Joseph Marshall Flint 27 
the line of separation between the respiratory (Figs. 4 Pg, 5'PA) and 
digestive systems (Figs. 4, 50) and extend from the region just below 
the gill pouches to the pulmonary anlage. In the upper portion, near 
the gill pouches, the lumen of the cesophageal part is somewhat larger, 
while, at the level of the pulmonary anlage, the respiratory segment is 
markedly dilated (Fig. 5 PA). Between these levels, the relationship 
between the two is practically equal (Fig. 4). Above, the epithelium is 
lower in the dorsal and ventral angles, shghtly so in the lateral fissures 
but somewhat thickened at the sides of both dorsal and ventral segments. 
In these thickened portions there is a double layer, in the angles a single 
layer of epithelium. In passing caudalwards, the epithelium of the 
respiratory anlage thickens as its lumen increases in size until a double 
row of columnar cells line the floor of the pulmonary groove (Fig. 5 PA), 
Thxt Wie. 5: Text Fic. 6. 
Text Fic. 5. Section of an embryo pig 4 mm. long, through the upper 
part of the pulmonary anlage. C—=Celom. PA=Pulmonary anlage. O= 
Digestive portion of the head gut. 
Text Fic. 6. Section of an embryo pig 4 mm. long through the lower 
portion of the pulmonary anlage. C—Celom. O= Digestive portion of the 
head gut. BD—Right stem bronchus. 
while at the sides, they are three cells deep. At the level of the pulmon- 
ary anlage, the asymmetry is again evident. The projection has now 
begun to extend lateralwards on each side to produce the main bronchi. 
To the left, the evagination is considerably higher than on the right 
and also less prominent. At the same time the asymmetry is exagger- 
ated by the anlage of the right bronchus (Fig. 6 BD) which points some- 
what caudally. The epithelium lining the two primitive bronchi is 
columnar and consists of several layers. Rapid mitosis is taking place 
chiefly in the inner row of cells. 
With the more marked symmetry of the head gut itself, there is also a 
greater symmetry of the mesodermal anlage (Figs 5, 6) of the lungs. 
