160 GEO. S. HUNTINGTON 
itive lung of the earlier stage. The entoderm is still throughout 
respiratory in character (fig. 7°). 
Any of the more simply constructed lungs of the Lacertilia 
and paludal Chelonia illustrates this phylogenetic stage in the 
evolution of the mammalian lung. The same stage is represented 
in the ontogeny of the mammal, due allowance being made for 
the smaller number of the secondary chambers and the increased 
intervals between their points of connection with the axial lumen, 
characters which foreshadow the future architectural plan of the 
mammalian lung. 
Continued extension of this evolutionary process leads in the 
well known way to the gradual production of a racemose organ, 
in which each ultimate component is the morphological equiv- 
alent of the single ancestral primitive lung-sac. Types are fur- 
nished to the phyletic series by the lungs of the soft-shelled 
chelonians (Aspidonectes, Eretmochelys, Sphargis). 
II. With this increase in the complexity of the lung the next 
phyletic advance is marked by a structural modification along 
definite lines both of the intrapulmonary epithelium and the 
related mesoderm, repeating at a number of points the earlier 
differentiations of these tissues through which the primitive 
entodermal lung-tube became divided into the extra-pulmonary 
bronchus and the primary lung-sac. First along the axial line 
of the central pulmonary cavum (fig. 7°), subsequently in the 
same way along the axial lines of the series of separate chambers 
opening into it (fig. 7°), the respiratory epithelium assumes the 
character of a conducting epithelium while the subjacent meso- 
derm differentiates into the supporting structures of a bronchus. 
These histogenetic changes begin at a number of separate points. 
The individual anlagen then becomes joined to form a tubular 
conducting system which connects with the extra-pulmonary 
bronchus. The lung has thus become invaded by an intra- 
pulmonary system of bronchial tubes, opening into an axial 
canal, the stembronchus, which in turn is now continuous with 
the extra-pulmonary bronchus and meets the corresponding 
structure of the opposite site at the bifurcation of the trachea 
(figs. 7°, 7°). The lungs of certain marine turtles (Chelonia 
