32 The Development of the Lungs 
the dilated sacs is different on the two sides, that on the right is larger 
and more nearly triangular in transsection (Fig. 10 BD). It projects 
further dorsalwards than the left (Fig. 10 BS). As yet there are no 
marked evidences of the production of lateral branches except a slightly 
more prominent angle at the upper lateral wall of the right sac and a 
similar irregularity of contour on the upper wall of the left. From these 
points, as we shall see in the next stage, the paired second lateral bronchi 
arise. Just above the origin of the stem bronchi, however, on the right 
side of the primitive trachea, one observes a slight bulging or outgrowth 
of its wall. At this level, the epithelium is a trifle thicker and numer- 
ous mitotic figures occur. The projection extends over an area of about 
80 mikra and represents the anlage of the first lateral bronchus (PI. I, 
Figs. 7, 8, L. 1). The process by which this structure is produced is 
apparently a simple evagination to be compared, perhaps, with the evag- 
ination of the pulmonary swelling from the primitive head gut, on the 
one hand, and the primitive bronchi from the pulmonary anlage on the 
other. ‘Thus we may consider the same process as repeating itself in the 
development of the first stages of the pulmonary apparatus. No similar 
evagination, however, can be observed on the left side. 
In the mesoderm of the lungs, the dorsal mesentery (Fig. 10 DM) 
now reaches as high as the forking of the trachea, while the Mesocardium 
posterior (Fig. 10 VM) extends as high as the anlage of the tracheal 
-bronchus. The mesodermic syncytium itself shows some differentiation, 
particularly under the pleura and in the region of the mesocardium and 
dorsal mesentery. Here the cells branch and anastomose and the differ- 
entiation of the exoplasmic portion into fibrils is in progress. About the 
cesophagus and pulmonary epithelium, however, there are dense masses 
of mesodermal cells without much differentiation. This group of cells 
is engaged in the production of the young basement membranes as the 
stems continue in their growth. In consequence of the more equal dila- 
tation of the sacs, the simple lung wings are more symmetrical than at 
any other period of early embryonic life. Differences, however, between 
the two sides on inspection of the reconstructions are readily made out. 
The right Recessus pleuroperitonealis extends slightly above the level 
of the lower end of the stem bronchus, while the left has disappeared. 
At this age we may express the derivations of the pulmonary anlage in 
the following tabulation : 
TRACHEA. 
Lateral 1. 
Right bronchus. Left bronchus. 
Right lung sac. Left lung sac. 
At 8.5 mm. the irregular contour of the lung sacs is lost and the two 
