I mentioned earlier that it now appears that, when monozygotic twinning occurs, an original embryo is formed at time of egg-sperm fusion, and then some of its cells break off at the blastula stage to form a second embryo, while the original embryo continues to exist, and can regenerate its missing cells.
Even if this picture turns out to be erroneous, and it turns out that monozygotic twinning erases the existence of the original embryo, and leaves two new embryos in its wake, this would still not prove that, before the twinning event occurred, there was not one individual embryo.
As an analogy to help demonstrate this clearly, let us consider the fact that, in principle, every single cell could be taken from an adult animal, such as an adult human's, body and a clone made from each one of them. This would have the result that there would be trillions of clones of the original adult, while the original adult would cease to exist. But by no means does this, somehow, retroactively negate the existence of the original adult as one individual organism, as opposed to merely a not-yet-individuated clump of cells, prior to its dismantlement and concurrent cloning.
When it is realized that, in any case, regardless of what happens to the original embryo when it splits to form identical twins, triplets, quadruplets, etcetera in the monozygotic twinning process, the exact same process could theoretically happen to an adult, as well, the legitimacy of this argument against the individuality of early embryos during the stage in which monozygotic twinning is possible gets effectively flushed down the toilet.