G

Valve of coronary sinus

Figure 11.14 Atrial septa at various stages of development. A. 30 days (6 mm).

B. Same stage as A, viewed from the right. C. 33 days (9 mm). D. Same stage as

C, viewed from the right E. 37 days (14 mm). F. Newborn. G. The atrial septum from the right; same stage as F.

obliterating the oval foramen and separating the right and left atria. In about 20% of cases, fusion of the septum primum and septum secundum is incomplete, and a narrow oblique cleft remains between the two atria. This condition is called probe patency of the oval foramen; it does not allow intracardiac shunting of blood.

Further Differentiation of the Atria

While the primitive right atrium enlarges by incorporation of the right sinus horn, the primitive left atrium is likewise expanding. Initially, a single embryonic pulmonary vein develops as an outgrowth of the posterior left atrial wall, just to the left of the septum primum (Fig. 11.15 A). This vein gains connection with

Interseptovalvular space Septum spurium

Right venous valve

Sinuatrial orifice

Septum primum \

Pulmonary veins

Crista ■ terminalis r i

Pulmonary veins

Crista ■ terminalis r i

Superior vena cava

Sinus Septum venarum

.secundum

Septum primum

Left venous valve A

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