：Given $M$ as the midpoint of the height $S O$ of a regular tetrahedron $S-A B C$ with edge length 1. A plane is drawn through $M$ parallel to the lateral edge $S A$ and the base triangle $B C$. The area of the section formed by this plane cutting through the tetrahedron is $\qquad$
$\qquad$ .
11. $\frac{2}{9}$.

It is proved that the obtained cross-section defines a rectangle, whose side lengths are $\frac{1}{3}$ and $\frac{2}{3}$.