Mo. Rev. Stat. § 510.350: Motion for new trial when based upon affidavits.

When a motion for new trial is based upon affidavits they shall be served with the motion. The opposing party has ten days after such service within which to serve opposing affidavits, which period may be extended for an additional period not exceeding twenty days either by the court for good cause shown or by the parties by written stipulation. The court may permit reply affidavits.