4 3 
of physiological function. Its phylogenetic 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 structures 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, the young are transferred 
to the pouch and compelled to carry on their own respiration when 
only the stem bronchus and its chief branches are formed. The or- 
dinary respiratory structures used in the adult stage, are produced at 
a later period. We have, thus, both a physiological and an ontoge- 
netic proof that the simple lungs correspond, in mammals, only to the 
stem bronchus and its chief branches. 
The great majority of mammalian lungs are asymmetrical, the 
asymmetry consisting in the presence of an unpaired Lateral 1 and 
an unpaired Ventral 2, both of which occur on the right side. Some 
mammalian lungs are symmetrical and considerable effort has been 
made to explain all the asymmetrical lungs on the basis of the minority 
of symmetrical ones. The asymmetrical lung, however, must be re- 
garded as typical for mammals. The two bronchi responsible for the 
asymmetry are, so far as we know, characteristic of the mammalian 
lung as similar bronchi have never been described in the lungs of 
lower animals. The cause for the asymmetry, apparently lies in the 
necessity of leaving space for the descent of the heart and great vessels, 
by the suppression of left Lateral 1, on the one hand, and to allow 
room for the shifting of the heart which draws the Vena pulmonalis 
to the left by the suppression of left Ventral 2, on the other. In 
those lungs where these two elements, which are usually missing, are 
found, they are apparently so placed as not to interfere with these 
features of the development of the heart. 
16. In the organogenesis of the lungs, we have the stem and main 
bronchi consisting of simple tubes lined by a double layer of epithelium, 
the inner which is columnar, while the outer is composed of smaller 
polygonal cells. This simple tube is surrounded by a Membrana pro- 
pria produced largely by the deposit of fibrils from the connective 
tissue syncytium, composing the mesoblastic portion of the lungs 
at this early stage. As the bronchi grow, a layer of spindle cells 
differentiate from the mesoderm, which are transformed into the 
muscular coat of the bronchi. Later still, a chondrification of the peri- 
muscular syncytium takes place from which the cartilaginous rings of 
2 
Vv 
Anat. Anz. Aufsätze. XXIX. 
