The usual way for new cells to come into being is by division of old cells. So the zygote, which is a—new—single cell formed from two, the sperm and ovum, is an exception. Textbooks of human genetics usually say that this new cell is beginning of a new human individual. What this indicates is that they suddenly forget about identical twins.

The process of embryogenesis poses numerous philosophical puzzles. Conceptual difficulties encountered while attempting to clarify the ontological and moral status of the fertilized ovum, for example, are compounded in the minds of some philosophers by the possible occurrence of monozygotic twinning during the earliest stages of embryological development. In light of certain conceptual complexities engendered by this possibility, G.E.M. Anscombe, for example, has come to believe that the pivotal metaphysical query in need of an adequate response is the following: Is (...) the human zygote a single human? The answer to this question is of interest to Anscombe primarily for the role it would play in solving the following puzzle: Suppose that zygote A divides into twins B and B1, such that neither B nor B1 is identical with its twin or with A. It then seems clear that either A, prior to its division, was somehow already B and B1; or A was a single human out of which another human grew ; or A, which was a single human, became two humans, much like an amoeba becomes two amoebae; or A is a "whole human substantial entity". (shrink)