PULMONARY EVOLUTION IN MAMMALIA 157 
transmission by inheritance of the varying extant bronchial 
patterns? I believe that at the close of our consideration of this 
problem we will stand squarely on the basis of the last named 
hypothesis. 
In compiling and interpreting the facts which have determined 
this conclusion it is necessary to preface the same with an outline 
of the phylogenetic and ontogenetic evolution of the mammalian 
lung. The respiratory apparatus of all pulmonate vertebrates 
consists essentially of a longer or shorter air-conducting tube 
derived from the ventral wall of the foregut and constituting the 
primitive pulmonary anlage. Its internal surface is lined through- 
out by an epithelium continuous with the entoderm of the in- 
testinal canal from which it is derived. Its ectal surface is sur- 
rounded by the splanchnopleural mesoderm of the pulmonary 
blastema, carrying the vessels of the organ. The distal end of 
this tube, differentiating in its ontogeny from the fore-gut in a 
caudo-cranial direction, divides into bilateral canals, the prim- 
itive lung-tubes of the right and left sides, upon whose surface 
the afferent and efferent pulmonary vessels ramify in a sub- 
epithelial capillary network, while the cranial portion of the canal 
remains undivided and forms the trachea, retaining a point of 
primitve connection with the foregut, the laryngeo-pharyngeal 
orifice. The primitive lung-tube, extending caudad on each 
side from the tracheal bifurcation, subsequently differentiates 
into two components, the lung-sac proper and the extrapulmonary 
bronchus (fig. 7*). The former expands and becomes demarked 
from the bronchus by its increased lumen. The ental surface 
is covered by respiratory epithelium. The bronchus retains its 
uniform caliber and, both inthe character of its lining epithelium 
and in the accessory differentiations of its enveloping meso- 
derm, conforms to the structure of the trachea. The smooth 
walled primitive lung proper forms the direct caudal continua- 
tion of the extrapulmonary bronchus, as seen in the Urodele 
lung (Necturus), and in the early ontogenetic stages of the 
mammalian organ. The hilus affording entrance to the bronchus, 
accompanied by the pulmonary vessels, occupies its cranial pole. 
The entire pulmonary apparatus thus divides into a conductory 
and a respiratory component. 
