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PROFESSOR D. WATERSTON ON 
left margin is a continuous line of large, very darkly staining cells similar to those 
seen in the endocardial lining of some portions of the ventricles, and which probably 
afterwards form Purkinje cells. 
The ventral attachments of both the right and left venous valves run into a mass 
of indefinite cushion tissue at the central portion of the atrio-ventricular junction, 
and especially into a mass of deeply-staining cells lying immediately to the left of 
the orifice of the coronary sinus, in the position of the atrio-ventricular node of the 
adult heart. This mass in turn is connected with a mass of similar cells lying in the 
interior of loose vacuolated tissue in the apex of the interventricular septum, similar 
to the structure seen in earlier specimens already described. This structure is 
Text-fig. 17. — Transverse section (slide 64, section 9) of heart of 28 , 5-mm. embryo. 
identical with that described by Mall as the rudiment of the atrio-ventricular bundle 
in the embryonic heart. 
The structure described above constitutes a continuous medium of communication 
between the nodal tissue, or rather pre-nodal tissue, which from an early stage is 
recognisable at the base of the right venous valve, and the definite atrio-ventricular 
node lying near the coronary sinus. 
The lower part of the septum primum forms a partition of considerable height, and 
it forms the lower left boundary of the sinu-atrial chamber. Its lower attachment is 
separate and distinct from that of the left venous valve, but the two structures 
gradually approach one another orally. 
The foramen ovale secundum is placed so far ventrally that its ventral boundary 
is the level of the wall of the atrium. Dorsally, however, the foramen does not 
extend to the dorsal wall of the atrium, but here the remains of the septum primum 
form a slight ridge, which merges into the left venous valve. 
