Cav) 
Or 
Joseph Marshall Flint 
narrows gradually until its lumen in cross-section forms an asymmetrical 
sagitally placed fissure. A short distance above the Ductus hepaticus 
(Fig. 3 DH) the pulmonary groove terminates caudalwards in an irregu- 
lar enlargement (Fig. 2 PA), the asymmetrical pulmonary projection 
forming the first unpaired anlage of the lungs. As yet, there is no trace 
of the main bronchi nor any evidence of a division. Ventralwards, 
it projects somewhat from the level of the ventral margin of the intestine 
below it (Pl. I, Fig. 1), while laterally it is more marked on the right 
than on the left side, an asymmetry more apparent from a transverse 
section (Fig. 2) or a dorsal view of the reconstructed intestine (PI. I, 
Fig. 2). Whether the cause of this asymmetry lies primarily in the 
anlage itself or is due to the influence of the heart as Minot suggests, it 
is impossible to determine from these specimens. Below the pulmonary 
projection, the head gut while still asymmetrical lies more in coincidence 
with the median longitudinal plane. 
PA. 
Text Fig. 1. Trexr Hie. 2. 
TExT Fie. 1. Section of embryo pig 3.5 mm. long, showing head gut in the 
region of the upper part of the Mesocardium posterior. C=Celom. SV= 
Sinus venosus. VM= Mesocardium posterior. 
TExT Fic. 2. Section of embryo pig 3.5 mm. long, through the pulmonary 
anlage. C—Celom. PA= Pulmonary anlage. 
At this stage, the epithelial lining of the head gut is quite variable in 
thickness. In the pulmonary enlargement (Fig. 2. PA) it is clothed 
by a columnar epithelium of several layers with mitoses taking place 
chiefly in the innermost row. In the dorsal segment of the head gut 
at this level, it is considerably lower especially at the dorsal angle where 
it consists of a single layer. Above the projection it is thinner in the 
bottom of the groove and thicker at its sides. The Mesocardium pos- 
terior (Fig. 1 Vif) begins just below the last gill pouch and extends 
down to a short distance below the pulmonary anlage. Between these 
points, the entire head gut is surrounded by a mesoderm composed of 
anastomosing cells in which the exoplasmic or fibrillar portion of the 
mesoderm is not well differentiated (compare Mall, 02, and chapter on 
