Missouri Revised Statutes Title XXXV. Civil Procedure and Limitations § 507.080. Third-party notice--when defendant may bring in third party--notice to plaintiff--third party to file petition, when

1. Before filing his answer, a defendant may move ex parte or, after the filing of his answer, on notice to the plaintiff, for leave as a third-party plaintiff to file a petition and serve a summons upon a person not a party to the action who is or may be liable to him or to the plaintiff for all or part of the plaintiff's claim against him. If the motion is granted and the petition is filed and summons served, the person so served, herein called the third-party defendant, shall make his defenses, counterclaims and cross-claims against the plaintiff, or any other party as provided in this (civil) code. The third-party defendant may assert any defenses which the third-party plaintiff has to the plaintiff's claim. The third-party defendant is bound by the adjudication of the third-party plaintiff's liability to the plaintiff, as well as of his own to the plaintiff or to the third-party plaintiff. The plaintiff may amend his pleadings to assert against the third-party defendant any claim which the plaintiff might have asserted against the third-party defendant had he been joined originally as a defendant. A third-party defendant may proceed under this section against any person not a party to the action who is or may be liable to him or to the third-party plaintiff for all or part of the claim made in the action against the third-party defendant.

2. When a counterclaim is asserted against a plaintiff, he may cause a third party to be brought in under circumstances which under this section would entitle a defendant to do so.