GARY PARKER / SCIENCE PHOTO LIBRARY GARY PARKER / SCIENCE PHOTO LIBRARY
Identical twins. Identical (monozygotic) twins develop when a single fertilised ovum divides completely and equally at an early stage of the embryonic phase. Identical twins share the same placenta, are always of the same sex and look remarkably alike. Nonidentical twins come instead from two separate ova which have been fertilised by two different spermatozoa. They may be of different sexes and look quite different.
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