In my last article, I discussed the early stages of embryonic development. In this article, I’m going to address how a fertilized egg can split into two embryos.

How Natural Twins Are Formed
There are two types of twins: monozygotic (one egg) which produces identical twins and dizygotic (two egg) which produces fraternal twins. With fraternal twins, there are two separate fertilization processes, each with its own sperm and egg. With identical twins, there is only one fertilization process, but at some point, the inner cell mass (ICM) splits apart to form two completely separate embryos.

Identical twins happen in about 1 in 400 births. About 33% happen before day 5 after fertilization, 66% happen between days 5-9, and splits after day 9 are rare and increase the risk of conjoined twins.

Prior to day 5 - If the ICM splits prior to day 5, each embryo will develop its own trophoblast cells, and 2 separate chorion will form.

Between days 5-9 - At this stage, the trophoblast cells have already differentiated themselves from the ICM, so if the ICM splits at this point, there will only be 1 chorion that will encase the two embryos.

At day 9, another layer forms around the embryo called the amnion. This will eventually become the amniotic sac which surrounds the embryo with amniotic fluid. If the twinning division has occurred before day 9 - and in most cases it has - each embryo will form its own amnion. If, however, the ICM splits after day 9, there will only be 1 amnion. With only one chorion and one amnion, the chance of the embryos conjoining and possibly sharing organs or body parts increased dramatically.

Stem Cells As the Precursor To All Cells
Looking at twin development, we can see that even if the cell mass divides between days 5-9, each division will go on to form a fully developed human being. The ICM remains undifferentiated during this time and can form any part of the embryo.

It’s also safe to say that at this stage, the mass of stem cells bears no resemblance to what we think of when we think of a human being. It’s just a bunch of cells, not unlike a cluster of skin cells or organ cells. If you saw it at this stage, you wouldn’t be able to tell that it would go on to become a human, nor does it possess any type of consciousness or any of the other characteristics we use to describe what makes someone a “human being” or “person.”

In my third installment, I’ll talk more about when cells start to differentiate during the 2-3 weeks of development and the neural tube starts to form.

Reference:
Bioethics and the New Embryology, ISBN: 0716773457. Picture, slightly modified, taken from pg 17.