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JANE I. EOBERTSON. 
almost a right angle with the sinus venosus and projects 
to the left into the pericardiac cavity; (2) a short auricular 
canal intervening between (1) and (3); (3) a ventral, trans- 
verse, ventricular part (Text-fig. 9 a, V.) directed from left 
to right, and (4) an anterior, ascending part, curving from 
right to left towards the middle line again — the forerunner 
of the bulbus cordis (Text-fig. 9 b, B). The long ventral 
and short dorsal walls of the 'transverse part of the tube 
may now, for brevity, be called the ventral and dorsal 
curvatures of the heart respectively. 
Auricnlo- ventricular Plug. — At Stage 27, before 
there is any definite division of the heart into separate 
Text-fig. 10. 
Section through the sinu-auricular junction at Stage 28. A. Auricle. 
av. r. Auriculo-ventricular ridge. S. V. Sinus venosus. 
chambers, a little knot of cells appears between the two 
layers of the heart-wall, at the left ventral margin of the 
opening of the sinus venosus into the auricle (Text-fig. 10, 
av.r.). 
This knot of cells forms a ridge extending in a ventral 
direction along the left posterior wall of the heart through 
the auricular canal, and reaching as far as the commencement 
of the ventral curvature (Text-figs. 9 a and b, av. r.) On the 
ventral curvature the tip of the ridge comes in contact and 
becomes continuous with the muscle-fibres appearing in the 
ventricle (Text-fig. 11, av. r.). This ridge inclines from left 
to right (compare fig. 15a, b, and c, av. r.) and divides the 
auricle into a larger right and a smaller left compartment, 
the former being situated at first somewhat posterior to the 
