Identical twins are
developed from a single fertilized egg which
possesses two growing points. Hence such twins
have substantially identical heredity, while
ordinary twins are merely brothers, sisters,
or brother and sister, borne two at a time, and
are no more alike than ordinary brothers and
sisters born several years apart. The comparison
of the two types of twins in different
environments has therefore been one of the most
fruitful means of studying human heredity.
