1. GARDNER, R. A., & GARDNER, B. T. 1967a. Acquisition of sign language by an infra-human primate. Unpublished proposal and progress report. Reno: Univ. of Nevada.
2. GARDNER, R. A., & GARDNER, B. T. 1967b. Teaching sign language to a chimpanzee. Parts I and II. Psychol Bull., 1 (2), 36.
3. HARRISSON, B. 1962. Orang-utan, London: Collins.
4. HAYES, C. 1951. The ape in our house. New York: Harper.
5. HAYES, K. J., & HAYES, C. 1951. The intellectual development of a home-raised chimpanzee. Proc. Amer. Philos. Soc., 95, 105–109.