Embryo question

[Merely]

The amnion forms before the chorion which forms before the placenta, is that correct?

If that is, why is it that if you split as an early blastocyst, you will have separate amnions but common chorions and placentas? If the placenta and chorion happen later than the amnion, it would make sense that you would have similar amnions then the split would happen then independent chorions and placentas? Im very confused would appreciate some insight! This is the process of monozygotic twinning im referring to

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[22031 Alum]

What actually is seen as an invading, "proper" placenta is apparent after amnion formation, but the precursor cells that become chorion/placenta come first. Or at least start down their final pathway of differentiation first. (I actually hate embryology, go figure.) Just remember, the order is trophoblasts (chorion) THEN yolk sac (amnion) THEN embryonic disc (body parts). That may be the source of your confusion.

For twinning the rule of thumb you can remember is that the later the split, the more is shared. First chance is at morula- you get two chorions and two amnions (di/di) twins as it's early enough for everything to differentiate separately. Second chance to split is at early blastocyst, when trophoblast has already formed, and you get MONOchorionic/ DIamnionic twins. Third chance is at late blastocyst, yielding MONO/MONO twins- everything has formed except the embryo itself, so splitting gives you two embryos in one sac. So conjoined twins "split" too late and share body parts.