea 
and larger bronchi. The epithelium of the latter is sometimes thrown 
into folds, is cylindrical, and composed of a double layer of cells. As one 
follows the branching to the end buds, it first becomes single layered and 
then of a low columnar type (Fig. 246). Chondral rings and bronchial 
cartilages are present only around the trachea and the upper part of the 
stem bronchi; the muscular coat, as one passes peripheralwards, thins 
out until it first consists only of a single layer of cells, and finally at the 
smaller branches and end buds is replaced by the young connective tissue, 
which, in the latter region, is engaged in the formation of the reticulated 
membranes. 
The most interesting change, however, lies in the further growth of the 
lymphatics, which, in the earlier stages, are found in the root of the 
lung in the neighborhood of the pulmonary vessels and large bronchi. 
As they grow in, they accompany these structures for a distance, then, 
approaching the end branches, they leave them and run in a plexiform 
manner midway between the bronchial tubes (Fig. 247) until they reach 
the pleura (Fig. 24). This gives the lung now an indefinitely lobu- 
lated appearance, in which the periphery of the simple lobule is indi- 
cated by the lymph vessels and the center by the bronchi. The lymphatics 
are lined by flattened endothelium, their walls are formed by the young 
connective-tissue fibrils, and, here and there, valves are beautifully shown, 
which, in general, point away from the pleura. The pleural epithelium 
(Fig. 24 p) is much flattened and now rests upon a thickened layer of 
young connective-tissue fibrils. 
Pig 13 em. long (Fig. 25). At this stage, we have the whole lung 
subdivided into a series of connective-tissue lobules with essentially the 
same characteristics as those shown in the preceding stage, namely, a 
peripheral plexus of lymph vessels with the bronchus in the center. The 
growth is centrifugal in so far as the bronchi are concerned and, in this 
sense, the lung at this stage may be compared in some respects with 
the younger stages of the salivary glands for example, where similar 
lobules without peripheral lymphatics are also formed from a centrifugal 
growth of the ducts. The framework at this stage (Fig. 25a) is con- 
siderably thicker than in the preceding embryo, the fibers denser and, at 
