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PROFESSOR D. WATERSTON ON 
and bulbus have generally the same features as in mine, their relative position is 
different — the ventricle lying caudally in his specimen, and the bulbus lying orally, 
the two chambers separated by a bulbo-ventricular groove which is nearly horizontal 
in direction. 
The condition in my specimen has been reached by a rotation of the bulbo- 
ventricular groove from the horizontal to the oblique axis, and later specimens show 
that this rotation is continued until the groove is vertical in direction. 
Heart of Embryo Si (6 mm. in Length). 
The external form and the internal structures are shown in Plate-figs. 2, 3, and 4. 
At this stage the sinus venosus has risen out of the substance of the septum 
transversum, and in place of the vitelline and umbilical veins there is a single large 
vessel (vena cava inferior) which opens into the right horn of the sinus venosus. 
The left duct of Cuvier runs into a comparatively narrow channel, and the transverse 
portion of the sinus venosus passes along the posterior surface of the heart and 
opens by a narrow orifice into the right horn. The right horn forms a small chamber, 
into which opens the right duct of Cuvier and the vena cava inferior. The sinus 
venosus opens into the atrium by a narrow slit-like orifice, measuring in the model 
25 mm. vertically and 5 mm. transversely at its widest part. The sinus wall is 
smoothly and evenly continued into the atrial wall. 
The right venous valve — the more prominent of the two — measures 8 mm. in 
length, and in structure consists of a more loosely reticular tissue than the adjacent 
heart wall, and vacuoles or spaces are present within it. The two venous valves 
united orally to the orifice form a long, narrow “ tensor valvulse,” prolonged on to 
the roof of the atrium (Plate-fig. 3). 
Atrium . — The shape of this chamber is considerably altered, and it forms a large 
crescentic cavity, enclosing in its bay the tubular portion of the bulbus cordis. The 
dorsal convex wall of the crescent is indented centrally by the dorsal body wall. 
Near the middle line there is an almost complete septum primum atriorum uniting 
