when he was just twenty-two years old. In it he de-
scribes a new science of organization that could be
used to explain how it is that new and more powerful
forms of knowledge can arise out of less powerful
ones. Throughout his long career Piaget continued to
explore this problem both in the thinking of children
and in the history of science.
The breadth of Piaget’s application of these ideas
can be seen in the titles of some of his major works:
Judgement and Reasoning in the Child; The Origin of Intel-
ligence in the Child; Construction of Reality in the Child;
The Child’s Conception of Number; Play, Dreams and Imi-
tation in Childhood; Intelligence and Affectivity; Biology
and Knowledge; Sociological Studies; Psychogenesis and the
History of Science; and Towards a Logic of Meanings. Rec-
ognition of Piaget’s contributions include honorary
doctorates from thirty-one universities and appoint-
ment to the Executive Council of United Nations Ed-
ucational Scientific Cultural Organization
(UNESCO).
See also: STAGES OF DEVELOPMENT
Bibliography
Bringuier, Jean Claude. Conversations with Jean Piaget. Chicago:
University of Chicago Press, 1980.
Smith, Les. Critical Readings on Piaget. London: Routledge, 1996.
In the Jean Piaget Society [web site]. Available from http://
http://www.piaget.org; INTERNET.
Chris Lalonde
PLACENTA
The placenta is a disk-shaped organ that serves as the
interface for maternal and fetal exchange of materi-
als. It is formed early in pregnancy when the outer
cell layer that envelops the developing embryo, the
chorion, fuses with the uterine wall forming fingerlike
projections called chorionic villi. Each villus, sur-
rounded by a pool of maternal blood, contains a net-
work of fetal capillaries through which nutrients and
waste products are transferred (although there is no
actual exchange of blood). Products harmful to fetal
development, such as nicotine, cocaine, alcohol, some
medications, and environmental pollutants, may also
be transferred to the fetus. Upon completion of the
pregnancy, the placenta is expelled from the uterus.
For the purposes of diagnosis in cases of complica-
tions, physicians may examine the placenta. Compli-
cations involving the placenta itself include
postpartum hemorrhage, placenta previa, pre-
eclampsia, and intrauterine growth restriction.
See also: BIRTH; PREGNANCY; PRENATAL
DEVELOPMENT
Jean Piaget’s fascination with children’s reasoning began with his
work on early Intelligence Quotient (IQ) tests. (AP/Wide World
Photos)Bibliography
Begley, David J., Anthony Firth, and Robin Hoult. Human Repro-
duction and Developmental Biology. New York: Macmillan, 1980.
Faber, J. J., and Kent Thornburg. Placental Physiology: Structure and
Function of Fetomaternal Exchange. New York: Raven Press,
1983.
Jansson, Thomas, and Theresa Powell. ‘‘Placental Nutrient Trans-
fer and Fetal Growth.’’ Nutrition 16 (7/8):500–502.
Vander, Arthur, Jane Sherman, and Dorothy Luciano. Human
Physiology: The Mechanisms of Body Function, 5th edition. New
York: McGraw-Hill, 1990.
Patricia Crane EllersonPLAY
All children play. From the infant squealing in delight
during a game of peek-a-boo to the older child play-
ing a game of basketball, children of all ages play and
they play in all kinds of ways.
Play is recognized as an important part of a
child’s development. In fact, it is an important topic
of study in many different disciplines. In the field of
early childhood special education, play is valuable in
assessing a child’s level of development and in pro-
viding intervention. In psychology, therapists oftenPLAY 309