  Interlayer single particle tunneling between the $Cu-O$ layers suppress the
in-plane short range magnetic order (which is modeled as spin density wave
(SDW) insulator). Doping over the SDW state kills perfect nesting of the Fermi
surface (FS) in certain directions and hence SDW gap reduces to zero in those
directions of the FS. Coupling between the planes through interlayer tunneling
($t_{\perp}$) further suppresses the in-plane magnetic SDW-gap and hence
becomes anisotropic. Superconductivity arises in the gapless regions of the FS
under the `modified spin bag' mechanism. We show that the highest $T_c$ can
only be obtained for non-zero $t_{\perp}$ based on this mechanism.
