Joseph Marshall Flint 97 
the lobules and the relatively slight amount of clear cytoplasm extends 
between them. The Bronchioli respiratorii (Fig. 27 br) are now readily 
recognized leading off from the bronchioli (Fig. 27>). They open into 
the dilated Ductuli alveolares (Fig. 277) from which the primitive Atria 
may be seen as lateral outgrowths. 
Shortly before birth, in a pig 27 cm. long (Fig. 28), the framework 
of the lung at the root, between the lobules and under the pleura, consists 
of definite trabecule composed of fibrils in the meshes of which le the 
connective-tissue cells. In the neighborhood of the root, the trabecule 
are thick and firm and thin out as the periphery is reached. The struc- 
ture of the stem bronchi is on the same plan as in the earlier stage, but 
the epithelium submucosa, muscularis, and cartilages are more developed. 
As the periphery is approached in this, as in the younger stages, they 
become essentially younger in structure, loosing first their cartilages, then 
the muscularis, and finally, before terminating, have only a thickened 
basement membrane which contains connective-tissue cells (Fig. 28D). 
The respiratory lobules are now fully formed, but are not as large or as 
complicated as in the stages after birth. In this section there are two 
Bronchioli respiratorii (Fig. 28 br) from the ends of which the Ductuli 
alveolares (Fig. 287) lead. These terminate in dilated Atria (Fig. 28 a) 
on the walls of which the Sacculi alveolares are now indicated as shght 
irregular outgrowths. While complete corrosions of the lungs in which 
the respiratory lobules are injected are of great service in interpreting 
the pictures found in sections, I have feared to trust these preparations 
for an exact description of the growth of these structures, owing to the 
possibility of artefacts. The nuclei of the respiratory epithelium now 
project often into the lumen of the air spaces. In general, the cells are 
extremely flattened and the nuclei elongated. A flat sheet of protoplasm 
extends out from either pole of the nucleus resting upon the mebrana 
propria. Here and there, where capillaries project into the lumen of the 
air passages, the nucleus lies in the angle formed by the capillary and the 
basement membrane with the protoplasmic portion of the cell projecting 
up over the capillary, like a non-nucleated plate. 
Adjacent Lobuli respiratorii impinge on each other, pressing the loose 
connective tissue, which has hitherto existed between the lobules into a 
thin membrane in which the capillaries run. This interalveolar mem- 
brane now consists of the membrana propria of the adjacent lobules, to- 
gether with the interalveolar connective tissue. The lymphatics in the 
various parts of the lung still show essentially the same relationships. 
After birth (Fig. 29) the development of the lung has advanced along 
‘ 
