162 GEO. S. HUNTINGTON 
buds from any point of its surface and to develop its intrinsic 
pulmonary organization in accordance therewith. The result- 
ing extensions of the primitive respiratory area are reflected in the 
corresponding modeling of the conducting system and give rise 
to the specific form of the bronchial tree pertaining to the type. 
Including thus within the limits of its anlage the common basic 
materials for the differentiation of both of its structural compon- 
ents, the lung is in a position to meet all the evolutionary 
demands put upon it by the environment and the purpose it 
serves in the organism. 
3. Compared with the early phylogenetic stages of the verte- 
brate organ (fig. 7*), in which the entire available surface of the 
primitive lung-sac is respiratory and supplied by the extrapul- 
monary bronchus, the evolution of the mammalian lung demands, 
in accordance with its ultimate design, extent and structure, the 
selection of a limited number of points of intensive epithelial 
budding in place of the preceding general use of the entire poten- 
tial surface. This selection is directed and regulated by the 
extent and location of the total peripheral respiratory area which 
will ultimately represent the culmination of the development 
derived from each of these points. This in turn is governed di- 
rectly by the amount and location of the intracoelomic space 
available for pulmonary extension and its topographical relations. 
The grouping of these selected areas of epithelial proliferation, 
and the pattern which they produce in their relation to each 
other and to the stembronchus, will determine the type of the 
bronchial tree characteristic for each form. Within this form 
the bronchial type is transmitted by heredity. 
After the primitive lung-sac has been invaded by the conduc- 
tory system, and the resulting intrapulmonary bronchus has 
replaced the antecedent central cavum pulmonale, the active 
epithelial budding is transferred to the lining of the secondary 
lung-sacs each of which is now connected by its own short afferent 
canal with the axial stembronchus (fig. 7”). The latter is thus 
provided with the anlages of its main derivatives, the ventral 
and dorsal primary bronchi. These act in turn as stembronchi 
for the individual secondary lung-sacs, repeating there the gen- 
