292 
PROFESSOR D. WATERSTON ON 
of the atrium of the right venous valve as it occurs in my own specimens, and his 
figure I can corroborate. 
The formation of the septum atriorum as illustrated in the models is as follows : — 
The Oral Surface of the Roof of the Atrium. — In the 6-mm. embryo there is 
a central slight indentation of the roof of the atrium at the point where the 
septum primum and left venous valve are present. This surface indentation is 
more distinct in the 8 -mm. embryo, in which there is a wide dorso-ventral groove 
of considerable depth, and on each side of it the roof of the atrium forms a large 
cupola. 
In the later stages these lateral cupolse increase greatly in size, and form narrow, 
long vertical chambers, quite separate from one another. 
At a still later stage the two adjacent walls of the two dilated portions meet 
and become slightly attached to one another. Thus the appearance is produced 
of a narrow communication between the right and left atria situated caudally, 
bounded cephalad and dorsally by an area of atrial wall of considerable extent. 
This area consists of a narrow margin — the edge of the “ limbus fossae ovalis,” 
formed by the fused left venous valve and the remains of the upper- portion of 
the septum primum, and of an area where the walls of the two atria are in 
coaptation. 
In this region there is no true blending of the atrial walls, for even in the 
adult heart the walls of the right and left chambers can be separated ' from one 
another without the division of any muscle tissue as far as practically to the 
margin of the limbus fossae ovalis. 
To show this it is only necessary to incise the epicardium on the dorsal wall 
of the atria along a vertical line between the right pulmonary veins on 'the one 
hand, and a line joining the left borders of the venae cavae inferior and superior 
on the other. The walls of the right and left atria can then be separated from 
one another by merely dividing a little loose connective tissue, and the separation 
can be carried as far as practically to the margin of the limbus fossae ovalis. The 
whole of the dorsal and oral portion of the so-called septum secundum atriorum 
is therefore merely the result of the coaptation of adjacent portions of the walls 
of the two atria, and is not a true septum comparable to the septum primum. 
To this extent I can confirm the view expressed by Retzer, in so far as he 
states that the septum secundum is not a true septum comparable to the septum 
primum. 
The sections of the more advanced embryonic hearts show the infolding of the 
wall quite clearly. 
4. The Atrial Canal and the BulHus Cordis. 
Born has pointed out the importance of the change which occurs in the position 
of the opening of the atrial canal into the ventricle. At first that canal lies on the 
