THE DEVELOPMENT OF THE HEART IN MAN 
285 
The dorsal attachments of the left venous valve and of the septum primum are 
fused, and at their origin from the wall of the atrium there is an infolding of the 
atrial wall, consisting of two layers of muscle tissue in apposition, separated only by 
a small amount of connective tissue. This slight invagination is the rudiment of the 
septum secundum. 
The embryo was specially examined to determine the presence of any conducting 
tissue between atrial and ventricular muscle in other regions. 
At the circumference of the atrial canal, at the junction of atrium and ventricle, 
an apparent infolding of the wall takes place, involving both the atrium and the 
ventricle ; and as a result a wedge-shaped mass of connective tissue is formed, its base 
Text-fig. 18. — Transverse section (slide 65, section 9) of heart of same embryo as fig. 17. x 25. 
at the free surface and its apex passing into the interior of the lateral valve flaps of 
the venous ostia. 
These lateral valve cusps are formed of embryonic heart muscle, and there is 
also present in each of them a mass of cushion tissue lying on the atrial surface 
of the cusps. 
This infolding was traceable around the lateral margins of the atria-l canal ; and as 
the muscle tissue present in the cusps is later transformed into connective tissue, 
there cannot be any continuity between the atrial and ventricular muscle at these 
levels. Orally and caudally there is not the same infolding,, but in these regions also 
there appeared to be a complete break between the muscle wall of the atria and that 
of the ventricle and bulbus. 
The Atrio-ventricular Valves . — The valve apparatus of the right and left sides 
forms a clear illustration of the formation of these structures. 
