158 GEO. S. HUNTINGTON 
In the subsequent stages increase in the extent of the respira- 
tory epithelial area is obtained by: A. Structural modifications 
Satis \ 
E HONG, fi -* 
fas  « 
Win Ze 
S 
eS : 
mand 
oss 
Ns 
y 
my); pom 
Cais Wh y SS 
ep a = 
sf 
os 
I 
Fig. 7 Schema of phyletic evolution of the vertebrate lung. Conductory 
paths cross-hatched; respiratory areas stippled. 
of the primitive lung-sacs. B. Structural modifications of the 
primitive extrapulmonary conducting canals of the trachea and 
bronchi. 
