Maria Montessori, Gerald Lee Gutek (2004). “The Montessori Method: The Origins of an Educational Innovation : Including an Abridged and Annotated Edition of Maria Montessori's The Montessori Method”, p.59, Rowman & Littlefield
Maria Montessori, Gerald Lee Gutek (2004). “The Montessori Method: The Origins of an Educational Innovation : Including an Abridged and Annotated Edition of Maria Montessori's The Montessori Method”, p.145, Rowman & Littlefield