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In a major nonfraternal
type of twinning, only one egg is fertilized; but during the cleavage
of this single zygote
into two cells, the resulting pair somehow become separated. Each
of the two cells may implant in the uterus separately and grow
into a complete, whole individual. In laboratory studies with
the zygotes of many animal species, it has been found that in
the two-cell stage (and later) a portion of the embryo, if separated
under the microscope by the experimenter, may develop into a perfect,
whole individual. Such splitting occurs spontaneously at the four-cell
stage in some organisms 
(e.g., the armadillo) and has been accomplished experimentally
with the embryos of salamanders, among others.
The net result
of splitting at an early embryonic stage may be to produce so-called
identical twins. Since such twins derive from the same fertilized
egg, the hereditary material from which they originate is absolutely
identical in every way, down to the last gene locus. While developmental
and genetic differences between one "identical" twin and another
still may arise through a number of processes (e.g., mutation),
these twins are always found to be of the same sex. They are often
breathtakingly similar in appearance, frequently down to very
fine anatomic and biochemical details (although their fingerprints
are differentiable).
Leslie
Gornstein
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