286 
PROFESSOR D. WATERSTON ON 
On the right side there is a single large lateral cusp attached to the septal wall 
and to the anterior and posterior walls of the ventricle. This cusp contains a portion 
of bulbar cushion B, and also the extension to the right of the septum of the bulb, 
and this cusp forms an investment for the right and for the oral margin of the 
right ostium. 
On the opposite side — namely, on the septal aspect — there is as yet no cusp, but a 
large oval prominence is present on the septal wall, which probably becomes under- 
mined later and forms the medial cusp. The moderator band of the adult heart is 
formed from a portion of the septal wall on which lay bulbar cushion A, which 
is gradually undermined. 
At the left ostium two cusps are present, forming practically the adult arrange- 
ment — the medial cusp derived from the endocardial cushions of the atrial canal, and 
the lateral from the infolding of the atrio-ventricular junction. 
These cusps are large, and there is already fully developed the apparatus con- 
nected with them of papillary muscles. 
The medial cusp extends from the oral to the caudal margin of the orifice, and 
its apex is widely separated from the septum. 
Comparison of this specimen with S4 shows the changes which have occurred in 
the structures found in the earlier specimen. As in the former, the right venous 
ostium has a valve of one cusp, viz. a large right or lateral cusp, representing the, 
two cusps, lateral and anterior, of the adult heart. The most oral attachment of this 
cusp is effected by muscular cords which pass to the infundibulum of the pulmonary 
artery, and are attached to the region of the former septum of the bulb. 
There is no proper septal cusp, but its position is indicated by prominent rounded 
elevation. 
On the left side the cusps of the mitral valve are attached, the medial cusp to 
a large anterior papillary muscle, to which the marginal cusp also is attached, and 
also to a small anterior papillary muscle. 
The interventricular foramen is barely closed. 
There is a large “ vestibule” in the left ventricle, below the root of the aorta. 
Discussion. 
The series of specimens is sufficiently comprehensive to afford an almost complete 
account of the successive stages in the development of the heart from the stage at 
which the heart tube is formed. 
The transition from the 3-mm. to the 6-mrn. stage is perhaps rather less com- 
pletely illustrated than the others, but the gap is adequately filled by specimens 
described in the literature of the subject, among which I would refer particularly to 
the 4'5-mm. embryo described by Ingalls, as well as by specimens figured by Tandler 
from the Vienna collection. 
