Biological Identities

Bruce Colbert (letter, Aug. 5) claims that a zygote, morula or blastocyst “retains its identity as an enduring being” and that “from conception, we are distinct and self-directing.” This is inconsistent with the fact that identical twins are formed by the splitting of a single embryo into two viable embryos, as occurs in about one-quarter percent of human births. This splitting, which can also give rise to triplets and more, can occur at stages prior to gastrulation--that is, in zygotes, morulas and blastocysts.

Thus, a zygote need not be an enduring being. It may give rise to more than one distinct being, one or more of which may survive, or to none at all.

It is self-directing in the same sense that a gnat zygote is--its development depends on its genes and on its environment.

William K. Purves

Professor Emeritus of Biology

Harvey Mudd College, Claremont