Joseph Marshall Flint 33 
bronchi continue their growth after the production of the first two paired 
lateral bronchi. ‘These appear as lateral evaginations from the walls of 
the primitive sacs. On the left side, however, the bud is directed more 
cranialwards owing to the horizontal position of the left stem. The 
trachea increases in diameter and length; the bronchi, however, still 
maintaining the same general relationships, have grown in both caliber 
and thickness. Now, the very slight evagination of the tracheal bronchus 
has increased considerably in size and projects from the right wall so 
as to be noticeable particularly in longitudinal sections from which the 
model shown in Pl. I, Figs. 9 and 10, was reconstructed. It is quite 
as apparent as the paired Lateral 2 and is separated from the one on the 
right side by a distance of approximately 380 mikra. ‘These three bronchi 
may be considered as practically contemporaneous branches of the prim- 
itive tree with the tracheal bronchus appearing as a very faint evagina- 
tion before the lateral bronchi as such can be definitely seen in the 
primitive lung sacs. 
The two stem bronchi now extend more caudalwards than in pre- 
ceding stages; of the left particularly is this true. They also preserve, 
although not to such a marked extent, the horseshoe-like dorsal curvature 
observed in a pig 7.5 mm. long. On their lateral surfaces are two slight 
evaginations, the anlage of the second lateral bronchi (PI. I, Figs. 9, 10, 
L. 2). Of these the right project lateralwards, while the left points 
upwards. These two projections do not appear from a terminal portion 
of the end bud, but from its lateral surface. They are, therefore, the 
productions of a monopodial growth The epithelial lining in these 
primitive buds is a trifle deeper than in the other parts of the tubes and 
in the inner row karyokinetic figures are more numerous than in the 
other parts of the respiratory endoderm. 'The mesoderm about the buds 
does not appear either thicker or thinner than that on other parts of the 
respiratory tube. It is impossible, therefore, that this tissue can exert 
any marked growth influence in the production of these lateral branches. 
Much more probable are the space relationships to which the tube adapts 
itself as, lateralwards in the ccelom, we have one point of least resist- 
ance, while caudalwards between the thorax wall and the liver, is another. 
The bending of the stomach anlage to the left (PI. I, Figs. 9, 10) for a 
time may have some influence on the growth of the left bronchus holding 
it in its more horizontal position. From this point the consideration of 
the development of the mesodermic portion of the lungs will be dis- 
cussed in a separate chapter. 
3 
