Mo. Rev. Stat. § 456.1065: Disposition of unappointed property under released or unexercised nongeneral power.

To the extent a powerholder releases, ineffectively exercises, or fails to exercise a nongeneral power of appointment:

(1) The gift-in-default clause controls the disposition of the unappointed property; or

(2) If there is no gift-in-default clause or to the extent the clause is ineffective, the unappointed property:

(a) Passes to the permissible appointees if:

a. The permissible appointees are defined and limited; and

b. The terms of the instrument creating the power do not manifest a contrary intent; or

(b) If there is no taker under paragraph (a) of this subdivision, passes under a reversionary interest to the donor or the donor's transferee or successor in interest.