32 
circumstances, never develop into distinct lobes. This is due to the 
more rapid growth of the first bronchi, to the gradual increasing den- 
sity of the mesoderm, and, lastly, to the environment of the several 
lobes of the lung. The right Lobus superior, containing Lateral 1 
does not belong to the dorsal region of the lung as some authors hold, 
but to the lateral. The characters which make it appear as a dorsal 
seginent are secondary and not primary. Likewise the portion of the 
left Lobus superior containing the apical bronchus belongs to the la- 
teral region and not to the dorsal. As in the case of the right Lobus 
superior, its dorsal characteristics are secondary. This segment is to 
be compared to the portion of the right Lobus medius which contains 
the main dorso- inferior bronchus. Moreover, the entire left Lobus 
superior is the ontogenetic equivalent of the right Lobus medius. The 
right Lobus superior is an unpaired lobe and has no equivalent in the 
lung. The same thing is true of the Lobus infracardiacus. 
Lobe formation varies greatly in different species. In the majority 
of mammals, there are three or four lobes on the right side, arising 
from Lateral 1, Lateral 2, Ventral 2, and the stem bronchus, while, 
on the left side, there are ordinarily two formed from Lateral 2, and 
the stem. Extremes of variation occur, however, between a lobeless 
lung in which none of the bronchi subdivide it and a multilobar lung 
in which most of the principal bronchi have segmented the wing into 
a series of small lobes. Apparently, the division of the lung into lobes 
is of no general morphological significance. 
15. In the light of recent researches on the reptilian, amphibian, 
and avian lung, it is possible to take a new view point for the de- 
velopment of the mammalian lung. The lungs of lower animals, we 
now know, are products of monopodial growth. The simple lungs of 
reptilia are capable of producing monopodially outgrowths in any di- 
rection (HESSER). These may become specialized in certain species 
and have a definite topography. As we mount the animal scale, the 
necessity of an increased respiratory surface finally results in the trans- 
formation of the original simple lung into a conducting apparatus, which 
is represented in the mammalian lung. by the stem bronchus and its 
chief branches. The simple lungs may no longer be compared to the 
Lobuli respiratorii of the mammalian lung for the latter represent new 
elements, which with the increased respiratory surface are added peri- 
pherally to the simpler lungs as the latter become transformed into 
bronchi. With the addition of these new elements, the respiratory 
function also wanders peripheralwards, so that the portion of the 
mammalian tree which represents the simpler lungs undergoes a change 
