  A tight-band model is employed for the $e_{2g}$-orbitals in manganites. It is
shown that a large intraatomic Hund's coupling, $J_H$ and the resulting
double-exchange mechanism lead to antiferromagnetic ordering along one of the
cubic axis stabilized by the cooperative JT effect which further decreases the
band energy of electrons. As a result, LaMnO$_3$ is a band insulator built of
2D ferromagnetic layers. The critical concentration $x_c\simeq 0.16$, for onset
of ferromagnetic and metallic behavior at low temperatures in
La$_{1-x}$Sr$_x$MnO$_3$ and the phase transition are treated in the percolation
approach.
