130 The Development of the Lungs 
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 com- 
pared to the portion of the right Lobus medius which contains the main 
dorsoinferior 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 left 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 viewpoint for the development of 
the mammalian lung. The lungs of lower animals, we now know, are 
products of monopodial growth. 'The simple lungs of reptilia are capa- 
ble of producing monopodially outgrowths in any direction (Hesser). 
These may become specialized in certain species and have a definite topo- 
graphy. As we mount the animal scale, the necessity of an increased 
respiratory surface finally results in the transformation 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 peripherally to the simpler 
lungs as these become transformed into bronchi. With the addition 
of these new elements, the respiratory function also wanders periph- 
eralwards, so that the portion of the mammalian tree which represents the 
simpler lungs undergoes a change of physiological function. Its phylo- 
genetic relationship to the simple lungs is shown by the monopodial 
growth of the mammalian stem bronchus and its principal branches, 
which recapitulate ontogenetically the growth process of the simple 
lungs before producing dichotomously the prepheral respiratory struc- 
tures which are used in mammalian respiration. In certain animals, 
moreover, the stem bronchus and its branches retain for a period in their 
life history their respiratory function. In monotremes and marsupialia, 
