THE DEVELOPMENT OF THE HEART IN MAN. 
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out that in the earlier specimens the sole outlet for the blood from the left ventricle 
was the interventricular foramen. With the closure of that foramen the left ventricle 
is brought into direct continuity with the aortic channel in the cylindrical portion 
of the right ventricle. 
External Form (model at 50 diameters magnification). — The heart is relatively 
small in comparison with that of the 12 '5 mm. specimen, and measures only 110 mm. 
vertically, 95 mm. transversely, and 130 mm. dorso-ventrally in the model. The 
atria are small and in systole. The left ventricle exceeds in vertical depth the right 
ventricle. The external form of the ventricular portion is similar to that part of 
S4, and there are similarly distinct interventricular furrow and notch. 
Interior of the Heart. — Sinus Venosus and Atrium. — The sinus is now largely 
merged into the atrium. The vena cava superior has a long oblique course through 
the dorsal wall. The orifice of the vena cava inferior is directly caudal to it. 
The right venous valve shows an arrangement similar to that in S4, and forms a 
long vertical and almost complete partition extending across the atrium from the 
right of the venas cavse to the left side of the right venous ostium. 
The inferior attachments of the right and left venous valves are completely 
separated from one another (Plate-fig. 12). 
Sinu-atrial Chamber. — This chamber is distinct, but is modified in its shape from 
that of S4, partly by the condition of systole. Into it there open the vena cava 
superior dorsally and the vena cava inferior and coronary sinus caudally, well 
separated from one another. The lateral boundaries are the venous valves. The 
left venous valve forms a ring-like fold, attached around its circumference to the wall 
of the atrium. The caudal attachment is blended with the septum primum, and 
orally the attachment of these two folds to the wall forms a stout mass. 
For the rest, the septum primum also forms a ring-like projection into the atrium, 
with a large central orifice, the foramen ovale secundum, towards which the lower- 
part of the septum is inclined. The interval between this deflected portion and the 
left venous valve constitutes the remains of the intersepto-valvular space. 
Ventricles. — The ventricles were apparently fixed in systole, and their cavities 
are extremely small, hence the specimen is not a favourable one for determining the 
arrangement of the atrio-ventricular valves. 
The position of the interventricular orifice is found to be occupied by a fibrous septum 
continuous orally with the septum separating the aorta from the right ventricle. 
The wall of the right ventricle is not so thick as that of the left ventricle. 
Within the upper part of the muscular interventricular septum there is a rounded 
mass of deeply-staining nuclei embedded in loose connective tissue which is a definite 
rudiment of the atrio-ventricular bundle, but its connection with other portions 
could not be traced. 
The heart in the following embryo (S3), of approximately a similar stage of 
development, affords a clearer picture of the condition of the chambers at this stage. 
