Joseph Marshall Flint alesyil 
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 ordinary respiratory structures used in the adult stage, 
are produced at a later period. We have, thus, both a physiological and 
an ontogenetic 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 asym- 
metry consisting in the presence of an unpaired Lateral 1 and an un- 
paired Ventral 2, both of which occur on the right side. Some mam- 
malian lungs are symmetrical and considerable effort has been made to 
explain all the asymmetrical lungs on the basis of the minority of sym- 
metrical ones. The asymmetrical lung, however, must be regarded as 
typical for mammals. The two bronchi responsible for the asymmetry 
are, so far as we know, characteristic of the mammalian and avian (Aeby) 
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 appar- 
ently 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 of which is columnar, while the outer is composed of smaller 
polygonal cells. This simple tube is surrounded by a membrana propria 
produced largely by the deposit of fibrils from the exoplasm of the con- 
nective-tissue syncytium, composing the mesoblastic portion of the lungs 
at this early stage. As the bronchi grow, a layer of spindle cells differ- 
entiate from the mesoderm, which are transformed into the muscular 
coat of the bronchi. Later still, a chondrification of the perimuscular 
syncytium takes place from which the cartilaginous rings of the trachea 
and the bronchial cartilages are formed. With these changes, the con- 
nective-tissue fibrils become grouped into trabecule about the bronchi 
and in the submucosa. Later, the mucosa is thrown into a series of 
longitudinal folds, while from the cuticular border of the inner row of 
cells, cilia develop. From the bottom of the crypt-like invaginations 
formed by the longitudinal folds of epithelium, glands begin to grow 
into the submucosa, which sometimes pass between the developing muscle 
