ARNO MASSEE / SCIENCE PHOTO LIBRARY ARNO MASSEE / SCIENCE PHOTO LIBRARY
Conjoined twins. Skeleton of conjoined (siamese) twins showing that each twin had its own head, neck and trachea (windpipe), but shared the rest of the body. The medical name for a two-headed infant is dicephalus dibrachius. Most of these are stillborn or die soon after birth, although a few have survived beyond that. As with identical twins, conjoined twins result from the fertilisation of a single egg cell, but in this case the spitting of the embryo into two identical ones is incomplete.
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