PULMONARY EVOLUTION IN MAMMALIA 163 
etic process which originally led to the establishment of the main 
primary intrapulmonary stembronchus (fig. 7"). Like the latter 
they develop the series of secondary lateral derivatives, each 
terminating in a tertiary lung-sac, which is the exact morpho- 
logical replica of the primary lung-sac on a smaller scale, and 
forms the anlage for the continuation of the process of pul- 
monary unfolding, on the original developmental plan. 
The primitive framework of the future bronchial tree has now 
been laid down. The actual details of its organization in any 
form will depend upon the number and position of the secondary 
lung-sacs budding from the primitive cavum pulmonale and of 
the primary bronchi based on them. The further the develop- 
ment of each of these is carried toward the periphery, and the 
smaller the number of the resulting ultimate respiratory areas 
becomes, the longer will be the unoccupied intervals of the stem- 
bronchus between the origins of the primary bronchi and the 
greater their initial caliber. 
In two lungs of equal volume and efficiency the stembronchus 
of one may carry a smaller number of larger derivatives, while the 
other will show a larger number of primary side branches, each 
supplying a smaller peripheral respiratory area. Thus the two 
lungs shown in schemata (fig. 8, A and B) belong to the same 
bronchial type, (A) having six ventral and eight dorsal side 
branches, reduced in (B) to three and four respectively. 
This numerical range of the bronchial organization prevails in 
the mammalian lung stem proper (lower lobe), which in general 
is characterized by the marked similarity in the arrangement of 
its lateral derivatives from the stembronchus, although their 
number varies considerably in the different genera and orders 
(from three to eight or even nine of the ventral branches). 
Typical examples of (A) are seen in the bronchial trees of the 
lungstem in the Bradipodidae [Choloepus] and in Hyraz, of (B) in 
that of the carnivores and lower primates. 
Based on the preceding conditions the following facts may be 
emphasized: 
1. The entoderm of the intrapulmonary bronchus retains the 
inherent potency, derived from its phylogenetic and ontogenetic 
