CONTENTS OF VOL. 4: Chap.1 Retention of Discriminations and an Analysis of Learning Set (Bessemer & Stollnitz). Chap. 2 Higher Mental Functions of a Home-raised Chimpanzee (Hayes & Nissen). Chap. 3 Two-Way Communication With an Infant Chimpanzee (Gardner & Gardner). Chap. 4 On the Assessment of Language Competence in the Chimpanzee (Premack).