Up to 14 days after fertilisation the early embryo (called a zygote) can divide into two separate identical genetic twins to produce identical genetic twin babies 9 months later. At 14 days a primitive streak, or ridge of cells, develops on the disc of the embryo and then generally identical twins can no longer occur.

But, now and then, more than one primitive streak may be formed, giving rise to identical twins, or, fortunately far less often, Siamese twins. A week later the streak is replaced by the neural tube and plate, a primitive spine.

Hence, up to 14 days, it is possible to have two Toms or (exclusive "or") two Marys. Which individual is to be regarded as the original Tom (or Mary)? If one says Tom ceased to exist at twinning to create two new individuals, d*ck and Harry, then d*ck and Harry no longer came into existence at fertilisation, and where did Tom disappear to when nothing had been lost?

Up to the eight cell zygote stage each single cell has the potential to produce at least one baby. That is, the eight cell embryo could become either one or up to eight babies, depending on whether twinning occurs. Sometimes it can go either way.

Fertilisation of two separate eggs by two separate sperm can occur at conception to produce two separate zygotes, say Betty and Kay. But on day 6 the two embryos can combine to produce only one zygote, known as a chimaera. (Remember souls are indivisible.)

Now is this zygote, Betty and/or Kay or someone else, a mixture of both Betty's and Kay's genes? Has Betty and/or Kay ceased to exist when nothing has been lost? (Extracted from Helga Kuhse, deputy director of the Monash University Bioethics centre, "The Age" 14/10/87).