@article{ref1,
title="Perceptual and cognitive (or operational) structures in the development of the concept of space in the child",
journal="Acta psychologica",
year="1955",
author="Piaget, J.",
volume="11",
number="",
pages="41-46",
abstract="<p>[Abstract unavailable]</p>",
language="",
issn="0001-6918",
doi="10.1016/S0001-6918(55)80062-5",
url="http://dx.doi.org/10.1016/S0001-6918(55)80062-5"
}