THE DEVELOPMENT OF THE HEART IN MAN. 
279 
The septum is more complete than in the former specimen, and the orifice of 
the foramen ovale II is only a small hole in a nearly complete partition, and it 
lies close to the oral border. 
The arrangement of these structures is further modified by the size and shape 
of the atria, which are of great vertical height. 
The wide bay which existed in the earlier specimens in the roof of the atrium, 
and which indicated the line of the division, is now replaced by a deep, narrow cleft, 
at the bottom of which the septum primum is attached. The apparent infolding of 
the wall of the atrium constitutes an early stage of the formation of the septum 
secundum, but it is produced not by a downgrowth of a septum but by the upgrowth 
of the atria. 
A large pulmonary vein opens immediately to the left of the septum primum ; 
and further to the left side there is an elevation of the floor of the left atrium, 
apparently marking off the quadrilateral sinus of the left atrium from the appendix 
(auricle). 
Atrio-ventricular Junction . — The atrio-ventricular junction shows now an 
almost complete separation of the muscle wall of the atrium from that of the 
ventricle. Connective tissue from the atrio-ventricular orifice passes deeply into the 
interior of the substance of the flaps forming the lateral boundaries of the atrio- 
ventricular orifices : so that the tissue on the atrial surface of the valve cusps is 
continuous with the atrial musculature, while that on the ventricular surface is 
continuous with the trabecular musculature of the ventricles. 
Rudimentary or embryonic muscle tissue is recognisable on the atrial surface 
of these valve cusps. The rudimentary medial or septal cusp of the tricuspid valve 
is present as a rounded projection on the right surface of the interventricular septum, 
consisting mostly of endocardial cushion tissue. 
The interventricular septum is now complete, but where it joins the region of 
the endocardial cushions its texture is very loose, while in the centre of this loose 
tissue is a very distinct rounded rudiment of the atrio-ventricular bundle. 
The medial (anterior or aortic) cusp of the mitral valve (not shown in the figure) 
is very distinct and projects from the left side of the atrio-ventricular septum, 
with which its ventricular surface is continuous ; but this flap throughout is less 
muscular in texture than the opposite cusp. 
The pulmonary artery and the aorta are completely separated from each other, 
and the semilunar valves of each of them are well formed. The semilunar valves 
of the aorta lie immediately above the level of the interventricular septum ; while 
the pulmonary valves, on the other hand, lie 50 mm. orally at the termination of 
the tubular part of the bulbus cordis. 
The condition of the whole of the right ventricle is very similar to the condition 
found in S4. There is, for example, the same narrowing of the lumen of the most 
distal portion near the root of the semilunar valves ; proximal to this a more dilated 
