Joseph Marshall Flint 101 
chioli. Later, the Bronchioli respiratorii are formed which have a 
progressively flattened epithelium, running 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. 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 passage as far as the Atria, where it 
stops in sphincter-like bands. 
The framework of the lung develops from a general syncytium form- 
ing the mesodermic anlagen of the two 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 the lobules the framework differentiates as the embryo 
grows, forming simultaneously basement membranes for the young 
bronchial buds. At the same time, the interlobular fibers, and those 
beneath the pleura, unite to produce trabecule. As the lobuli respira- 
torii towards the end of foetal life begin to impinge on each other, the 
interalveolar framework and the two adjacent basement membranes are 
pressed together into a single wall or septum in which the blood-vessels 
run. These lobules remain until adult life, and correspond in the pig 
apparently to those described by Laguesse and d’Hardiviller, 98, and 
Councilman, o1, in the human lung. Noteworthy, however, is the fact 
that they may become compound by the loss of the interlobular septa and 
the subsequent confluence of several adjacent lobules. This usually 
takes place at the base leaving the periphery of the compound lobule 
separated by partial septa. 
The lymphatics appear at the root of the lung in an embryo 4-5 cm, 
in length. Accompanying the bronchi and vessels, they gradually grow 
in for some distance and until the smaller air passages are reached, 
they leave these structures and grow towards the pleura in the inter- 
spaces between the smaller bronchi, aiding in the differentiation of the 
connective-tissue lobules. The reason for this course is not entirely clear, 
but it may be due to the increasing density of the framework about the 
bronchi, which forces the later-appearing lymphatics into the interlobu- 
lar spaces as a Locus minoris resistentia. Upon reaching the pleura, 
they turn and form a plexus in the subpleural connective tissue. Here 
and there they may be seen penetrating into the lobules, but cannot be 
followed for any distance in them. At 23 cm. the first evidence of the 
