LEPIDOSriiEN PAKADOXA. 
93 
ends of the primitive heart-tube to the pharyngeal rudiment 
will be recalled. 
The pharyngeal rudiment (Text-fig. 14, extends 
dorsally across the narrow pericardiac cavity, and, with its 
sheath, remains for some time closely applied to the sinus 
venosus behind the pericardium (Text-fig. 14, S. F.). When 
the cellular proliferation that gives rise to the auriculo- 
veutricular ridge occurs, its cells are continuous with those 
of the pharyngeal sheath along the left sinu-auricular angle 
(Text-fig. 15, av. r. and ph.s.). 
A little later, about Stage 31, the pharynx becomes split 
off from the dorsal surface of the anterior part of the sinus, 
but this split occurs dorsal to a little column of cells (Text- 
fig. 15 E, F and G,p.v.) lying obliquely across the roof of the 
sinus, and arching down anteriorly across its left wall, where 
it is continuous with the auriculo-ventricular ridge and 
the interauricular septum (Text-fig. 15 d and e, . The 
splitting-off process occurs gradually from before back- 
wards synchronously with the development of the pharynx 
and lung. Posteriorly, the little column of cells remains 
continuous with the ventral surface of the pharyngeal and 
lung rudiments (Text-fig. 15 G, ph.s.). Finally, it becomes the 
scaffolding along which tlie common pulmonary vein crosses 
the roof of the sinus venosus, and arches ventrally and to the 
left to reach the auricle from the ventral surface of the lung 
(Stage 31). Once in the auricle the pulmonary vein opens 
through a slit in the left side of this little cell mass, dorsal to 
the auriculo-ventricular ridge, and the margins of the opening 
presently project a little, forming a hood-like fold (Text-fig. 
17, P. F.). The pulmonary vein therefore develops in the left 
wall and roof of the sinus venosus, but later, owing to the 
expansion of the auricles and the rotation of the heart, its 
terminal portion comes to lie deeply between the two auricles 
(Text-fig. 13, P. /. and P.V.). The pulmonary fold, to which 
reference has been made in the account of the adult heart 
(PI. 5, fig. 1, P. /.), is thus formed by the right wall of 
the terminal portion of the pulmonary vein that passes for- 
