Mo. Rev. Stat. § 491.070

A party to a cause, civil or criminal, against whom a witness has been called and given some evidence, shall be entitled to cross-examine said witness (except where a defendant in a criminal case is testifying in his own behalf) on the entire case, but this shall not be construed to entitle a defendant who has pleaded a counterclaim or setoff in a civil case to cross-examine a plaintiff's witness in respect thereto, but as to said counterclaim or setoff such witness (if examined by defendant in relation thereto) shall be deemed defendant's witness and be so examined in the course of the trial.