264 
PROFESSOR D. WATERSTON ON 
The interval between these orifices, or venous ostia, represents the single atrial 
canal of the former specimen, which has moved to the right and lies almost ventral 
to the sinu-atrial inlet into the atrium. 
The atrial septum, prolonged forward, would lie nearer to the right than to the 
left orifice. The fused endocardial cushions form a mass, extending into the floor of 
the atrium to the base of the septum primum. 
The left orifice opens into the left ventricle, and the right into the bulbus cordis ; 
and the right margin of the right atrio-ventricular opening is limited by one of the 
two bulbar cushions (Plate-fig. 3). 
Ventricular and Bulbus Portions. — This portion shows a partial division into 
right and left ventricles by a thick, rounded muscular partition which projects into 
the interior from the ventricle wall, and is represented on the surface by a groove 
on the ventral aspect and a notch upon the caudal margin of this portion of the 
heart. Of the two chambers, the left ventricle forms a spherical, thick-walled 
chamber, with reticulated muscle . tissue in its interior, receiving the left atrio- 
ventricular orifice in its dorsal cephalic wall. The interventricular foramen forms 
the sole outlet for this chamber, and by it the blood can pass to the distal portion 
of the bulbus cordis (Plate-fig. 3). 
The wall of the right ventricle is of less thickness, and is not so fully reticulated. 
The ventricle is flask-shaped, and the lower portion, dilated and capacious, leads into 
a narrow tubular portion, where the lumen shows incomplete division into two 
channels. Within the lower portion are found the terminations of the two chief 
bulbar cushions, one on the interventricular septum, the other forming a prominence 
on the right of the right atrio-ventricular orifice. 
Within the tubular portion are found the distal portions of the bulbar cushions. 
The spiral course and the general shape and arrangement of these cushions may 
be gathered from the reconstruction figured in Plate-fig. 4. 
The ultimate derivatives of these two portions of this chamber are very different, 
and the nature of the two portions is discussed later. 
For the present, the names of conical portion and tubular portion may be given 
to distinguish the two parts from one another. 
Heart of Embryo Bl (8 mm. in Length). Plate-figs. 5,6, and 7. 
External Form. — The heart in this embryo and that in embryo Ml (9 mm.) 
resemble one another very closely indeed. 
The large ventricular portion shows clear indication upon its surface of the 
internal division into right and left chambers by the interventricular furrow and 
the notched caudal border. 
The interventricular furrow continues in a caudal direction the left margin of 
the tubular “ bulbus cordis” portion. 
