Joseph Marshall Flint 99 
about which are numerous connective-tissue cells. There is as yet, how- 
ever, no differentiation of this layer into muscle fibers. From this arise 
the short Bronchioli respiratorii (Fig. 29 br) where the cubical epithe- 
lium flattens as the passages run into the Ductuli alveolares (Fig. 290). 
From these structures, the Atria (Fig. 29a) are formed, which in turn 
produce the Sacculi alveolares (Fig. 29sa). The air sacs which were 
only indicated in a pig 27 cm. long are now distinctly seen. It is pos- 
sible that they are even more developed before birth than is shown in 
Fig. 28, as I have frequently found embryos in utero 29 cm. long. 
Unfortunately, I have been unable to obtain good sections from speci- 
mens of this age. This makes, however, no essential difference as the 
whole respiratory lobule is produced before the pig is born. Following 
the use of the lungs for respiration, there is a dilatation of the various 
structures of the lobule (cf. Figs. 28 and 29) which is accompanied by a 
still greater flattening of the connective tissue between the alveoli, yield- 
ing practically a single membrane containing the blood-vessels between 
the two layers of respiratory epithelium. ‘This, however, as we have seen, 
ontogenetically consists of the two basement membranes and the inter- 
alveolar framework of the adjacent alveoli. The larger connective-tissue 
lobules still retain their general relationships, increasing in size with 
the growth and dilatation of the respiratory lobules of Miller. The 
lymphatics (Fig. 297) still have their regular relationships. 
In a half-grown pig, one observes the thickening of the framework, 
which in the main septa at the root and under the pleura is now made up 
of well-formed trabecule, consisting of connective-tissue fibrils. The 
bronchi have developed peripheralwards taking on an older type, 1. @., 
adding muscular layers, submucous glands, and bronchial cartilages, 
which may be traced as far as the larger intralobular branches. From 
this point peripheralwards, gradually thinning, the muscle layer extends 
to the opening of the atria in the Ductul alveolares. The lymphatics 
in the interlobular septa are difficult to see as they are pressed together 
by the growth and distension of the connective-tissue lobules. No 
marked changes occur between this and the adult stage, save that the 
lobules are sometimes less apparent owing to their larger size and the 
fact that the septa may become thinned out in the later stages of growth. 
They may be demonstrated as definite anatomical structures in the pig 
by thick sections stained by Mallory’s method or better still by complete 
Wood’s metal injections. When a lung has been distended for a short 
time with air to its maximum, Wood’s metal will pass into all the indi- 
vidual alveoli. After digestion, we have a cast of granular appearance 
