Joseph Marshall Flint 21 
the 1st ventral or lateral bronchus on the left side. Concerning the 
so-called accessory bronchi, Merkel seems to be in accord with the older 
observers in looking upon them as derivations of the dorsal and ventral 
lateral bronchi, and apparently follows Narath, instead of His, regarding 
the Bronchus infracardiacus as a possible derivative either of the first, 
second, or even third ventral bronchus instead of an independent branch 
of the stem. 
The comparative embryology of the lungs in vertebrates has been 
studied by Moser, 00, whose material consisted chiefly of the lower 
vertebrates amplified to some extent by sections of rat, mouse, and rabbit 
embryos. All vertebral lungs, according to Moser, are developed through 
a common principle consisting in a general increase in size due to an 
increase of their constituent tissues. The epithelium is the principal 
factor which originates from the endoderm and passes as a single tube 
into a solid mass of connective tissue forming the framework of the 
lung. If this connective tissue is thin, the growth of the epithelium 
produces a widening of the intrapulmonary bronchus with simple pro- 
jections on its walls as in amphibia. On the other hand, if the con- 
nective tissue is dense and resistant, the epithelial increase is localized 
in certain places, the cells are packed together until they force their 
way into the connective tissue forming buds such as we find in the lungs 
of all vertebrates from reptiles up. Certain points on the walls of the 
lung are more resistant and remain in the lung cavity as septa. At the 
same time, as we ascend the scale, the number of buds of the second 
order constantly increase. According to Moser, we may also observe 
at this time a gradual increase in the mass of connective tissue in pass- 
ing from lower to higher vertebrates, and we obtain, in consequence, a 
system of long canals or bronchi passing through a connective tissue sac. 
The division of the bronchi is always and exclusively by monopodial 
growth, and is a main bronchus, the intrapulmonary bronchus, which is 
a direct continuation of the extrapulmonary bronchus passes through 
the lung from the root to its distal end. 
By means of the reconstruction method, Bremer, 04, studied the 
lung of the young opossum (Didelphys virginiana) and compared it 
with older stages. His youngest specimen measured from 10.5 to 12.5 
mm. and were taken from the same pouch. Older specimens, 14 cm. 
long, and three adults were also used for comparison. In five out of 
six of the new-born animals Bremer found an eparterial bronchus on 
both sides, except that the one on the left bronchus is always smaller 
and placed slightly lower than the eparterial branch on the right. The 
air chambers supplied by it, however, do not form the apex of the lung. 
