Comment on G. Herranz: The timing of monozygotic twinning: a criticism of the common model. Zygote (2013).

The topic of monozygotic twinning (MT) on which the recent paper by Herranz (Herranz, 2013) focuses is indeed a fascinating one, not only for the embryologist. And, as pointed out by the author, it may on principle also be relevant for ethical discussions in the context of embryo research and stem cell work in humans. Obviously, this is the reason why the paper was published in Zygote, in spite of the fact that it concentrates on history and speculation. When reading through the paper, however, I have ended up disappointed. The paper does have one merit: it is based on very careful reading of old, in part classical, descriptive literature, including original papers, reviews and textbooks, and it depicts meticulously the history of the emergence of the generally accepted model of MT in the human. It is indeed fun reading that first, historical part of the paper. The text correctly states that the universally accepted idea about the various modes of MT (addressed as ‘the model’) is based on few experimental data, which is a known fact and not surprising because embryo experimentation meets (and must meet) restrictions in humans. Those passages of the paper that come after this main part, however, are very disappointing: The Conclusions are very poor, and so is the Addendum – presenting an alternative hypothesis proposed by that author. His hypothesis consists, in substance, of nothing more than the assumption that the mechanism of MT is based solely on separation of (the first two) blastomeres [i.e. just one of the mechanisms which the author addresses in the main part of the paper, specifically under the heading ‘Part (a)’], while suggesting that the other