1. Bühler, K. 1930.The mental development of the child. Translated from the fifth German edition by Oscar Oeser, xi + 170New York: Harcourt, Brace.
2. Eng, Helga. 1931.The psychology of children's drawings, Edited by: Hatfield, H. S. viii+223New York: Harcourt, Brace.
3. Jespersen, O. 1922.Language, its nature, development and origin, 448New York: Holt.
4. Luquet, G. H. 1927.Le dessin enfantin, 260Paris: Félix Alcan.