76 
JANE I. EOBERTSON. 
Pulmonary Veins. — Posteriorly the pulmonary veins lie 
along the outer borders of the lungs, but turn inwards at about 
the anterior third of those organs across their ventral surfaces, 
and join to form the common pulmonary vein (Text-fig. 3, 
c. P. F.) somewhat to the right of the middle line. The 
common pulmonary vein passes forwards and to the left on the 
dorsal wall of the anterior portion of the posterior vena cava, 
and entering the pericardium reaches the roof of the sinus 
venosus. It then runs obliquely to the left in the roof of the 
sinus venosus and curves ventrally across the anterior end of 
the sinus, to open into the left auricle on the left side of a 
fold that projects into the auricles, across the middle of the 
dorsal surface of the auriculo-ventricular plug (PI. 5, fig. 
1, P./.). This pulmonary fold is continuous dorsally with the 
auricular roof, anteriorly with the muscular part of the auri- 
cular septum, and ventrally with the auriculo-ventricular 
plug. The pulmonary aperture is a rounded opening in the 
h ft wall of this pulmonary fold, and is guarded by a hood-like 
fiap that serves to guide the blood directly to the left on 
entering the auricle (PI. 5, fig. 2, P. F. and P. /.). 
Coronary Vein. — A small coronary vein passes from 
about the middle of the dorsal surface of the right ventricle 
to reach the floor of the sinus venosus a short distance from 
the sinu-auricular aperture; its opening is guarded by a val- 
vular flap whose free border is directed posteriorly. The 
exact point at which this little vessel passes from the ventricle 
to the sinus varies considerably in different specimens (Plate 
5, fig. 1, C. F), 
III. Development op the Heart. 
Pericardiac Space. — At Stage 23, in the development of 
Lepidosiren, when there are twenty-four segments present, 
the lateral mesoderm is extending ventrally and inwards over 
ihe surface of the yolk ou either side of the endodermic 
pharyngeal rudiment, and already two chink-like coelomic 
