278 
PROFESSOR D. WATERSTON ON 
Heart of 20-mm. Embryo (S3). 
Model of the Heart magnified 50 diameters (Plate-fig. 13). 
Dimensions. — 1. Vertical (of ventricles) . 
150 mm. 
50 „ 
60 „ 
165 „ 
2. Transverse width | ri g htVentricle 
l left 
3. Dorso-ventral, whole heart 
The heart was modelled within the pericardium and the body wall, and the 
external appearances reproduce accurately the shape of the heart within the 
embryo. 
In external form the heart resembles the former specimen, but is somewhat 
flattened in an oblique direction. The impression on the under aspect of the right 
ventricle is caused by the projection of the right lobe of the liver against the floor 
of the pericardium. 
The heart appears to have been fixed in systole, and the cavities are small, 
particularly those of the ventricles. 
The atria form vertically elongated narrow channels enclosing the dorsal half 
of the upper portion of the heart. 
The right and left ventricles are separated by an oblique furrow, and the upper 
part of the right ventricle forms a bulging prominence whose upper margin lies 
horizontally. 
The aorta occupies very much the same position as in the adult heart. 
Interior of the Heart (Plate-fig. 13). — The incorporation of the sinus and 
atrium is far advanced, and the venous valves and the septum primum are so 
prominent that in cross-section the three-chambered appearance of the atrium is 
very striking. At each side lie the expanded lateral portions of the atria, and 
between them lies the sinu-atrial chamber. 
The vena' cava superior opens by an oblique channel through the dorsal wall, 
and then turns forwards ; and the vena cava inferior and the coronary sinus open 
separately oil the floor. 
The right venous valve again forms a large vertical septum stretching dorso- 
ventrally across the right atrium, perforated only by a small orifice which allows 
blood to escape from the sinu-atrial chamber into the right atrium proper and 
so to the right ventricle. The condition of the left venous valve is not easily 
determined in this specimen, for only a small portion of it is at all distinct. It 
forms a small flap on the left side of the orifice of the vena cava superior. That 
orifice is a narrow one, and the small flap blends with the right venous valve both 
oral and caudal to the opening. 
The septum primum is separate and distinct. Its lower part forms an S-shaped 
fold, the bay of the S leading to the foramen ovale II, and directing the blood 
stream from the vena cava inferior directly to it. 
