132 The Development of the Lungs 
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 differentiation of the framework into distinct fibrous 
trabecule takes place. As we follow the bronchi peripheralwards, they 
become simpler and essentially younger in structure and yet, develop 
their adult characteristics 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 dila- 
tation of the growing ends of the bronchi. These represent the bron- 
chioli. Later Bronchioli respiratorii are then formed, having a pro- 
gressively flattened epithelium, which runs over into Ductuli alveolares. 
These are present at the age represented by a pig 22 cm. long. Subse- 
quently, 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 fenestree which communi- 
cate between adjacent respiratory lobules. The latter form independ- 
ently 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 differ- 
entiation of connective-tissue fibrils from the exoplasmic part of the 
syncytium, the framework becomes denser and, finally, at 8 cm., a sug- 
gestion of lobulation is obtained about the end branches of the growing 
bronchi. Within these connective-tissue lobules, the framework differ- 
entiates as the embryo grows, forming simultaneously basement mem- 
branes for the young bronchial buds. At the same time, the interlobular 
fibers and those below the pleura, unite to form trabecule. As the Lobuli 
respiratorii, towards the end of foetal life, begin to impinge on each 
other, the interalveolar framework and the two adjacent basement mem- 
branes are pressed together into a single wall or septum in which the 
