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PROFESSOR D. WATERSTON ON 
On a transverse section the atria are crescentic in shape, and they are very 
capacious — many times more so than the ventricles. It is probable that the atrinm 
has been fixed in diastole and the ventricle in systole. 
The septum primum is not complete, for there is a small foramen ovale primum 
close to the region of the atrial canal. The septum is S-shaped in section, and shaped 
so as to direct the inflowing blood from the vena cava inferior towards the foramen 
ovale secundum, an orifice of some size in the upper part of the septum, with 
irregular margins. 
Atrio-ventricular Openings . — These orifices resemble closely the orifices seen in 
Si. They are very small indeed, and lie on either side of the fused mass of the 
Text-fig. 4. — Transverse section (slide 10, section 20) of heart of 8-mm. embryo. 
endocardial cushions. The extent and arrangement of these cushions are shown 
diagrammatically in text-fig. 5. 
The upper cushion is of loose texture, and the nuclei are widely separated from 
one another, but there are small areas where they are closely packed together. 
The margins of the cushion stain more deeply and are marked off distinctly from 
adjacent muscular tissue. The upper endocardial cushion begins high up in the 
ventricle and on the ventral surface of the septum primum of the atria. At this level 
it lies embedded in the muscle substance of the ventricles. Traced downwards, it 
passes to the free margin- of the interventricular septum and projects on each side 
into the lumen of the right and left ventricles. Its right margin extends to blend 
with the lateral bulbar cushion above the level of the right atrio-ventricular 
orifice, and to the left it joins the trabecular musculature of the left side of the 
ventricle above the left atrio-ventricular orifice. On the left of the left atrio- 
ventricular orifice there is a small patch of cushion tissue. Slightly caudally this 
upper endocardial cushion diminishes in size, and the right and left atrio-ventricular 
