260 
PROFESSOR D. WATERSTON ON 
• Chambers . — The sinus venosus consists of a short and comparatively narrow 
transverse portion, and of a right and left horn. The left horn, the smaller of the 
two, lies in the upper margin of the left portion of the septum transversum, whence 
the transverse portion passes to the right and terminates in a large right horn, which 
is partly embedded in the septum transversum and partly has risen out of it. 
The right horn of the sinus venosus lies dorsally and to the right of the atrium. 
It is roughly triangular on section, and its wall is continuous with that of the 
atrium (text-fig. l). 
Sinu-atrial Orifice . — The communication between sinus and atrium is a vertical 
slit-like orifice, opening from right to left, and slightly ventrally. The ventral 
Text-fig. 1.— Transverse section (slide 103, section 3) of heart of 3-mm. embryo, showing the chambers 
and the narrow endothelial tube within the myo-epicardial mantle, x 100. 
junction of sinus venosus and atrial walls -is infolded, forming a prominent right 
venous valve, but there is no left venous valve. 
Atrium . — The atrium is a large, capacious chamber, lying dorsal and oral to the 
ventricle, and extending at its oral end across the whole' width of the heart ; the 
sinu-atrial opening is placed at the dorsal right margin, while the atrial canal opens 
from the caudal and left ventral corner. 
The dorsal wall is incurved towards the cavity, but there is no trace of the 
atrial septum. 
The muscular wall is deficient over a small area of The dorsal wall; the “area 
interposita ” and the myocardial and endocardial walls are in contact. The muscle 
wall is extremely thin. 
Atrial Canal . — This short channel extends from the left ventral and caudal corner 
of the atrium to the oral left portion of the ventricle. Its width in the model, at 
200 diameters magnification, is 15 mm., and it is almost cylindrical. 
In this portion is found that separation of the myocardial and epicardial walls by 
