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.
Mo. Rev. Stat. § 509.450
Counterclaim set up by amendment, when
Effective: 28 Aug 1943; (L. 1943 p. 353 § 76)
Official source: Missouri Revisor of Statutes. Reproduced from public-domain Missouri statutes; confirm against the official source for the current text. Not legal advice.