Artificial twinning

Artificial twinning is relatively simple and was first done successfully (in
sheep) in 1979. A single fertilized egg was used, meaning that the resulting
offspring was the result of sexual reproduction. Zygotes from normally fertilized
(sexually produced) eggs were harvested from ewes (female sheep). The
zygote was allowed to divide up to the 16-cell stage (see the “Cloning before
Dolly: Working with sex cells” section earlier in the chapter). The 16 cells
were then divided into two groups. The separate groups of cells went right on
dividing, and after they were implanted into the reproductive tract of the
ewe, they resulted in twins. The twins were genetically identical to each other
because they were produced from the same fertilized egg.