THE DEVELOPMENT OF THE HEART IN MAN. 
291 
In the embryo of 30-mm. length the left venous valve is merged at its root with 
the septum primum, but at a higher level it projects as a crescentic ridge along the 
roof of the atrium, and extends to the ventral wall. 
3. The Formation of the Atrial' Septum. 
The specimens show the origin of the septum primum as usually described and 
the formation of the foramina ovalia primum and secundum. 
The septum primum is fairly completely formed, even at the 6-mm. stage, and 
it forms an almost vertical septum, joining the dorsal and ventral walls of the 
atrium to one another near their middle. The septum appears to form orally 
and grows downwards to the floor of the atrium, and it unites with the dorsal 
portion of the floor of the atrium at an early stage. It is, however, for some time 
incomplete caudally and ventrally, where it fails to reach the floor of the atrium, 
leaving a channel between the two halves (foramen ovale primum). This orifice 
is shown in the 6- and 8-mm. embryos, but has closed at 12'5 mm. The lower 
margin of the foramen ovale primum is formed by the cushion tissue of the endo- 
cardial cushions of that atrial canal, and its closure is probably due largely to 
the increase in size of those structures and their extension dorsally. 
I have, however, found a specimen of the heart of a fully-developed human 
embryo in which the foramen ovale primum had not become closed, but persisted 
with a foramen ovale secundum (Plate-fig. 16). 
The foramen ovale secundum is formed, even at the 6-mm. stage, as an irregular 
orifice with ragged edges in the upper part of the septum primum. It gradually 
increases in size, and at 8 mm. it is large, and only a small portion of the septum 
primum is found inferiorly to it. It is, in many specimens, placed in the hollow of 
a bay in the septum by which the current from the vena cava inferior is conducted 
to the foramen. 
In structure the septum primum seems for a long time to be fibrous instead of 
muscular. 
It is convenient here to describe the exact conditions of the fold known as the 
septum secundum atrium. 
Retzer states in regard to it that in the pig “ the septum secundum in Born’s 
sense does not exist. Search for it was made in vain in the embryonic hearts of 
man, monkey, and rabbit. 
“ The conus arteriosus is the fixed point physically, and the growing atria, in 
their efforts to expand and form auricles, grow around the fixed point and cause 
a bulging inwards of the wall. It is, therefore, a passive formation, and never 
attains such a size that one is justified in calling it a septum. The septum primum 
of Born remains the ultimate septum.” 
In the same paper Retzer describes and figures the attachment to the floor 
