Mo. Rev. Stat. § 509.450
509.450 Counterclaim set up by amendment, when.
509.450. Counterclaim set up by amendment, when. — When a pleader fails to set up a counterclaim through oversight, inadvertence, or excusable neglect, or when justice requires he may by leave of court set up the counterclaim by amendment.
(L. 1943 p. 353 § 76)