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PROFESSOR D. WATERSTON ON 
At the stage of the 8-mm. embryo the vena cava inferior and the coronary sinus 
open into the sinus venosus dorsal to the atrium. The widening of the sinu-atrial 
orifice caudally allows the opening of both of these vessels to move forwards so that 
their orifices come to lie in the floor of the sinu-atrial chamber near the atrio- 
ventricular opening. 
This attachment extends as far as to the atrio-ventricular junction, and thence 
on the ventral wall of the atrium, and divides into two limbs. This splitting is con- 
nected with a change in the position of the opening of the coronary sinus, which 
moves forwards in the floor of the sinu-atrial chamber and comes to open between the 
two limbs. Its orifice therefore is separated from that of the vena cava inferior by 
the left limb of the attached margin. This limb forms the valve of the coronary 
sinus, while the right limb forms the valve of the vena cava inferior. 
The vena cava superior does not move forwards to the same extent, nor do the 
venous valves diverge so much from one another orally. For a time, at about the 
20-mm. stage, the orifice of the vena cava superior appears to be guarded by a 
definite valve, formed by the right venous valve and a portion of the left venous 
valve. This condition is well illustrated in the figure of the 20-mm. embryo. The 
left venous valve later is displaced to the left, and the narrow orifice becomes a 
wide one. 
There can be little doubt that the terminal parts of the venae cavae superior and 
inferior are formed from portions of the sinus venosus, for the orifices of these vessels 
widen out and merge very gradually into the atrium. 
Left Venous Valve . — In the 6-mm. embryo the left venous valve forms a small 
flap united to the right valve at its upper and lower ends. 
In the 8-mm. specimen the two valves are more widely separated. Subsequently 
their union caudally is divided and each valve is attached separately to the floor of 
the atrium. 
The left venous valve is at first separated from the septum primum of the atrium 
by an interval which forms a small bay in the posterior part of the atrium, the inter- 
septo-valvular space. , This interval gradually disappears, and the left venous valve 
and septum primum come into contact with one another. In the 12'5-mm. specimen 
these two folds are united caudally for a short distance and form a single vertical 
fold. Above the level of this union the left valve is continued along the roof of the 
atrium, and forms there a ridge which extends as far forwards as to the atrio- 
ventricular junction. In the 16-mm. embryo the left valve is attached caudally to 
the septum primum, but orally passes to the roof of the atrium and on to the ventral 
wall, forming a complete ring separating the sinu-atrial chamber from the inter- 
septo-valvular space. In the 20-mm. embryo a portion of the left venous valve 
forms a small flap limiting on the left the opening of the vena cava superior into the 
sinu-atrial chamber. Caudally it is fused with the base of the septum primum, which 
now forms the left margin of the orifice of the vena cava inferior (Plate-fig. 15). 
