34 
the trachea and the bronchial cartilages are formed. With these 
changes, the connective tissue fibrils become grouped into trabeculae 
about the bronchi and in the submucosa. Later, the mucosa is thrown 
into a series of longitudinal folds, while from the cuticular border of 
the inner row of cells, cilia develop. From the bottom of the crypt- 
like invaginations formed by the longitudinal folds of epithelium, 
glands begin to grow into the submucosa, which sometimes pass be- 
tween the developing muscle bundles into the deeper layers of this 
coat. As this process takes place, there is a differentiation of some 
of the epithelium into goblet cells, a process, which one also observes 
in the glands, giving rise to a series of submucous glands with partly 
serous and partly mucous cells. While these changes occur in the 
mucosa, the cartilages are also growing, and with them a further differ- 
entiation of the framework into distinct fibrous trabeculae taken place. 
As we follow the bronchi peripheralwards, they become simpler and 
essentially younger in structure and yet, develop their adult character- 
istics in precisely the same way. The epithelium soon becomes single 
layered and of a columnar type as the periphery is reached. Finally 
it takes on a distinct, flat, cubical form. The Lobuli respiratorii begin 
to develop in pigs about 19 cm long by a slight dilatation of the 
growing ends of the bronchi. These represent the Bronchioli. Later 
Bronchioli respiratorii are then formed, having a progressively flattened 
epithelium, which runs over into Ductuli alveolares. These are present 
at the age represented by a pig 22 cm long. Subsequently, Atria, 
Sacculi alveolares, and Alveoli pulmonis form in the prenatal period, 
all of which have the characteristic flattened respiratory epithelium. 
And finally, after birth, there is a dilatation of the lobules and a 
further flattening of the epithelium occurs, and before the pig is half 
grown, a muscle layer develops about the air passages as far as the 
atria, where it stops in sphincter like bands. One finds at no period 
in the life history of the pig’s lung, openings or fenestrae which com- 
municate between adjacent respiratory lobules. The latter form in- 
dependently at the growing ends of the tree and as they approximate 
each other, the interalveolar framework can always be demonstrated 
between them without interruptions suggestive of fenestrae connecting 
adjacent alveoli. 
17. The framework of the lungs develops from a general syncytium 
forming the mesodermic anlagen of the lung wings. By a gradual 
differentiation of connective tissue fibrils from the exoplasmic part of 
the syncytium, the framework becomes denser and, finally, at 8 cm, 
a suggestion of lobulation is obtained about the end branches of the 
