Joseph Marshall Flint 
oo) 
-2 
way of review, suffice it to say that the syncytium forming the primi- 
tive framework of the lungs differentiates slowly until 10 mm. is reached 
when, in the neighborhood of the root of the lung and the Mesocardium 
posterior, the fibrils begin to appear and the cells become more isolated 
from each other. About the young bronchi, however, they are still in 
close apposition during the formation of the reticulated membrane about 
the tubes, which, in Mallory preparations, may be seen as a dark blue line. 
At 13 mm. (Fig. 21) these conditions are well shown. The stem 
Lronchus (Fig. 210) and its chief lateral branches is seen in longi- 
tudinal section lined, by an epithelium consisting of a row of inner 
nxn Ge 22. 
Text Fic. 22. Section of the lung of an embryo pig 3 cm. long. Same 
preparation as used with tissue shown in Fig. 21. xX 70. p=pleura. a= 
connective tissue. 6 —bronchus. 
columnar cells with smaller polygonal cells beneath them. The epithelial 
tube is surrounded by a simple reticulated membrane which is in pro- 
cess of formation. Above, at the root of the lung (Fig. 21 a), the trans- 
formation of the exoplasm into young connective-tissue fibrils has taken 
place, while in the lower portions of the Lobus inferior (Fig. 21 ¢), the 
framework consists of a mass of anastomosing syncytial cells without 
any particular differentiation. About the basement membrane, the cells 
are thickly packed and under the primitive pleura (Fig. 21p) the 
epithelium of which has begun to flatten, we have a distinct blue line 
indicating the formation of a membrana propria. 
