100 The Development of the Lungs 
which maintains absolutely the form of the lungs. This may now be 
broken up into the lobules, as the splitting always occurs along the septal 
lines and, thus, the entire connective-tissue lobular system may be re- 
vealed. It should be observed that the lobules may become compound 
through a failure of the septa to persist, a process similar to that which 
takes place in the submaxillary gland where the whole series of primitive 
lobes, which are first formed in the embryo and separated by well-marked 
septa, disappear and are indicated in the adult only by irregular septa, 
without distinct relationships, passing in from the capsule. Usually, 
however, these lobules in the pig’s lung not only persist, but may be 
easily demonstrated by any of the ordinary connective-tissue stains. 
Recapitulation of Organogenesis.—In recapitulating the growth of the 
main structures of the lungs, we have stem and main bronchi origi- 
nating in the primitive lung sacs as an epithelial tube with a double 
layer of epithelium, the inner of which is columnar, while the outer 
is composed of smaller polygonal cells. This simple tube is surrounded 
by a membrana propria formed by a deposit of fibrils from the exoplasm 
of the connective-tissue syncytium. As the bronchi grow, a layer of 
spindle cells differentiates from the mesoderm, which is transformed 
into the muscular coat of the bronchi. Later still, a chondrification 
of the perimuscular syncytium takes place from which the cartilla- 
ginous rings of the trachea and the bronchial cartilages are formed. 
With these changes the connective-tissue fibrils become grouped into 
trabecule 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 erypt-like invaginations formed by the longitudinal folds of epith- 
elium, glands begin to grow down into the submucosa, which some- 
times pass between 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 also takes place 
in the glands, giving rise to a series of submucous glands with partly 
serous and partly mucous cells. While these changes are taking place 
in the mucosa, the cartilages are also growing, and with them, a further 
differentiation of the framework into distinct fibrous trabeculae. As we 
follow the bronchi peripheralwards, they become simpler and essentially 
younger in structure and yet develop their adult characteristics in pre- 
cisely the same way. The epithelium soon becomes single layered of a 
columnar type, and then of 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- 
