Joseph Marshall Flint 93 
layer, which then becomes lower until, in the lobules, it forms a lower 
columnar epithelium. Still further out in the growing terminal buds 
(Fig. 25 ¢), it now has a distinct cubical form. About these, the mem- 
brana propria is formed from the connective tissue of the lobule. 
The lymphatics (Fig. 257), forming a plexus around the bronchial 
veins and arteries at the root of the lung, accompany them towards the 
periphery, giving off branches to the interlobular spaces en route. Their 
walls, owing to the increasing differentiation of the framework, are 
thicker. On reaching the periphery of the lung, they leave these struc- 
tures and pass out as in the preceding stages to the pleura. They have 
a plexiform arrangement and may (Fig. 25) be traced at times into the 
substance of the lobules. This course may also be observed in the deeper 
lobules of the lung as well as those on the surface under the pleura. 
In the period of embryonic hfe between pigs 13 and 19.5 em. in length 
there are no marked changes of the relationships we have,thus far de- 
scribed. In the larger bronchi, a gradual development has occurred. 
The epithelium now possesses well-marked cilia springing from the 
cuticular border of the inner layer of epithelhum, between the elements 
of which, goblet cells appear here and there, partly filled with mucous. 
These are clearly seen first in the stem bronchi of pigs between 15 and 
17 cm. long. The folds, which have already been described running 
longitudinally with the bronchus, now look in cross-sections hke regular 
papille with a core of submucosa. That they are regular structures of 
the bronchi and not shrinkage products is shown by the impressions they 
leave on corrosion specimens which are injected under considerable 
pressure as well as their appearance in distended lungs. The muscu- 
laris mucose is more developed and the bronchial cartilages are well 
formed. In general, the relations of the lymphatic system has not 
changed; lymph glands may be observed forming in the neighborhood of 
the root of the lung, and large bronchi in pigs as young as 12 cm. ‘They 
naturally increase in size and number with the age of the embryo. With 
the other changes, there has been a gradual flattening of the epithelium 
in the growing ends of the tree, until, in an embryo 18 cm. long, the 
end buds are lined by a very flat form of cubical cells with spherical 
nuclei. The cytoplasm, which in the earlier stages was granular, is now 
clear and transparent. 
At 19 cm. (Fig. 26), some notable changes have been inaugurated in 
the structures. The ciliated epithelium of the stem bronchi possesses 
a great number of goblet cells. In the submucosa, the muscularis has 
gathered into distinct bundles, while from the fundus of the crypt-like 
